Ecology and Physiology
Autecology and phenology of Arctic Plants
EA: Plant Sciences
(Thorvald Sørensen)
AUTECOLOGY AND PHENOLOGY OF ARCTIC PLANTS
Conditions of plant life in the Arctic are justly considred unfavor–
able. The rapid decrease in number of species toward the extreme North
can probably be taken as a proof of a corresponding deterioration of the
conditions. The high-arctic vegetation, poor as it is, represents one
of the outposts of plant life on earth. Ecologically, therefore, it holds
a position of its own. In the following paragraphs, the main causes of
the poverty of vegetation and some general trends in the adaptation of
plants to the unfavorable conditions will be discussed. Observations
made by the author in northeast Greenland between 70° and 77° N. latitude
form the main basis of the present paper.
Plant distribution within an area is largely determined by the two
leading ecological factors: temperature and access to water. In the true
Arctic these factors have to be looked upon in a manner somewhat different
from that to which we are accustomed in the milder southern latitudes.
The constantly frozen subsoil acts as an unbroken bedrock. Water communi–
cation is restricted to comparatively thin layers which are determined by
the depth to which the soil thaws in summer. The immature soils are strongly
affected by solifluction, which tends to be inimical to plant life. A
closed plant cover is found only in localities with stable conditions in
EA-PS. Sørensen
L
^
:
^
Autecology and Phenology
the upper soil strata
[:
]
- for example, on mature moist or wet soils
which have attained a state of equilibrium through the sorting by frost
of the coarse material into a framework of stone rings, and the subse–
quent removal of the fine-grained material by the action of silting water.
Apart from this, the climax vegetation of the high-Arctic is the “speck
tundra” where plant life is hampered by solifluction.
Apparently, plant species of a chamephytic life from (that is, having
the perennating buds at the soil surface, the aerial parts dying down in
autumn) are best suited for keeping pace with the movements of the soil
and preventing the shoot spices from being buried in the mud. Moreover,
the cooling influence of the convection currents within the mud polygons
should not beneglected. Only plants with very modest temperature require–
ments can thrive in wet soil that is constantly cooled by the water
ascending from the melting ice beneath. On theother hand, sloping terrain
supplied with flowing or percolating water from distant snowbanks can
contribute to an improvement of the temperature conditions and harbor
relatively luxuriant vegetation composed of more exacting species.
The North Greenland climate is
[:
]
characterized by: (
1
)
extremely low winter temperatures, which sometimes fall to about −50°C.,
(
2
) cool summer temperatures, the mean during the warmest month scarcely
rising above 5°C. and (
3
) a very short season of only 2 to 3 months
when the temperature normally remains above freezing point throughout
the 24 hours.
From field observations on the occurrence and distribution of the
individual species, it must be inferred that the winter cold does not
seriously hamper plant distribution in the Arctic. Snowfall in northern
EA-PS. Sørensen: Autecology and Phenology
Greenland often does not set in until the soil surface has been exposed
to very low temperatures; thus, all plant species of the region, including
the plants of the snow patches, can endure extreme cold without injury.
On the other hand, it is evident that temperature conditions of the
growing season, and their local variations with exposure, water supply,
and geographical location, play an important part in plant distribution.
The duration of the season with sufficiently warm temperatures for develop–
ment and growth, flowering and fruiting, seems to be the ecological factor
which, for the majority of plant species, comes closest of the critical
minimum. The survival of the individual plant is not as important a
problem as the maintenance of the stock, i.e., reproduction.
The climate is indeed harsh. However, the meteorological data avail–
able from these northern regions provide a somewhat exaggerated impression
of severity because they do not give a reliable picture of the conditions
under which the plants actually live. This is illustrated by the following
information from latitudes 72° to 74° N. in northeast Greenland. During
the favorable season the temperature of the surface soil (i.e., within
the uppermost 5 centimeters) is 5° to 7°C. above the meteorological mean
air temperature. In the spring, the mean temperature of the surface soil
passes the 0°C. line 3 to 4 weeks earlier than the meteorological mean.
In the autumn, however, the mean temperature of the air and the surface
soil will intersect the 0°C. line at about the same time. In places
with a rather slight southern exposure, the soil is commonly thawed to a
depth of about half a meter by the time the mean air temperature in the
spring reaches 0°C.
These data show to what degree the real plant climate is influence
s
^
d
^
EA-PS. Sørensen: Autecology and Phenology
by insolation. North of the Arctic Circle, the unbroken daylight during
the
[:
]
summer causes a relatively high summer temperature. Thus for
example, in East Greenland, the meteorological summer temperature of
Denmark Harbor (about lat. 77° N.) does not differ substantially from
that of scoresby Sound (about lat. 70° N.). Nevertheless, a surprising
decline in the flora can be noted. About one-fourth to one-third of the
flowering plants of Scoresby Sound have disappeared at Denmark Harbor.
It has been shown that during midsummer the irradiation at ground level,
coupled with the high transmission coefficient which is found in northern
Greenland, does not decrease toward the north. In higher latitudes,
however, an increasing fraction of the irradiation falls upon the north–
facing slopes. In other words, the temperature conditions of the favored
southern hill slopes rapidly become less favorable toward the north as com–
pared with the meteorological air temperature. While southern exposure
has an extremely favorable influence on plant climate in the eastern
coastal part of central Greenland, it is of relatively minor importance
in the northernmost parts.
A few phonological observations may serve to throw some light upon
the problems of plant climate (see Table I).
|
Table I. Calendar Time in Relation to Phenological Data
|
|
Location
|
Year of
observation
|
Mean 24-hour temp–
erature around 0°C.
|
Breaking of
rivers
|
Saxifraga oppositifolia
in flower
|
|
Denmark Harbor
|
1907
|
June 3 to 10
|
June 20
|
June 4
|
|
(lat. 77° N.)
|
1908
|
June 8 to 16
|
June 18
|
June 7
|
|
Clavering Island
|
1935
|
June 11 to 18
|
June 3
|
June 11
|
|
(lat. 74° N.)
|
|
|
|
|
|
Scoresby Sound
|
1892
|
June 6 to 13
|
May 15
|
May 23
|
|
0
^
(
^
lat. 70° N.)
|
|
|
|
|
EA-PS. Sørensen: Autecology and Phenology
The mean air temperature at the two main observation stations,
Denmark Harbor and Scoresby Sound, passes the 0°C. line at almost the
same time. The breaking up of the rivers occurs about 35 days apart.
At Denmark Harbor the break-up happens one or two weeks after, at
Scoresby Sound about one month before, the 0°C. mean has been reached.
It should be noted that the breaking up of the rivers indicates a large–
scale melting of the snow masses and also the disappearance of the slight
snow cover from the favorably exposed slopes. These phenological data
undoubtedly give a clue to the understanding of the abrupt decrease in
the number of species between 70° and 77° N. latitude.
In East Greenland at 72° to 74° N. latitude, i.e., immediately south
of the region most pronounced floral decline, the following succession of
aspects may be noted. The vernal aspect, from the beginning of June to
about June 25, is characterized by the flowering of
Saxifraga oppositifolia,
Cerastium
P
alpinum
species of
Draba
,
Melandryum (Lychnis)
, and
Potentilla,
growing upon rock ledges and hills that are bare of snow in winter.
This aspect is also characterized by the coming into leaf of the deciduous
dwarf shrubs, mainly
Betula nana
and
Vaccinium uliginosum.
The aestival aestival aspect, from the end of June to the middle of
July, or somewhat longer, is characterized by the flowering of
Cassiope
tetragona
and
Dryas
, and of the plants which are associated with them in
heaths, etc.
The serotinous aspect, from the latter half of July to about August 10,
is characterized by the flourishing of the most luxuriant grass and herb
slopes, mainly harboring the southern species of the region. During this
season also the Salix arctica heaths, which may only recently have become
EA-PS. Sørensen: Autecology and Phenology
free from snow, are adorned with innumerable candle-shaped catkins. In
depressions and revines the snow-patch vegetation flourishes step by
step.
The autumnal aspect, in the second half of August, is characterized
by intense coloring of the foliage of the deciduous dwarf bushes.
The vernal plants,
Saxifraga oppositifolia
and its companions, are
so hardy that one could visualize them holding on to the North Pole if
only as much as a bird’s resting place of bare ground could be found:
As early as the middle of the short summer, they are often subjected to an
extreme drought, causing an interruption of their life functions long before
the autumnal frost sets in. They represent the xerophytes among the high–
arctic plants. Conversely, the plants contributing by their flowering to
the serotinous and autumnal aspects are mainly hygrophytes. From an
ecological viewpoint, two main types of hygrophytes can be roughly dis–
tinguished: (
1
) the inhabitants of watered slopes which thaw moderately
early and those of the real swamps, Gramineae and Cyperaceae predominating;
(
2
) the constituents of the real snow-patch vegetation, dicotyledcnous
plants predominating but the families Saxifragaceae, Cruciferae, Caryophyl–
laceae, and Ranonculaceae being abundantly represented. Only the last type
proceeds far to the north.
From the phenological data it appears that the flowering of
Saxifraga
oppositifolia
at latitude 77° N. is delayed by a fortnight as compared
with latitude 70° N. In other words, the vernal aspect is delayed by a
fortnight, i.e., a much smaller retardation over seven degrees of latitude
than is observed in temperate regions. The rapid advance of the phenologic
springtime in the North is not followed up by the next season. As a
EA-PS. Sorensen: Autecology and Phenology
consequence of the retarded snow-melt toward the north in areas subjected
to a moderate snow cover, at least as compared with most areas of lower
latitude, and especially their southern slopes, the summer aspect is
subjected to a greater calendar retardation with increasingly high latitude.
Strictly speaking, in the extreme North no real summer aspect as defined
above is found, in fact, the vernal and the autumnal aspects meet.
The vegetation types associated in East Greenland with a moderate
snow-covering and a rather early thawing, such as some
Cassiope tetragona
heaths, are characteristic of the middle Greenland landscape. To the north,
these vegetation types withdraw to narrow belts on the southern slopes
skirting the hilltops; or they may disappear entirely, giving way to a
vegetation found farther south in the depression having a prolonged snow–
covering. Accordingly, the plant communities which by their flowering
contribute to the summer aspect, are markedly diminished in area. The
communities whose culmination characterizes the serotinous aspect to the
south, i.e., the luxuriant grass and herb slopes, have here disappeared.
The constituents of these exacting communities represent the abrupt floral
decline which has been mentioned.
The increasing retardation of the summer aspect in a south-to-north
direction as compared with the vernal aspect, is paralleled by a similar
retardation in a west-to-east direction, i.e., from the heads of the
fjords toward the coast land in East Greenland. The last-mentioned re–
tardation is caused by frequent fogs in the coastal regions, especially
after the breaking up of the Arctic Sea drift ice in early summer. Thus,
in the spring there is no essential phenological difference between inland
and coast, while in the summer the flowering time of one and the same
EA-PS. Sørensen: Autecology and Phenology
species may differ by a fortnight. This general phonological retardation,
which mainly
[:
]
influences the late-flowering species, probably accounts
for the well-known fact that in the coast lands the northern plant communi–
ties proceed far to the south.
From the phenological typification of the northern Greenland vegetation
briefly outlined above, it may be inferred that plant distribution and
differentiation of the vegetation are dependent mainly upon the duration
of the growth season. The faster a plant can complete its yearly life cycle,
the farther to the north and the deeper into the snow-covering sequence will
it be able to thrive. However, it can be demonstrated that, generally the
growth rate of the northern species is rather slow as compared with more
southern species. This characteristic might impede a rapid flowering,
although the peculiar developmental morphol
g
^
o
^
gy of the high-arctic plants
compensates for this drawback.
The following peculiarities of arctic plants should be emphasized:
(
1
) an advanced development of the floral organs during the year (or years)
preceding flowering; (
2
) proleptic or anticipating development of innova–
tion shoots which causes a distribution of the yearly leaf production
over several few-leaved shoots within one and the same sympodium. The
result
s
is a simultaneous expansion of the foliage and a possibility
for the sympodium to develop flowers yearly in spite of the inconsiderable
growth of the individual shoot apices; (
3
) irregular or at least a
periodic development: a rather large number of species, particularly
those inhabiting the late-thawing snow patches, show quite a periodic
and almost continuous initiation of shoots and floral organs. Such plants
have no real resting period. They winter in a torpid state, with floral
EA-PS. Sorensen: Autecology and Phenology
organs in all or at least in strongly varying developmental stages which
range from scarcely observable initials to flowering and fruiting states.
Generally, only flower buds which have not passed a cer
g
^
t
^
ain developmental
stage are able to continue their growth in the following spring. Never–
the le
a
ss this development secures an effective utilization of even the
shortest favorable period for the purpose of producing reproductive organs.
Even if the flowers are apt to be destroyed year after year, some favorable
summer may yield sufficient time for the flower buds that have wintered in
a proper stage to complete their fruiting. It should be pointed out that
only one aperiodic plant,
Braya humilis
, was found to winter uninjured,
even on ground bare of snow, in all stages, and to continue its growth and
development during the following summer. Many northern species, especially
the aperiodical ones, are evergreen and have no bud protection at all.
Hence, the shoot apices look rather alike all the year round. (
4
) Vivipary:
a number of species, most of them belonging to the families
Saxifragaceae
,
Caryophyllaceae,
Gramineae, propagate by means of vegetative “diaspores”
such as bulbils and detached shoot apices. The vegetative diasporas seem to
endure the winter frosts in every
[:
]
stage of development. However, no
marked correlation between vivipary and speriodicity could be demonstrated.
In contrast to the decidedly northern species just mentioned, the
species of a more southern distribution, in East Greenland reaching
their northern limit slightly north of the 70th parallel, are usually distinguished
by the following characteristics: no
[:
]
proleptic shoots; concentration of
the yearly leaf production to a single shoot generation, in some cases to
the flowering shoot proper, and, consequently, successive development of
leaves during the summer of flowering. Because the flower buds winter in
EA-PS. Sørensen: Autecology and Phenology
a young state, if initiated at all before wintering, plants of this
developmental type require a relatively long prefloration period. Aperiodi–
cal development, as described above, is scarecely met with among the southern
species. Evergreen herbs are rare. Bud protection by scale leaves is
frequent.
The two developmental types pointed out above, the one attached to a
northern, the other to a southern distribution, cannot be distinguished
sharply Typification may serve to make clear some general trend of adaptive–
ness to the unfavorable conditions which the northernmost plants on earth have
to endure. The pronounced tendency to a proleptic formation of innovations
connected with an inconsiderable yearly increment of the individual shoots,
as demonstrated for the one type, results in the well-known cushion shape of
many high-arctic plants. Such a growth form, it may be inferred in this
instance, represents a secondary adaptation to an extremely short summer.
Slow growth and small size make the high-arctic plants unable to com–
pete for space with faster-growing and bigger southern plants. Evidently
the northern species will suffer the fate of being exterminated by compe–
tition, if for climatic reasons the southern species alone are able to
live and propagate. Thus, the southern species have the opportunity of
reaching their definite climatic northern limit. The high-arctic plants,
on the other hand, will not be found so far to the south as their climatic
tolerance would allow. They are the outsiders of the plant world. Their
acquired adaptiveness to environment is mainly one-sided, as differentiation
of conditions on inhabitable land is lowered toward the extreme north.
Size is unessential because high-growing exacting competitors are absent
from most of the domain of high-arctic plants. Their chief virtue is
EA-PS. Sørensen: Autecology and Phenology
contentment. Their demands are small in every respect. Evidently they
live in a state of constant hunger if we judge from their altering size
and habit as a response to manuring round lemmings’ holes and foxes’
earths. High-arctic plants act on the defensive, so to speak. Their
principal goal is to shorten their yearly life cycle at the expense of
the vegetative equipment. Their developmental peculiarities, their habit
and size, and their devious geographical area all have a phenological
background. Their kingdom is characterized as being the land where time
is all and where lack of time is constantly threatening continuation of
life.
EA-PS. Sørensen: Autecology and Phenology
BIBLIOGRAPHY
1. Kjellman, F.R. “Ur polarväxternas lif.” Nordenskiőod:
Studier och
Forskningar
. Stockholm, 1883.
2. Resvoll, Thekla. “Om planter som passer til kort og kold sommer.”
Archiv. Fűr Mat. Nat
. vol.35, 1917.
3. Sørensen, Thorvald. “Temperature relations and phenology of the north–
east Greenland flowering plants,”
Medd. Grønland
, vol. 125,
no. 9, 1941.
Thorvald Sørensen
The Polar Tree Line
EA-PS. Ilmari Hustich
THE POLAR TREE LINE
The approximate course of the polar tree line is shown in Figure 1.
The boundary on the map attempts to include to its south the scattered trees,
sometimes occurring as bushes only, which lie outside the forest region proper.
In general, coniferous tree species go farther north than deciduous species.
In continental areas the tree line tends to
[:
]
lie further north than in mari–
time
[:
]
areas. This is the reason for the eccentric course of the tree line
around the North Pole.
The general factors which influence the climate also, of course, influence
the course of the tree line; the northern situation of the tree line in Scandi–
navia, for instance, is due to the ocean current in the Atlantic. The course of
the tree line coincides fairly well with the course of the 10°C. July isotherm,
and the marked northwest-southeast direction of the tree line in North America
coincides completely with the general direction of the isotherms.
The appearance of the tree species at their polar limit shows a general
similarity all over the polar
[:
]
area, in spite of the fact that different species
form the tree line in different parts of the continents. The some stunted tree
form and low creeping branches that are found in extremely windy localities,
and the some marked difference between the
[:
]
branches which are covered with
snow in winter and those lying above the snow (the height of
[:
]
the snow cover is
usually indicated by a branchless part of the stem) can be seen on almost all
polar tree-line individuals.
In arctic America the following species from the polar tree line: white
spruce (
Plcea glauca
), black spruce (
P. mariana
), and tamarack or black larch
EA-PS
Hustich: Tree line
(Larix laricine)
. It seems that in areas with sedimentary bedrock, or where
the glacial till is of less acid origin, as also on alluvial soils, white spruce
forms the polar tree line. Black spruce dominates in granite-gneise areas,
and in moist places, the northernmost trees are tamaracks. In Alaska balsam
poplar (
Populus balsamifera
) reaches the polar tree line, as also on the Labra–
dor coast. White b
ri
^
ir
^
ch and aspen reach almost as far.
In Eurasia several species appear at the tree line. In Europe, going from
west to east, Scotch pine (
Pinus silvestris
), Norway spruce (
Picea excelsa
),
and the closely related
P. obovate
, are found along the line. Outside this
coniferous tree line, low mountain birches (
Betula tortuosa
agg. and related
species) occur, particularly in northern Scandinavia and on the Kola Peninsula.
In the Pechora region the tamarack-dominated Sibe
e
rian polar tree line begins; the
tamarack species is
Larix sukatschewii
, a species that was some years ago sepa–
rated from
L. sibirica
, which forms the tree line in the central part of north–
ern Siberia. In eastern Siberia a
[:
]
third tamarack species,
L. dahurica
,
appears at the tree line. A poplar,
Populus suaveolens
, goes as for north as the
tamarack in subarctic eastern Siberia. The tree line on Figure 1 inclu
ed
^
de
^
s in
easternmost Siberia the bushy pine species,
Pinus pumila
. The fir species (
Abies
),
such as balsam fir and Siberian fir, do not teach the tree line, and pine is also
generally absent there, whereas birth, aspen, and balsam poplar go as far, or
nearly as far, as the tree line.
The forest region proper begins some distances from the tree line. The polar
tree line in the north, and the northern limit of the continuous forest, the
forest line, border a zone which in places is a hundred miles wide, being espe–
cially broad on the northern plains of the Soviet Union. This zone, the forest
tundra, is ecologically interesting; here arctic and forest plant and animal
EA-PS
Hustich: Tree li ne
species intermingle. Economically, however, this region is of little value;
the trees grow slowly and are stunted and deformed. The treeless tundra
easily invades a cleared area in this forest-tundra region where the forest goes
[:
]
north only in the shape of narrow patches or scattered trees in sheltered valleys,
these timbered tracts being bordered by large open tundra areas. This forest–
tundra region was probably narrower in earlier centuries, but
[:
]
the fuel and timber
requirements of the nomadic northern tribes, such as the Samoyeds in the north–
ern Soviet Union, may have formed large open tundra areas in the northernmost
parts of what were once continuous forests.
It seems appropriate to apply the common expression “timber line” only
to the northern limits
[:
]
of forests with good reproductive capacity, and this
timber line
^
,
^
then, does not coincide with the forest line. The different ex–
pressions used here - tree line, forest line, and timber line - are illustrated
by Figure 2. Generally speaking, the northernmost trees flower richly, forming
great quantities of cones, but the seeds do not germinate every year. An
early investigation by Renvall of the Scotch pine forest in northernmost Finland
showed, for instance,
^
that
^
there are 60 to 80 years between good seed years at the
polar tree line. In
this
^
the
^
author’s opinion the same
[:
]
condition could, at least
in some degree, be found on other polar tree lines.
In the north, the summer is short and its temperature varies greatly from
one year to another, a
[:
]
good warm summer with rich flowering commonly alterna–
ting with cold inhospitable ones - a fact well known to everyone who has spent
more than one summer in the Arctic. In some places north of the present polar tree
line one may find remnants of trees, indicating warmer periods in earlier centuries;
one may also, in exposed forest areas that have been cut, observe a rate of re–
growth too slow to enable the trees to hold their own. However, in the last
EA-PS
Hustich: Tree line
decades a slight climatic change, confirmed by several glaciologists and
meteorologists, has
[:
]
caused the forest line and tree line to move northward.
This is the case particularly in areas near latitude 70° N., where continuous
daylight may accentuate a rise in temperature. The seed years at the polar tree
line have been more frequent in the latest decades than previously, and the growth
in thickness of the northern forest trees has slightly increased.
At the polar tree line the trees are on their climatic border and thus, ac–
cording to a familiar
^
e
^
cological principle, react more freely to changes in cli–
mate. The polar
[:
]
tree line should
^
,
^
therefore, the considered not as something
stable but as a dynamic phytogeographical border line
^
that
^
clearly oscillates in
accordance with climate fluctuations.
Ilmari Hustich
Phytoplankton
EA-Plant Sciences
(R. Ross)
PHYTOPLANKTON
CONTENTS
|
|
Page
|
|
Marine Phytoplankton
|
1
|
|
Distribution of Marine Phytoplankton
|
9
|
|
The Polar Basin
|
9
|
|
Greenland and Barents Sea
|
10
|
|
Davis Strait and Baffin Bay
|
11
|
|
Hudson Bay and the Canadian Arctic Archipelago
|
12
|
|
Freshwater Phytoplankton
|
13
|
|
Bibliography
|
16
|
EA-Plant Sciences
(R. Ross)
PHYTOPLANKTON
The plant life of the sea is divided into two main ecological groups:
the benthos which lives on a solid substratum (whether this be the bottom
of the seas, other plants or animals, or, in polar waters, ice), and the
plankton which floats unattached. The term plankton is used to include
both the plant and animal life which occurs free in the waters of the
ocean and whose mobility is so limited that it drifts at the mercy of the
currents. It is the plants of this community which constitute the phyto–
plankton. Most bodies of fresh water larger than puddles also possess a
phytoplankton. Both marine and freshwater phytoplankton consist entirely
of members of the Algae; but there are great differences in their composi–
tion and the factors affecting their distribution and abundance in the two
types of water, which are accordingly treated separately.
MARINE PHYTOPLANKTON
Almost all species of algae which to form the marine phytoplankton
are unicellular, but a number of these unicellular types form chains or
colonies. The groups of algae found in the marine phytoplankton of the
Arctic, and the approximate numbers of species in each group, are as
follows: Bacillariophyceae (diatoms), about 250 species; Dinophyceae,
EA-PS. Ross: Phytoplankton
about 200 species; Chrysophyceae, about 15 species; Chlorophyceae (Volvocales),
2 species; Zanthophyceae (Heterokontae), 1 species.
It is impossible in the present state of our knowledge to give more
than approximate figures for the numbers of species in each group. There
are certainly many more species to be found than have hitherto been de–
tected. This is particularly true of the small unarmored flagellate forms
usually spoken of as the “nannoplankton,” the investigation of which is
still in its early stages and is beset with great difficulties. Most of
the species are so small that they pass through the finest note, and so
delicate as to be extremely diffi
d
^
c
^
ult to preserve. A start has been made
on their study by concentrating them, after killing, either by settling or
by the use of the centrifuge. In coastal waters in the neighborhood of
research stations, living material has also been examined and culture
methods tried. From these studies it appears that these unarmored flagel–
lates are an important element in the phytoplankton, particularly in the
oceanic regions.
The figures which have been given show that the diatoms and the
Dinophyceae are by far the most important groups in numbers of species in
the marine phytoplankton of the Arctic. They also form the bulk of the
individuals, for
Phaeocystis pouchetii
and
Dinobryon pellueidum
of the
Chrysophyceae, and
Halosphaera viridic
of the Xanthophyceae, are the only
species belonging to other groups that have
[:
]
so far been found in any
great numbers.
Most of the algae of the phytoplankton, in the manner of plants in
general, manufacture their own food from carbon dioxide, water, and various
inorganic salts, using the energy of sunlight for the purpose. Hence they
EA-PS. Ross: Phytoplankton
can only live in the upper layers of the sea where sufficient light for
their needs penetrates. The
L
lowest level at which they can maintain
themselves indefinitely is that at which the two processes of phytosynthesis,
or the building up of food substances, and respiration, which is the break–
ing down of those substances to provide the energy necessary for the
various processes of life, just balance over the twenty-four hours. This
depth, known as the compensation depth, varies with turbidity of the
water and the strength and direction of the incident light. The turbidity
of the water is itself greatly influenced by the amount of phytoplankton
present. In the Arctic, because of the obliquity of the light, the frequent
cloudiness, and the high turbidity due to an abundant phytoplankton, the
compensation level is comparatively shallow, being often only about 20
meters, while in low altitudes with almost vertical incidence of light and
low plankton populations, it is often as deep as 200 meters. Another result
of the need which plants have for light is that the abundance of phytoplankton
in the Arctic shows great seasonal variation - in contrast to the tropics
where the amount of the standing crop in any one place changes little except
when hydrographic conditions alter.
Storms and heavy seas have a detrimental effect on the growth of
phytoplankton because of the turbulence in the upper layers of the sea
which they cause. By means of this turbulence a proportion of the
p
phyto–
plankton organisms are carried down below the illuminated layer. Some of
them are able to regain the upper layers, but in s
i
^
o
^
doing they have to
expand energy which is accordingly no longer available for growth. Further–
more, no growth is possible until the upper layers are once more reached.
In addition, the total stock is depleted by those individuals which are unable
to rise again above the compensation level.
EA-PS. Ross: Phytoplankton
Of the other requirements of phytoplankton organisms, water and carbon
dioxide are always present in adequate, or more than adequate, quantities
in both marine and freshwater habitats. On the other hand, the supply of
mineral salts, of which nitrates and phosphates are the most important to
the phytoplankton, often limits its growth. In the sea, nitrates and
phosphates go through a complex cycle of assimilation by the phytoplankton,
then consumption by animals, and later regeneration by bacterial action from
the dead bodies of the plants or animals, or from animal feces. Much of
this regeneration takes place below the illuminated zone, and consequently
the maintenance of a supply of these salts in the upper layers of the sea,
where the phytoplankton can use it, is dependent upon mixing of the upper
and lower layers of the water or upon upwelling. During the winter in high
latitudes, the cooling of the upper layers of the sea increases their
density and thus reduces the stability of the water column. In those
areas they are free from ice, mixing is further facilitated by the strong
winds. At the same time, lack of light prohibits the growth of phytoplankton
and considerable concentrations of nitrates and phosphates are built up in
the surface waters. Even where the ice prevents the wind from having any
effect, mixing does occur when the upper layer are cooled and their salt
concentration is increased by ice formation, and there is a consequent
increase, albeit a rather lesser one, in the nitrate and phosphate content
of the upper layers.
Throughout the Arctic
a
the development of the phytoplankton follows
a similar cycle, but its timing varies considerably from place to place.
The presence of ice, and especially of snow-covered ice, so reduces the
light intensity in the water below it that the growth of phytoplankton
EA-PS. Ross: Phytoplankton
is almost entirely prevented. In the spring or early summer, when the
ice begins to melt and open out, a rapid growth of phytoplankton occurs.
This first outburst consists chiefly of diatoms, although these are
preceded at times, and perhaps generally, by an abundance of minute
flagellates belonging to the Dinophyceae, Chrysophyceae, and Chlorophyceae
(2). These have been so little investigated, however, that the extent of
their occurrence is virtually unknown. This rapid growth of diatoms
depletes the water of nutrient salts and is followed by a decline in the
number of diatoms, often equally rapid, to which grazing by the then
abundant zooplankton contributes considerably. The diatom maximum is
usually followed by a considerable growth of Dinophyceae which do not
require such high concentrations of nutrient salts and which need a higher
temperature for rapid growth.
Ceratium arcticum
is an especially important
species among these. Certain less exacting diatoms may also be found
in considerable numbers at this stage.
This change from a predominantly diatom phytoplankton to one in which
the Dinophyceae are the major element, normally occurs at about the time
at which the surface layers become heated to such an extent that mixing
with deeper waters by vertical circulation is stopped (6). The time at
which the growth of phytoplankton begins in the spring appears to be
controlled by temperature and light intensity, for it gets progressively
later as one moves north. Thus in the Gulf of Maine and the English
Channel, the diatom outb
t
u
^
^
rst begins in late February or rarely March,
while in Baffin Bay and the waters around Spitsbergen it does not start
until late May or early June, and about the center of the polar basin
it is as late as August.
EA-PS. Ross: Phytoplankton
High concentrations of diatoms are found chiefly along the coasts
and close to the edges of the melting ice. In such places the numbers
present are very great and only equallyed by those found in the Antarctic
at the corresponding period of the year. Densities of the order of one
million cells per liter have been recorded. Many factors combine to
cause this high production of phytoplankton. The high concentration of
nutrient salts built up by mixing during the long winter when lack of
light prevents their utilization has been mentioned. The continuity of
illumination when growth is taking place is another cause. Also it has
been shown (9) that, other things being equal, the growth of diatoms is
more rapid in waters that have recently been frozen. This is attributed
to the stimulating effect of trihydrol, the trimolecular polymer state of
water found in greater proportion in such circumstances. A further facto
^
r
^
d
is the greater density of the cold polar waters, resulting in the expen–
diture of less energy in maintaining the phytoplankton in the upper layers
of the sea, so that a greater proportion of the food manufactured is
available for growth.
Protoplasm and the various constituents of the cell walls of the
phytoplankton organisms are
[:
]
denser than sea water and hence, although
many plankton organisms have structures (giving “form resistance”) which
retard their rate of sinking by increasing drag, they can only maintain
themselves near the surface by active processes. Gross and Zeuthen (8)
have recently shown that diatoms, which have no means of swimming, keep
their density equal to that of the sea by reducing the density of the
vacuolar sap - probably by eliminating bivalent ions such as calcium
and sulphate - while maintaining it isotonic with sea water.
EA-PS. Ross: Phytoplankton
In addition to these favorable physical factors, two biological factors
operate in the same direction. During the long arctic winter, when virtually
no phytoplankton is present, the zooplankton population is also drastically
reduced so that, when the phytoplankton begins to grow rapidly in the
spring, there are few animals to graze on it. Some weeks are required
befo
d
^
r
^
e the zooplankton population has increased sufficiently for it to
exercise a large effect on the numbers of phytoplankton organisms. This
is in contrast to the conditions in warmer seas where the zooplankton is
present throughout the year and is exercising a steady grazing pressure on
a virtually constant phytoplankton population.
Another necessity for the rapid increase of phytoplankton when condi–
tions become favorable after having been unfavorable for a period, is the
presence of sufficient individuals to initiate the increase. Many planktonic
diatoms from resting spores, which sink to the bottom with the onset of
unfavorable conditions. These species, which are normally found only in
coastal and shelf waters shallow enough for the resting spores to be able,
after germination, to reach the illuminated zone, are called “neritic”
species, and plankton composed of them is termed neritic plankton. Others,
which live in deep water and form no resting spores but maintain themselves
in the upper layers of the sea throughout the year, are described as “oceanic.”
In waters that can be colonized by neritic species, resting spores provide a
considerable initial population, and the building up of large numbers with
the onset of more favorable conditions is much more rapid than in oceanic
waters. This is one reason why, in temperate waters, larger vernal growths
of phytoplankton are found only in coastal waters.
In the Arctic the resting spores of neritic species are able to overwinter
EA-PS. Ross: Phytoplankton
not only on the bottom in coastal waters but also frozen into the ice.
When this melts the re
[:
]
ting spores are set free and large population is
able to accumulate rapidly. While many oceanic species are found within the
Arctic, particularly in the Greenland and Barents Seas and in Davis Strait,
into which they are carried by Atlantic water, the main bulk of the phyto–
plankton, particularly during the spring maximum, consists of species
which have overwintered in the ice. At the beginning of the spring out–
burst, the most prominent diatoms in the phytoplankton are often species
which can live equally well in the plankton or growing attached to ice
floes.
Melosira arctica
,
Fragilaria oceanica
, and
Achnanthes taeniata
are the commonest of these.
As a result of the operation of these factors, large populations of
phytoplankton dominated by neritic species of diatoms (especially
Melosira
arctica
and
Fragilaria oceanica
) are found in the open waters of the Arctic
during the spring and early summer. These high concentrations are usually
localized, but where they occur they discolor the water noticeably and give
it a characteristic smell. This type of water is the “black water” of the
early whalers who used to seek it out because they knew that in and around
areas of it they would find the largest concentrations of whales. These
were attracted by the high concentration of planktonic animals feeding on
the dense phytoplankton population (3).
The boundary regions between ocean currents flowing in different
directions constitute another type of situation in which large concentra–
tions of phytoplankton are often found. This is due to the turbulence
which is found in these regions as a result of the mixing along the edges
of the two water masses, and which brings water rich in nutrient salts to
EA-PS. Ross: Phytoplankton
the surface. Also it is often the case that one water mass, whose
nutrient content has been greatly reduced by a “flowering” of phytoplankton,
can contribute the initial population, while the other contributes the
necessary salts. The nitrate and phosphate content of water masses can
change considerably in the course of a season, and accordingly the role
played by each mass in a boundary region may differ from month to month.
Distribution of Marine Phytoplankton
In reaind this account it must be remembers that, for most of the
areas, the observations on which it is based refer only to one or two
years and conditions may vary considerably from one year to another.
Accordingly, although the available information is presented here in
generalized form, the annual cycles described may well not be typical,
and almost nothing is known of their variability from year to year. Also,
this account does not refer to waters close inshore, nor to the fjords
and narrow channels along the coasts. Observations on these show that the
phytoplankton may very greatly over comparatively short distances, and it
is clear that local conditions and topography are apt to be the determining
factors in such localities. There are indications, however, that the
diatom-dinophycean cycle is a normal feature here as in more open water.
The Polar Basin
. The results of Nansen’s expedition in the
Fram
(5; 11), and the observations made by Sverdrup during the drift of the
Maud
(15), suggested that there was very little development of phytoplankton
in the permanently ice-covered regions about the center of the polar basin.
During Papanin’s drift, however, a moderately large development of phyto–
plankton was observed to begin during early August (14). This plankton
is poor in species and consists principally of diatoms,
Chaetoceros socialis
EA-PS. Ross: Phytoplankton
being the dominant form; Dinophyceae are unimportant in it. Active
growth takes place between the depths of 3 and 30 meters. The start of
this growth of phytoplankton coincides with the melting of the snow on
the ice floes, which renders them much more transparent. This suggest
that light is the determining factor.
Around the edge of the polar basin, similar conditions are found,
with the start of the outburst coming earlier in lower latitudes. Off
Wrangel Island it is still around the beginning of August, but in the
Chuk
t
^
o
^
tsk Sea it occurs during July. In these areas, and apparently
throughout the polar basin, there is a large phytoplankton population
until the onset of winter. Thus Nansen (11) reported that north of the
New Siberian Islands there was abundant
Chaetoceros
in October, with
many individuals being frozen into the newly forming ice, and the
Belgica
found large phytoplankton populations to the
N
^
n
^
orth of Novaya Zemlya in
September
^
.
^
( ).
Greenland and Barents Seas.
Owing to the influx of comparatively
warm Atlantic water, the spring growth of diatoms occurs early in the
eastern part of the Greenland Sea and in the Barents Sea. Thus dense
populations of phytoplankton, consisting mainly of diatoms, with
Chaetoceros
spp. Forming the most prominent element, are found in late
April and early May extending northward from the region of the Faeroes
and Iceland to Jan Mayen and Spitsbergen and into the Barents Sea. In
the latter sea, at least, the growth of diatoms occurs earlier and is
denser near the coast and the ice edge than it is in the center of the
area of Atlantic water which is ice-free throughout the winter (10).
Throughout this area the diatoms are succeeded by a population dominated
EA-PS. Ross: Phytoplankton
Dinophyceae, of which
Ceratium arcticum
is the most prominent form, and
in which species of
Peridinium
are often frequent.
Halosphaera virdis
is also a
[:
]
frequent member of this community. It is a southern
species which comes in with Atlantic water during the summer, and may also
maintain itself throughout the year in the ice-free parts of the Barents Sea.
In the western part of the Greenland Sea, in the waters of the south–
ward-flowing East Greenland current, the seasonal cycle begins with the
development of a phytoplankton consisting of minute naked flagellates,
some being Dinophyceae and some Volvocales. This is found under close
and thick pack ice near the Greenland coast of Denmark Strait at the end
of June (2), and is succeeded, as the ice opens and the upper layers of
the sea become better illuminated, by a rich growth of diatoms.
Achnanthes
taeniata
and
Fragilaria oceanic
are the first dominants in this diatom
growth, followed by
Thalassiosira gravida
accompanied by
Chaetoceros
species
in large numbers. This growth of diatoms depletes the upper layers of
nutrients and, owing to their dilution by melting ice, the water column in
this area at this time is very stable and replenishment from deeper layers
is probably negligible. This population accordingly lasts only a few weeks.
In this East Greenland current it is followed by a phytplankton community
dominated by
Detonula confervacea
, a much less exacting species of diatom
than which precede it. The larger Dinophyceae are apparently less important
in the later part of the phytoplankton cycle in these waters than elsewhere
in the Arctic. This is possibly due to the very low surface temperature
even in August when it scarcely reaches 3°C.
Davis strait and Baffin Bay
. As the ice breaks up along the west
coast of Greenland in the spring, a rich plankton consisting principally
EA-PS. Ross: Phytoplankton
of neritic species of diatoms develops (7). It is probable that this is
preceded by a development of flagellates as on the east coast, but this
point has nto been investigated. In the southern part of Davis Straft
the chrysophyceae
Phaeocystis pouchetii
is prominent along with the diatoms.
After a period of between one and two months, the nutrients in the upper
layers are much depleted and the plankton becomes much poorer in individuals,
the Dinophyceae becoming the group principally represented in it.
During the summer the West Greenland current penetrated further and
further north, and eventually, in September, reaches Smith Sound. As it
does so the same cycle of plankton development is followed. Meanwhile
cold water is flowing through Smith Sound and the straits between the
islands to the west. Being largely ice-covered, this is very poor in
plankton but quite rich in mineral salts. Along the boundary between the
two water
[:
]
masses large phytoplankton populations develop, with
Ceratium arcticum
the most prominent species, and as the summer advances
and the surface temperature of the water rises slightly, this community
spreads throughout the whole area of the cold water mass. Just south of
Cape York at the beginning of August a phytoplankton dominated by
Detonula
confervacea
has been recorded. This may correspond in its place in the
annual cycle to that observed in the East Greenland current by Braarud (2).
Hudson Bay and the Canadian Arctic Archipelago
. Almost nothing is
known of the marine phytoplankton of this area. Davidson (4) investigated
the diatoms of some plankton collections made in Hudson Bay in August and
early September.
Chaetoceros compressus
,
C. lacinosus
, and
C. convolutus
were the most important species. The density fell off from the north and
northeast of the bay toward the southwest, the distribution of diatoms
EA-PS. Ross: Phytoplankton
being very similar to that of salinity and temperature, with the densest
populations in the colder and more saline waters. In addition Polunin (12)
reports a phytoplankton consisting principally of
Chaetoceros
and
Thalassiosira
species as occurring at the end of August off Akpatok Island
in Hudson Strait. This again was apparently not very rich.
FRESHWATER PHYTOPLANKTON
Our knowledge of the phytoplankton of arctic fresh wa
r
^
t
^
ers is much less
than of that from marine sources. It is limited to floristic lists from
various localities and more or less subjective estimates of the abundance
when the collections were made. For the most part only single collections
have been made from the various lakes. The only set of observations made
at intervals over a year are those of Vanhöffen (16), who made a series of
collection from November to July in a small lake close to the Karajak Fjord
just north of latitude 70° N. on the west coast of Greenland. In this lake
Dinobryon bavaricum
var.
vanhoeffenii
was the most important planktonic plant,
being present in small numbers under the ice from November to March; in late
May, when the lake was still ice-covered, and in early July, when the water
was open, it was present in considerable quantity.
Peridinium cinctum
,
one of the Dinophyceae, was also present in considerable numbers in
November in January, more sparsely in March, and again in considerable
numbers in July. The diatoms, of which
Eunotia
and
Tabellaria
species
were the most important, apparently take longer to develop after the melting
of the ice, for none were found in July, although they had been frequent
the previous November and January. The only other planktonic alga of any
importance was the desmid
Xenthidium fasciculatum
, which was found not
infrequently in July.
EA-PS. Ross: Phytoplankton
The floristic lists which are available for a number of lakes in
Greenland (1), Novaya Zemlya, and Franz Josef Land (14) show that there
is considerable variation in the phytoplankton of the arctic lakes. No
work has, however, yet been published which enables a correlation to be
made between plankton types and the hydrological features of the lakes,
nor has the effect of the long periods of ice cover on the physical and
chemical conditions yet been elucidated. It is possible, nevertheless,
to make some suggestions based on analogies with conditions farther south
in the Subarctic and North Temperate Zones. The majority of arctic lakes
have a poor phytoplankton and desmids are frequently the most important
group contributing to it. Lakes with a plankton dominated by desmids are
also found quite commonly in western and northern Scotland and in the
mountain regions of Norway. Such lakes are usually very poor in mineral
salts and have rather said waters. Lakes in which the main component of
the phytoplankton is one of the species of
Dinobryon
are also frequently
met with. These are especially the shallower lakes with muddier substrata,
and may be somewhat richer in nutrients than those characterized by desmids.
Diatoms do not seem to
[:
]
be
^
a
^
predominant element in the plankton of any of
the lakes investigated. Almost always the phytoplankton has been poor
in quantity; but a water bloom, due to the colonial chrysophycean
uroglens volvox
, was reported from a shallow lake in Greenland by Bachmann (1)
The approximate numbers of species of the various groups of algae
recorded up to the present from freshwater plankton in the Arctic is:
Chlorophyceae, about 100 species; Bacillariophyceae (diatoms) about 50
species; Cyanophyceae, about 20 species; Dinophyceae, about 10 species;
Chrysophyceae, about 10 species; Xanthophyceae, about 5 species. These
EA-PS. Ross: Phytoplankton
figures are very tentative since it is not always possible to be certain
which of the gatherings examined by Bachmann (1) and Shirshov (14) should
be regarded as consisting of plankton. All further work will undoubtedly
add many species.
EA-PS. Ross: Phytoplankton
BIBLIOGRAPHY
1. Bachmann, H. “Beiträge zur Algenflora des Süsswassers von Westgrönland,”
Mitt.Naturf.Ges.Luzern
, vol.8, pp.1-181, 1921.
2. Braarud, T. “The Øst expedition to the Denmark Strait, 1929.”
Skr
.
vol.10, pp.1-173, 1935.
3. Brown, R. “On the nature of the discoloration of the Arctic Seas,”
Trans.Bot.Soc.Edinb
. vol.9, pp.244-52, 1868.
4. Davidson, V.M. “The planktonic diatoms in Hudson Bay (Biological and
oceanographic conditions in Hudson Bay, 5)”
Contrib.Can
.
Biol
. and
Fish
., n.s. vol.6, pp.497-509, 1931.
5. Gran, H.H.
Diatomacaae from the Ice-Floes and Plankton of the Arctic
Ocean.
London, N.Y., Longmans, Green, 1904. Norwegian North
Polar Expedition, 1893-1896.
Scientific Results,
vol.4,
no.11, pp.1-74, 1900.
6. Gran, H.H. “Quantitative plankton investigations carried out during
the expedition with Michael Sars, July-Sept.1924,”
Conseil Perm.Internat.Explor.Mer.
Rapport
vol.56, no.5,
pp.1-50, 1939.
7. Grøntved, Jul. and Seidenfaden, Gunnar.
The Phytoplankton of the Waters
West of Greenland
. København, Reitzel Medd.Grønland vol.82,
no.5, pp.1-380, 1938.
8.
[:
G
]
ross, F., and Zeuthen, E. “The buoyancy of plankton diatoms: A problem
of cell physiology,”
Proc.Roy.Soc
. London, B, vol.135,
pp.382-89, 1948.
9. Harvey, H.W. “Substances controlling the growth of a diatom,”
J.Mar.Biol.Ass.U.K
., n.s. vol.23, pp.499-520, 1939.
10. Krebs, E., and Verbinskaya, N. “Seasonal changes in the phosphate
Barents Sea,”
J.Cons.Int.Explor.Mer
. vol.5, pp.329-46, 1930.
11. Nansen, F.
The Oceanography of the North Polar Basin
. Norwegian North
Polar Expedition, 1893-1896.
Scientific Results
, vol.3,
no.9, pp.1-429, 1902.
12. Polunin, Nicholas. “The vegetation of Akpatok Island. Part II,”
J.Ecol
.
vol.23, pp.161-209, 1935.
13. Shirshov, P.P. “Ecologic geographical essay on the fres
y
^
h
^
/-water algae
Novaya Zemlya and Franz-Joseph Land,” Leningrad.
Arkticheskii Nauchn.-Issled Inst.
Trudy
, no.14, pp.73-162,
1935.
EA-PS. Ross: Phytoplankton
14. ----. “Oceanological observations,”
C.R.Acad.Sci. U.R.S.S
., vol.19,
pp.569-80, 1938.
15. Sverdrup, H.U. “The waters on the North-Siberian Shelf,” Norwegian
North Polar Expedition “Maud” 1918-1925.
Scientific Results
vol.4, no.2, pp.1-131, 1929.
16. Vanhöffen, E. “Die fauna und flora Gr
ö
nlands,”
Grönland-Expedition
der Gesellschaft für unde zu Berlin 1891-1893
. Berlin, 189
8
^
7
^
.
vol.2, pp.1-383.
Robert Ross
Arctic Plant Physiology
EA-Plant Sciences
(J. Warren Wilson)
ARCTIC PLANT PHYSIOLOGY
CONTENTS
|
|
Page
|
|
Growth
|
1
|
|
Photosynthesis and Respiration
|
4
|
|
Nitrogen Relations
|
13
|
|
Experimental Culture of Plants
|
15
|
|
Observation of the Effect of Fertilization
|
15
|
|
Observations of Late-Snow Areas
|
16
|
|
Plant Analysis
|
17
|
|
Water Relations
|
19
|
|
Summary and Conclusions
|
20
|
|
Bibliography
|
22
|
EA-Plant Sciences
(J. Warren Wilson)
ARCTIC PLANT PHYSIOLOGY
The foremost feature of arctic vegetation is perhaps its poverty; the
most obvious factor peculiar to the arctic environment is low temperature.
One of the main objects of the physiologist must be to find the mechanism
of the relationship (if any exists) between these two features. An exami–
nation of growth rates appears likely to be a profitable approach.
Practical difficulties have so discouraged work with plant physiology
in the Arctic that only four real contributions have been made - those of
Müller and Wager in Greenland, and of Russell and Warren Wilson on Jan Mayen
Island. These four investigations form a rather inadequate basis for generali–
zation about the physiology of plants in the Arctic as a whole. Low-temperature
studies made in laboratories of temperate countries can be used to supplement
field investigations in the Arctic; but as temperature is only one of a complex
of factors affecting arctic plant growth, such studies have a limited value in
this connection. Several aspects of arctic plant physiology remain completely
uninvestigated.
Growth
Wager (40), by weighing pressed plants belonging to various age groups,
found that in several species growing at Kangerdlugssuaq, East Greenland
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
(lat. 68° N.P, the weight was approximately doubled each year, and in especially
warm or moist habitats it was trebled. Warren Wilson (44) estimated annual
growth to be of the same order at Jan Mayen (lat. 71° N.). Such increases
are found normally in a single week’s growth of plants in temperate regions.
These low values are related to the short growing season, which lasts
only some two or three months for arctic plants. A slow rate of growth even
at the height of the season is probably an additional factor. Few data are
available on this point. Warren Wilson (44) determined the relative growth
rates at midsummer of plants growing at Jan Mayen. Even in favorable habitats,
values for
Saxifraga tenuis
were less than one-tenth of those typical of tem–
perate plant growth, and those for
Koenigia islandica
about one-third. Plants
growing in unfavorable habitats gave lower figures, falling in one especially
barren area to a negative value.
The relative growth rate is a measure dependent not only on the rate of
assimilation; it is also a function of plant form — of the relationship of
total weight to leaf area. The net assimilation rate (increase in dry weight
in grams per square decimeter of leaf area per week) has the a
s
^
d
^
vantage of being
independent of form; it measures the efficiency of what might be termed the
photosynthetic apparatus.
Heath and Gregory (13) believe “that the net assimilation rate in the
most diverse types of plant under very various conditions approximates to a
constant mean value” — about 0.55. Watson (46), on the other hand, found
seasonal trends and differences between species. It is of some general interest,
therefore, to find out whether values under the extreme conditions of the Arctic
approximate to the “usual” value.
Russell (29) obtained a value of 0.30 for the net assimilation rate of
EA-PS.
^
Warren
^
Wilson: Arctic Plant Physiology
Oxyria digyna
in mid-August at Jan Mayen. Because of various deficiencies in
his experimental technique (a detached leaf method was used), he regarded this
value as minimal, and concluded that “the net assimilation rates of arctic
plants are at least comparable with those of plants in temperate regions.”
Similar studies, in greater detail, gave values near 0.55 for four species
and indicated, for
Oxyria digyna
(45); (
1
) a downward trend from 0.9 at the
end of June to 0.4 at the end of August; (
2
) low net assimilation rate, 0.2 to 0.3,
in midseason of leaves from ce
d
^
r
^
tain especially poor habitates; and (
3
) a 25% lower
net assimilation rate in an exposed as compared with a sheltered locality doing
leaves from one source.
Net assimilation rates are usually determined on whole plants, and the use of
detached leaves gives spuriously high results, as it neglects losses due to respi–
ration in stem and root. Thus net assimilation rates of detached leaves under
temperate conditions are of the order of 1 to 2 (7; 32). The approximation to
0.55 of net assimilation rates as determined at Jan Mayen on detached leaves
suggests therefore that estimates on whole plants would give low values. Deter–
minations for whole plants of three species growing in favorable habitats gave
net assimilation rates between 0.18 and 0.23; even lower values were found for
plants in poor habitats (44).
At Jan Mayen, therefore, and probably in the Arctic generally, plants appear
to have abnormally low net assimilation rates which are subject to further
depression by poor conditions of soil or climate.
The figures quoted suggest that although
Koenigia islandica
and
Saxifraga
tenuis
have roughly similar net assimilation rates, the former has a much higher
relative growth rate than the latter. This discrepancy is attributable to
differences in form. In the minute
Koenigia
(at Jan Mayen, plants are about
EA-PS.
^
Warren
^
Wilson: Arctic Plant Physiology
5 mm. tall), a higher proportion of material can be devoted to photosynthetic
structures than in the larger
Saxifraga
, which necessarily has a greater propor–
tion of structural tissue. The ratio of leaf area to total dry weight is some
four times higher in
Koenigia islandica
than in
Saxifraga tenuis
, and the
Saxifraga
in turn gives a ratio rather higher than those typical of crop plants
in temperate regions. The relative growth rates, for a certain net assimilation
rate, vary correspondingly. This association of high relative growth rate with
small size may determine in part the smallness of arctic plants generally; it
may also explain the capacity of
Koenigia
to grow farther north
a
than any other
annual.
Wager (41) has pointed out that the ability to become active rapidly in
spring is a characteristic feature of arctic plants which enables them, by an
early expansion of leaves, to exploit the short arctic growing season to the
full. Sørensen (34) has reached the same conclusion; he mentions two ancillary
features — the production of leaves simultaneously on several shoots (as opposed
to successive expansion on one shot), and the aperiodical development of organs,
which allows even short favorable periods to be utilized.
Such considerations are clearly related to the dominance of perennial plants
in arctic regions; their rapid spring growth is made possible by food stored in
perennating organs. Annual species with no such store are at a disadvantage
unless, as Russell (28) points out, the seed contains a large proportion of
stored food relative to the total growth of the plant.
Photosynthesis and Respiration
Net assimilation and growth
^
depend
^
on the balance of photosynthesis and respira–
tion. Arctic conditions may be expected to affect these processes both directly
and also through genetic changes in the physiology of arctic species. Müller (23)
EA-PS.
^
Warren
^
Wilson: Arctic Plant Physiology
and Wager (41) have studied gas exchange at controlled temperatures in East
and West Greenland, respectively latitude 68° N., and their results appear
to lead to an appreciation of the more important effects of arctic conditions
on phytosynthesis and respiration. Müller and Wager used temperatures of
10° C. and 20°C., and less frequently of 0°C. Observations on the balance
of phytosynthesis and respiration under field conditions of temperature and
illumination are lacking for arctic regions, apart from a single observation
on detached leaves of
Oxyria digyna
at Jan Mayen (45). The ratio of photosyn–
thesis to respiration obtained at midsummer was roughly 6: 1. Anderson (2)
usefully summarizes the literature on gas exchange at low temperatures, and
the results of his experiments with winter wheat confirm those of Müller and
Wager with arctic plants.
Kostychev, Chesnokov, and Bazyrina (17) at Portshnicha (lat. 69° N.) on
the Murmansk coast estimated rates of
[:
]
photosynthesis by a method of their
own, which has been criticized by Müller and Wager, and which gave unusually
high results with temperate material. Their observation of lower rates of
photosynthesis in four arctic species than in temperate plants does not agree
with results of other workers and must be treated with reserve.
Using gas analysis methods, Müller and Wager found that the maximum rates
of photosynthesis (under high light intensity and at 20°C.) of arctic and
temperate plants were similar. Lower temperatures, however, reduced the rate
of photosynthesis less for arctic than for temperate species; thus Wager found
the Q
10-20
of arctic and temperate plants averaging about 1.2 and 2.0,
respectively. These figures are from several sources.
Respiration behaves quite differently. Stocker (35), using Muller’s figures,
and Wager (41) have shown that, at a particular temperature, respiration is
EA-PS.
^
Warren
^
Wilson: Arctic Plant Physiology
more rapid for arctic than for temperate and tropical species. Wager, using
figures from several sources, obtained the following averages for respiration
at 20°C: Arctic, 2.3 milligrams per square decimeter per hour; Temperate,
1.25 mg. / sq.dm. / hr., and Tropical, 0.5 mg. / sq. dm. / hr. The same
trend is apparent when a fresh-weight basis ms used. Especially striking is
the fact pointed out by Anderson that the respiration of arctic plants in
Greenland at 10°C. may have the same intensity as that of tropical plants in
Java at 30°C.
The Q
10
of respiration, on the other hand, appears to be similar for
arctic and temperate species, at least when active summer material is used (41).
Winter material has given slightly higher values, probably owing to saturation
of enzyme systems by the high carbohydrate concentrations known to occur in
arctic plants in winter; analysis of Wager’s figures in fact shows a correla–
tion of high respiration rate with high Q
10
.
Thus temperature has a greater effect on respiration than on photosynthesis.
This is confirmed many times in literature. Zakharova (50) considers assimila–
tion in
Pinus sylvestris
and
Picea excelsa
to be independent of temperature over
a wide range around zero centigrade; Müller (23) found for
Epilobium (Chamaenerion
)
latifolium
and
Salix glauca
that the Q
10-20
for photosynthesis at high light
intensities was 1.25 and 1.48, respectively; but for respiration, 2.5 and 1.8
in the same plants, Stålfeld (36) found for certain lichens a Q
2-10
of photo–
synthesis of 1.23 and 1.20, but of respiration, 2.29 and 2.80; Anderson (2),
investigating the hardening process in winter wheat, stated that “the respira–
tion is far more influenced by the temperature than the assimilation.”
The observations mentioned above permit the following general conclusions:
arctic plants are capable of photosynthesizing rapidly — as “sun” plants --
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
given relatively favorable conditions of temperature and light intensity;
under such conditions, loss of energy in rather rapid respiration is not
dangerous to the plant; it may indeed be related to a capacity for rapid
growth in the arctic spring. With lower temperatures, the rate of photosyn–
thesis falls, but, as the respiration rate falls more markedly, the efficiency
of the over-all exchange is decreased less than proportionately. Synthesized
materials appear as accumulating sugars rather than as new growth.
Müller (23) has shown that the compensation point falls with temperature.
Thus at low temperatures, relatively little illumination may “saturate” the
photosynthetic apparatus. Müller demonstrated, in various arctic species under
low light intensities, an apparent assimilation more rapid at 10°C. than at
20°C. Stålfelt (36; 37) has found the same effect in a number of mosses and
lichens. It is clearly of importance in relation to assimilation under arctic
conditions, where temperatures are low, and where light, though of low inten–
sity, is more or less continuous.
Anderson (2) found in winter varieties of wheat and rye many of the
physiological features of arctic plants: a maximum assimilation rate at
15°C. of the same order as in sun plants, and the rate at 2°C. only 25%
less rapid; a very high respiration /
maximum assimilation
quotient at low temperatures;
a normal compensation point at 15°C., and a low or very low compensation point
(as for a shade plant) at 2°C. He traced the physiological changes accompanying
lowered temperatures more thoroughly for his winter varieties than has been
done for arctic plants; as the same main features are likely to occur in both
groups, his observations are summarized here.
He found that at 7° to 8°C. growth practically ceased, and carbohydrate
formation and utilization balanced. A decrease in temperature disturbed
th
d
^
e
^
balance, resulting in a surplus of assimilates and an increase in the
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
sugar content of the plant. After some time the rate of photosynthesis
began to fall off gradually and at the same time the respiration rate began
steadily to increase. The daily surplus consequently diminished, finally
reaching zero, so that a balance between production and consumption was
again reached. The level at which this new balance was reached depended
upon light and temperature and also upon inherent characters of the plant,
both genetic and historical.
Anderson showed that the depression in photosynthesis and increase in
respiration which he found at low temperatures were caused, directly or
indirectly, by accumulation of sugar. This is interesting in view of a
comparison by Russell (28) of plants of
Oxyria digyna
growing at Jan Mayen
under continuous daylight, with some occurring at 3,800 meters in the
Karakoram-Himalaya where there was an eleven-hour night. Temperatures in
both localities were low. A surprisingly close similarity was found in con–
tents of starch and of reducing and nonreducing sugars, suggesting “that when
the level of carbohydrate is high it may tend to reduce the assimilation rate.”
The accumulation of sugar in plants at low temperatures is well estab–
lished as a result of investigations on frost hardening (2). The sugar
is derived from hydrolysis of starch and also directly from photosynthesis;
the former source is probably the more important. The only observation on
carbohydrate levels of plants under arctic conditions are those of Russell (29)
at Jan Mayen.
Russell analyzed
Oxyria digyna
roots at four stages of development,
from the start of activity in spring to the fruiting condition of mid-August.
His values are expressed as percentages of alcohol-insoluble material (see
Table I). The earliest estimate suggested that during the winter, with
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
|
Table I. Analysis of Oxyria Digyna Roots.
|
|
Development stage
|
Total
sugar, %
|
Starch, %
|
Total
carbohydrate, %
|
|
Break of dormancy
|
36.6
|
1.9
|
38.5
|
|
Flowering
|
22.8
|
3.2
|
26.0
|
|
Fruiting
|
12.4
|
6.0
|
18.4
|
|
Late fruiting
|
13.8
|
13.0
|
26.8
|
temperatures averaging −5°C., starch was largely converted to sugar. During
rapid growth, in spring and early summer, (the third sample was taken on
July 21), sugar was used. Subsequently carbohydrate was built up again in
shoot as well as root. Throughout the season, startch content gradually
increased, even when the total carbohydrate level was falling; summer tempera–
tures were apparently sufficiently high to promote the conversion of sugar to
starch.
Indirect confirmation of these results is provided by the work of Wager
and Wager (42) which showed that, in seven species growing at latitude 68° N.
in East Greenland, osmotic pressures were high during the winter, fell rapidly
in spring to a low summer value, and built up again d
r
^
u
^
ring
[:
]
autumn and winter (owing, it was suggested, to physiological drought as well
as to temperature effects). Arrhenius and Sőderberg (3) found high osmotic
pressures in mountain plants growing at Abisko in Swedish Lapland (lat. 68° N.)
at temperatures around zero centigrade.
Russell’s analyses of
Polygonum viviparum
showed carbohydrate levels
rather lower than in
Oxyria digyna
, but yet quite high. Leaves of
Ranunculus
glacialis
contained about 50% total carbohydrate (alcohol-insoluble material
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
basis), mostly as reducing sugar. He concludes from these high carboh
d
rate
levels that arctic plants do not suffer from carbohydrate deficiency; and
that the hypothesis of Wager (40), that the poor vegetation of
fjeldmark
is related to carbohydrate starvation, is false.
Russell (29) found a diurnal rhythm in carbohydrate content during the
arctic summer day, when light, though of low intensity at midnight, is con–
[:
]
tinuous. In mid-August at Jan Mayen, sugar and starch levels in
Oxyria
digyna
leaves rose during morning and afternoon, and fell again during evening
and night. In the absence of data on translocation, interpretation in terms
of assimilation is difficult. It seems likely that at midsumer, when light
intensity varies less, the diurnal rhythm would be less marked.
Curtel (9), using rye an
[:
]
Hieracium pilosella
, investigated assimilation
during the “night” (August 1) at 900 meters in mountains of Norway (lat. 62° N.);
but his method did not give reliable quantitative results, and his estimates do
not cover the darkest period of the night. Kostychev (16), working at
St. Petersburg (lat. 60° N.), found that assimilation, as indicated by gas
analysis, had ceased half an hour after sunset in four herbaceous species, though
not in
Pinus strobus
and
Abies sibirica
. Kostychev, Chesnokov, and Bazyrina (17),
at Portshnicha (lat. 69° N.), found assimilation generally continuing throughout
the 24-hour day in July.
Kjellman (15) provided 24-hour daylight artificially and found that arctic
species could use the additional light, whereas other plants grew only slightly
better in continuous than in interrupted light. Fries (12) at Abisko in
Swedish Lapland (lat. 68° N.) agreed with Kjellman that plants could assimi–
late during a light “night”, but found that temperate as well as arctic species
were capable of doing so. His results were confirmed by Polunin (25a) working
in Norwegian Lapland.
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
It seems likely that arctic plants have low optimum and minimum tempera–
tures for assimilation as compared with temperate plants. Direct evidence is,
however, lacking. Lundegårdh (20) states that many plants can assimilate
at temperatures below zero; he mentions the low temperature-assimilation
optima (around 10°C.) found by Henrioi for alpine plants. Beliakov (4)
claims that his figures for two varieties of barley show a difference of 10°C.
in assimilation optima, corresponding to differences in geographical distribution.
Kreusler (19) found assimilation at temperatures just below zero in several
species native to warm regions. Jumelle’s observations (14) of assimilation
in certain conifers and lichens at or near −40°C. are probably false, owing
to certain features of his method. Matthaei (21) demonstrated photosynthesis
in
Prunus laurocerasus
at −6°C., and Zakharova (50) showed that there was
photosynthesis at temperatures several degrees below zero in
Pinus sylvestris
and
Picea excelza
. Walter (43), using winter wheat and varley, detected assimi–
lation at −2° to −3°C., and respiration at −6° to −7°C. It seems likely that
the cold-enduring plants of arctic regions will, as Matthaei suggests, photo–
synthesis at even lower temperatures. Indeed, Wager’s figures (41) suggest
that
Ranunculus glacialis
,
Oxyria digyna
, and
Sexifraga cernua
in Greenland
assimilate almost as fast between 0° and 1°C. as at 10° or 20°C. In this
connection also it is known that leaf temperatures under field conditions are
generally higher than air temperatures, because of radient energy absorbed
from insolation. Thus Ehlers (11) has shown that at temperatures between
−20° and +5°C. pine leaves may be 2° to 10°C. warmer than the surrounding air.
Müller (23) at Disko, East Greenland, and Kostychev, Chesnokov, and
Bazyrina (17) at Portsnicha, Murmansk, both at latitude 69°N., have found
stomata wide open in all plants examined throughout the 24-hour daylight
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
of arctic summer. Stålfelt (38) at Abisko (lat. 68° N.) found some species
with open stomata and some with stomata partly closed at midnight in July.
Russell (29), using a porometer, found that at Jan Mayen (lat. 71° N.) during
July and August the stomata of Oxyria digyna and
Taraxacum
[:
]
lapponicum
(
T. croceum
) were always open, but less widely so at night. Polunin (25a),
on the other hand, found in plants of Norwegian Lapland, stomata open at night
but closing during the day in relation to dryness of the woil and atmosphere.
Stomata of snow-covered plants remained inactive and for the most part closed.
Nitrogen Relations
Available information suggests that the growth rates of plants in the
Arctic are low or very low; also, that carbohydrates are in excess rather than
in short supply. Some factor other than reduced assimilation rate must there–
fore be looked to for the limitation of growth.
Summerhayes and Elton (39) describe how, in bird-manured areas of Svalbard,
the general dward-shrub heath is replaced by rich herbaceous (often grassy)
communities. Such enriched vegetation around localities frequented by birds,
foxes, or men is a general feature of arctic regions (27; 31). Summerhayes
and Elton ascribed the change to increased available nitrogen, stating that the
supply of nitrogen in arctic soil
d
^
s
^
is very small They had no direct analytical
evidence and mentioned that other factors (e.g., phosphate) might be involved.
Russell
et al
. (30) also believed that nitrogen deficiency was widespread
and of foremost importance in controlling the density and floristic composition
of arctic vegetation. They attributed the lack of available nitrogen mainly to
slow rates of bacterial activity, due in turn to low temperatures and small
nutrient supply.
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
The only available estimates — those of Acock (1) for Svalbard, and
those of Russell
et al
. (30) and Warren Wilson (44) for Jan Mayen — suggest
that total nitrogen contents of arctic soils lie between 0.05 and 0.1% dry
weight, though higher and lower values are occasionally found. These figures
are somewhat lower than those characteristic of temperate soils, which usually
fall between 0.1 and 0.4%. Soluble nitrogen contents of Jan Mayen soils
(which are exceptionally young) are of the order of 10 parts per million of
which only about
[:
]
one-tenth is inorganic.
These low values of soil nitrogen correspond to the small quantities of
nitrogen present (per unit area of ground) in the generally sparse and low
vegetation. Russell
et al.
(30) have shown a correlation of soil nitrogen
content with density of vegetation; and these two features are in turn core–
lated with numbers of microorganisms and rates of biological processes —
respiration, nitrification, etc. — in the soil (44).
Such a correlation of vegetation with soil
[:
]
nitrogen would be expected,
if nitrogen supply were inadequate; but a deduction in the reverse direction
(that if a correlation is found, nitrogen supply must therefore be deficient)
cannot be made. The presence of vegetation, controlled by whatever factor it
may be, will promote the accumulation of soil nitrogen by stabilizing soil
and reducing leaching effect, and by improving conditions for microorganism
activity through the presence of moisture and humic substrates, and by
rhizosphere effects.
Luxuriance of arctic vegetation seems to be generally related not only
to soil nitrogen but also to local microclimatic and topographic effects - –
such as shelter (reinforced by snow cover), aspect, and moisture (5; 26; 45).
It seems that where climate, vegetation, and nitrogen supply are correlated,
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
the primary differentiating factor is most likely to be climate, which will
affect vegetation and hence soil nitrogen; interaction between these three
features will of course occur. However, it is not yet shown at all clearly
that such a relationship is existent. Work concerned with evaluating the
relative importance of climatic severity and nitrogen deficiency in limiting
arctic vegetation has so far been confined to Jan Mayen, and has not been
carried much beyond a preliminary stage. Further work, along lines which are
indicated in this article, and in the various parts of the Arctic, is much needed.
In general, the following methods have been used.
Experimental Culture of Plants
. Winter rye, grown simultaneously at Jan
Mayen and in England, under identical condit
^
i
^
ons of sand culture, showed
development some six times as fast in England as at Jan Mayen. The arctic climate
slowed down hydration of the seed; translocation of materials from endosperm to
embryo; and, when leaves had expanded, assimilation. These differences were
even more marked when spring rye was used instead of the winter variety.
Turnip seedlings, culture on an exposed hilltop and in a sheltered
lowland habitat at Jan Mayen, grew some 50% faster in the more favorable
lowland climate (44; 45).
Though the results of such experiments would be more significant if species
native to the Arctic were used, it is clear that climatic effects along (even
at midsummer) may seriously hinder arctic plant growth.
Observation of the Effect of Fertilization
. Manuring by bird, fox or man
results in increase in total and soluble nitrogen content of the soil, and in
numbers and rates of biological activity of soil microorganisms. Accompanying
these effects are characteristic changes in floristic composition of the
vegetation. Plant growth also becomes more dense — as it does with manuring
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
in temperate countries — but appears not to attain as rich a condition
(measured as dry weight of plant material per unit area) as is found in more
favorable, unmanured habitats. This is especially brought out by the fact
that luxuriant vegetation examined by Russell
et al
. (30) in bird-manured areas
owes its richness in fact to favorable aspect coupled with outcropping of a
rock type producing particularly suitable soil. Moreover, while the
[:
]
rich vegetation developed in naturally favorable (but unmanured) localities
persists, that developed under manuring gradually shows less and less differen–
tiation from the general vegetation, if the addition of manure ceases.
Seedlings of several species, cultivated on various types of soil at
Jan Mayen, grew much faster on bird cliff soils than on others. Growth was
also increased, on all soils, by the addition of phosphate, potassium, and
especially nitrogen fertilizers. In the field, however, these fertilizers
rarely had any effect. The appearance of differences in culture was probably
related to the absence of competition (44; 45).
Observations of Late-Snow Areas. R
ussell
et al.
pointed out that the
level of available nitrogen in the soil as measured at any one time is dependent
on the balance between formation and utilization, and forms a poor basis for
assessing deficiency. The measurement of rates of turnover at different
seasons being impossible, Russell ingeniously compared the level of inorganic
nitrogen at the edge of a late-snow area with that 20 meters away, where the
plants had been free of snow for some three weeks and had developed rapidly.
Values of inorganic nitrogen of 0.39 and 0.02 parts per million were found,
and the difference was attrib
y
^
u
^
ted to nitrogen uptake by the growing plants
proceeding to a point at which concentrations were too low for further absorption.
Warren Wilson (44) failed to find this effect.
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
Late-snow areas are also of interest at Jan Mayen in that, where they
face south, they support the richest vegetation found on the island. This
is correlated with high soil nitrogen contents, and with air and soil tempera–
tures as much as 2° and 6°C., respectively, above those of surrounding un–
sheltered areas (45). While such microclimatic differences will come into
force as soon as a late-snow depression is formed, and will favor plant growth,
it is difficult to understand why a high soil nitrogen content should be
differentiated in such places except as a result of successful plant growth.
Plant Analysis
. Analyses of
Oxyria digyna
leaves (44) showed progressively
better growth and higher total nitrogen content correlated with successively
more favorable habitat conditions as regards shelter and soil nitrogen content;
soluble nitrogen was, however, lowest in leaves of the best-developed plants,
and progressively higher (a three-fold difference) in the poorer plants. This
suggested that poor growth was not a product of nitrogen deficiency.
Polunin (personal communication), using the diphenylamine test for nitrates
plus nitrites on Cornwallis Island, north of Baffin Island, found in 1949 “no
marked deficiency, at least in the plants of some habitats.”
Russell
et al.
(30) believe that the activity of soil microorganisms in
the Arctic must be limited for a considerable part of the year by cold;
observations on temperate microorganisms are quoted, which suggest that
activities are slight below 5° to 7°C. However, Warren Wilson (44) has shown,
in microorganisms isolated from Jan Mayen, soils, that there is activity at
rather lower temperatures than in their temperate counterparts. Fungi had
cardinal points about 5°C. lower, and ceased activity around zero centigrade
(mean for 20 isolates); for bacteria the optimum (mean of 10) was 18°C.,
certain species having an exceptionally low optimum of 11°C. All bacteria
examined appeared to remain active at temperatures several degrees below zero
centigrade.
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
Observations on microorganisms in late-snow areas give additional infor–
mation; for while snow persists, conditions in the soil beneath must be
similar to those generally prevalent during winter. Warren Wilson (44)
found that numbers of fungi in soil beneath the melting snow (at 0°C.) were
very low’ numbers continued low a few meters beyond the edge of the snow, and
then increased suddenly (approximately sevenfold) to a steady high value. This
increase presumably corresponded to germination of fungal spores, which is
depressed by cold even more than is vegetative growth. Bacteria, on the other
hand, were present in considerable numbers under the snow, and increased
approximately twofold immediately on melt, to a steady figure. These results
suggest that, during winter, soil fungi may be largely inactive, but that soil
bacteria (which are physiologically more important than fungi in the soil) may
remain to a considerable extent active.
As, moreover, arctic soil temperatures are usually several degrees higher
than air temperatures, owing to insolation (1; 34; 45), it seems by no means
certain that limitation of the activity of microorganisms occurs to such an
extent as to cause nitrogen deficiency.
Although numbers of microorganisms are relatively low in many soils at
Jan Mayen, rates of biological activities appear to be reduced less than
proportionately (47).
It is clear that both climatic and edaphic factors may limit the growth
of arctic vegetation; but the evidence is too scanty to permit of a decision
as to which factor is generally the more important. An interesting possibility
is revealed by physiological studies on frost hardening. In plants at low
temperatures, soluble nitrogen content increases at the expense of protein
nitrogen. This change has been demonstrated for winter wheat by Mudra (22)
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
and by Dexter (10); for the bark of certain trees by Siminovich (33); for
va
d
^
r
^
ious crop plants by Wilhelm (49); and in a number of other instances (24).
Accumulation of soluble nitrogen to the abnormal level of 50% of total nitro–
gen in plants of
Oxyria digyna
and
Koenigia islandica
under arctic conditions
is indicated by the results of Warren Wilson (44). This soluble nitrogen may
be accumulated as nitrate (24) or as amino acids (49). Accumulation of sugars,
previously, occurs simultaneously with that of soluble nitrogen, and is in fact
attributed by Nightingale to decreased utilization in organic nitrogen synthesis.
Put in oversimplified terms, protein synthesis involves the combination
of simple carbohydrates with soluble nitrogenous compounds. The availability
of these substances determines the rate of synthesis, which is reduced when
one or other group of materials becomes limiting. The situation discovered
in plants at low temperatures and in the Arctic, of accumulation both of simple
carbohydrates and of soluble nitrogen, is therefore surprising. It seems
possible that protein synthesis or some related process may be limited by low
temperatures. Protein synthesis is an immediate requisite for protoplasm forma–
tion and growth, and its depression would explain the reduction of growth rates
observed by Tumanov (48) and Andersson (2) to occur at about 6° to 7°C. and to
be associated with carbohydrate accumulation.
This working hypothesis, that low growth rates in arctic plants are caused
by slowing down of protein synthesis (in the widest sense) by low temperatures,
seems worth examination; the progress of one such attempt was reported by the
author in 1950 (45).
Water Relations
Low temperatures impede water uptake, owing mainly to decreased permeability
of root membrances and increased viscosity of water (18). Water contents of
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
plants may thereby be reduced (2; 22) and transpiration rates lowered; wilting
may be caused. Whitfield (47) and Clements and Martin (8) believe that low
soil temperatures may limit growth of plants at high altitudes in this way.
Such effects appear to commence at temperatures below about 10°C; however there
is adaptation to low temperatures in species typical of cold habitats, and in
the absence of direct evidence it is impossible to assess the importance of
this effect in arctic regions.
Polunin (25) has shown the conduction of water in roots of
Betula odorate
through 50 centimeters of frozen soil at 0°C.; absorption was occurring in soil
at a temperature just above zero. Conduction through roots at 4° to 5°C. was
little if at all more rapid than that through roots at 0°C.
Bonnier (6) compared the anatomy, in certain species, of individuals from
arctic and temperate localities. He concluded that the differences in structure
sometimes to be found under arctic conditions (thinner cuticle, larger inter–
cellular spaces, etc.) were symptomatic of a more humid environment.
Summary and Conclusions
The limited evidence at present available suggests that arctic conditions
affect plant physiology as follows. Growth rates are generally reduced much
below those typical of temperate plant growth, and are subject to greater
depression by locally unfavorable conditions of soil or climate. These low
growth rates result in accumulation of sugars and probably of soluble nitrogenous
compounds, and may be caused, in part at least, by retardation of protein
synthesis or some related process by low temperatures. High sugar levels are
probably responsible for the low rates of assimilation which have been
observed. There is no evidence of carbohydrate shortage in arctic plants, in
spite of the fact that respiration rates are higher, at a particular temperature,
EA-PS
^
Warren
^
Wilson: Arctic Plant Physiology
in arctic species than in temperate or tropical species. Physiological adap–
tation is shown by low Q
10
s of assimilation, by low compensation points at
low temperatures, and probably by low optimum and minimum temperatures for
photosynthesis. Photosynthesis has been shown in some cases to continue
throughout the 24-hour arctic day. Morphological adaptation also is present
in the form of leaf area /
high total dry weight rations, in the rapid expansion of leaves
at the start of the short growing season, and in the dominance of the perennials
of small growth habit. There is some evidence of lack of available nitrogen,
owing perhaps to low activity of soil microorganisms, but it is not clear how
important this effect is.
EA-PS. Warren Wilson: Arctic Plant Physiology
BIBLIOGRAPHY
1. Acock, A.M. “Vegetation of a calcareous inner fjord region in Spitsbergen,”
J.Ecol
. vol.28, pp.81-106, 1940.
2. Andersson, Gősta.
Gas change and frost hardening studies in winter cereals
.
Lund, 1944.
3. Arrhenius, O., and Sőderberg, E. “Der osmotische Druck der Hochgebirgspflanzen,
Svensk Bot. Tidsk
. Vol.11,
4. Beliakof, E. “Uber den Einfluss der Temperatur auf die Kohlensäureassim–
ilation bei zwei klimatischen Pflanzenrassen,”
Planta
, vol.11,
p.727, 1930.
5. Bőcher, T.W. “Studies on the vegetation of the east coast of Greenland between
Scoresby Sound and Angmagssalik,”
Medd. Grønland
, vol.104, no.4,
1933.
6. Bonnier, G. “Les plantes arctiques comparees aux memes especes des Alpes et
des Pyrenees,”
Rev. egen. Bot
. vol.6, p.505, 1894.
7. Brown, H.T. and Escombe, F. “Researches on some of the physiological processes
of green leaves, with special reference to the interchange of
energy between the leaf and its surroundings,” Roy.Soc. Lond.
Proc
. B, vol.76, p.29, 1905.
8. Clements, F.E. and Martin, E.V. “Effect of soil temperature on transpiration
in
Helianthus annuus
,”
Plant Physiol
. vol.9, p.619, 1934.
9. Curtel, M.G. “Researches physiologiques sur la transpiration et l’assimilation
pendant les nuits Norwegiennes,”
Rev. egen. Bot
. vol.2, p.7, 1890.
10. Dexter, S.T. “Grwoth, organic nitrogen fractions and buffer capacity in
relation to hardiness of plants,”
Plant Physiol
. vol.10, p.149, 1935.
11. Ehlers, John H. “The temperature of leaves of
Pinus
in winter,”
Amer. J. Bot
.
vol.2, p.32, 1915.
12. Fries, T.
Experimentella undesokningar őver dit arktiska Gusklimaets infly
—
tarde paa vaxterna, Flora och Fauna
. Stockholm, 1918.
13. Health, O.V.S. and Gregory, F.G. The constancy of the mean net assimilation
rate and its ecological importance.
Ann. Bot
. 2, p.811, 1938.
14. Jumelle, Henri. “Recherches physiologiques sur les lichens,”
Rev. egen. Bot
.
vol.4, p.305, 1892.
EA-PS. Warren Wilson: Arctic Plant Physiology
15. Kjellman, F. Ur polarväxternas liv. Stockholm, 1884. (Quoted by Müller,
1928)
16. Kostychev, S. “Studien űber Photosynthese. III. Findet eine Kohlensäureas–
similation während der Sommernächte in der subarktischen Region
statt?” Deutsch. Bot. Ges.
Ber
. Vol.39, p.334, 1921.
17. Kostychev, S., Chesnokov, W., and Bazyrina, K. “Untersuchungen űber den
Tagesverlauf der Photosynthese an der Kűste des Eismeeres,”
Planta
, vol.11, p.160, 1930.
18. Kramer, Paul, J.
Plant and soil water relationships
. New York, 1949.
19. Kreusler, U. “Beobachtungen űber die Kolhlensäureaufnahme und -Au
^
s
^
gabe
(Assimilation und Atmung) der Pflanzen,”
Landwirtschaftl. Jb
.
vol.16, p.711, 1887.
20. Lundegardh, H.
Enivronment and plant development
. London, 1931.
21. Matthaei, L.C. “Experimental researches on vegetable assimilation and respire–
tion. III. On the effect of temperature on carbon-dioxide assim–
ilation,” Roy. Soc. Lond.
Philos. Trans
. B, vol.197, p.47, 1905.
22. Mundra, Alivs. “Zur Physiologie der Kälteresistenz des Winterweizens,”
Planta
,
vol.18, p.435, 1932.
23. Müller, D. “Die Kohlensäureassimilation bei arktischen Pflanzen und die Ab–
hangigkeit der Assimilation von der Temperatur,”
Ibid
. vol.6,
p.22, 1928.
24. Nightingale, G.T. “The nitrogen nutrition of green plants,”
Bot.Rev
. vol.3,
p.85, 1937.
25. Polunin, Nicholas. “Conduction through roots in frozen soil,”
Nature
vol.132,
1933, p. 313.
25a. ----.
Contributions to arctic botany
. D. Phil. thesis, Oxford Univer–
sity, 1935.
26. ----. “The vegetation of Akpatok Island. Part I,”
J.Ecol
. vol.22, p.337, 1934.
27. ----. “The vegetation of Akpatok Island. Part II,”
Ibid
. vol.23, p.161, 1935.
28. Russell, R.S. “The effect of arctic and high mountain climates on the carbohy–
drate content of
Oxyria digyna
,”
Ibid
. vol.36, p.91, 1948.
29. ----. “Physiological and ecological studies on an arctic vegetation.
III. Observations on carbon assimilation, carbohydrate storage
and stomatal movement in relation to the growth of plants on
Jan Mayen Island,”
Ibid
. vol.28, p.289, 1940.
30. ----,
etx al
. “Physiological and ecological studies on an arctic vegetation.
EA-PS.
[:
]
Warren Wilson: Arctic Plant Physiology
II. The development of vegetation in relation to nitrogen
supply and soil micro-organisms on Jan Mayen Island,”
Ibid
.
vol. 28, p.269, 1940.
31. ----, and Wellington, P.S. “Physiological and ecological studies on an
arctic vegetation. I. The vegetation of Jan Mayen Island,”
Ibid
. vol.28, p.153, 1940.
32. Sachs, J. Ein Beitrag zur Kenntniss der Ernahrungshatigkeit der Blätter.
ärb. Bot. Inst. Wurzburg, 1884.
33. Scarth, G.W. “Cell physiological studies of frost resistance: a review,”
New Phytol
. Vol.43, p.1, 1944.
34. Sørensen, Thorvald. “Temperature relations and phenology of the northeast
Greenland flowering plants,”
Medd.Grønland
, vol.125, no.9, 1941.
35. Stocker, O. “Assimilation und Atmung Westjavanischer Tropenbäume,”
Planta
,
vol.24, p.402, 1935.
36. Stalfelt, M.G. “Der Gasaustausch der Flechten,”
Ibid
. vol.29, p.11, 1938.
37. ----. “Der Gasaustausch der Moose,”
Ibid
. vol.27, p.30, 1937.
38. ----. “Die Lichtokonomie der Arktischen Pflanzen,”
Svensk. Bot. Tidskr
.
vol.19, p.192, 1925.
39. Summerhayes, V.S., and Elton, C.S. “Further contributions to the ecology of
Spitzbergen,”
J.Ecol
. vol.16, p.193, 1928.
40. Wager, H.G. “Growth and survival of plants in the arctic,”
J.Ecol
. vol.26,
p.390, 1938.
41. ----. “On the respiration and carbon assimilation of some arctic plants
as related to temperature,”
New Phytol
. vol.40, p.1, 1941.
42. Wager, H.G., and Wager, E.M. “Annual changes in the osmotic value of some
arctic and temperate plants,” Roy.Soc.Dublin
Sci.Proc
. vol.21,
p.641, 1938.
43. Walter, H. “Über die Assimilation und Atmung der Pflanzen im Winter bei
tiefen Temperaturen,” Deutsch. Bot. Ges.
Ber
. vol.62, p.47, 1949.
44. Warren Wilson, J.
Botanical Results, Oxford University Expedition to
Jan Mayen
, 1947.
45. ----.
Botanical Results, British Expedition to Jan Mayen
, 1950.
46. Watson, D.J. “Comparative physiological studies on the growth of field
crops. I. Variation in net assimilation rate and leaf area
between species and varieties, and within and between years,”
Ann. Bot
. vol.11, p.41, 1947.
EA-PS. Warren Wilson: Arctic Plant Physiology
47. Whitfield, C.J. “Ecological aspects of transpiration. II. Pikes Peak and
Santa Barbara regions: edaphic and climatic aspects,”
Bot. Gaz
.
vol.94, p.183, 1932.
48. Whyte, R.O.
Crop production and environment
. London, 1946.
49. Wilhelm, A.F. “Untersuchungen űber die Kaltresistenz winterfester Kultur–
pflanzen unter besonderer Berucksichtigung des Einflusses
verschiedener Mineralsalzernährung und des N-Stoffwechsels,”
Phytopath. Z
. vol.8, p.111, 1935.
50. Zakharova, T.M. “Über den Gastoffwechsel der Nadelholzpflanzen im Winter,”
Planta
, vol.8, p.68, 1929.
J. Warren Wilson
Frost Resistance
EA-PS
(J
acob
Levitt)
FROST RESISTANCE
Even in temperate and semitropical climates, plants are injured by the in–
frequent, mild frosts. The occasional loss of a major part of the citrus fruit
crop is a case in point. Arctic plants must therefore possess pronounced adaptations
to survive the extremely severe and long-lasting frosts to which they are exposed.
In other words, they must be frost-resistant to the highest degree.
One of the earliest attempts to explain such frost resistance was the caloric
theory, which maintained that resistant plants produced enough heat to keep them
from freezing. But it was soon discovered that plants are not like warm-blooded
animals. They are unable to keep their temperatures appreciably above that of
their environment, except insofar as their temperature lags behind during a change.
Thus bulky plant parts such as tree trunks may be temporarily several degrees above
or below that of the air, depending on whether the air temperature is falling or
rising (22; 26). But it is the cambium layer of tree trunks that is most in
need of frost resistance, as future growth of the tree is dependent upon (and to
some extent takes place in) this layer. The readiness with which cambium tempera–
ture changes is shown by the fact that on the sunny side of a tree on a cold winter
day it may be 60° to 80°F. higher than on the shady side, when the latter is about
0°F. (6).
Frost resistance therefore cannot be due to any ability of a plant to keep
its temperature above that of its environment. In fact, even the early observers
noticed that resistant plants do freeze solid during winter without being injured.
The term resistance is therefore used in the sense of tolerance. Furthermore, a
EA-PS Levitt: Frost resistance
resistan
ce
^
t
^
plant must be able to tolerate very rapid temperature fluctuations,
such as occurred in the case of the cambium layer described above.
That the speed of temperature change is an important factor has been re–
peatedly shown. Even plants with a high degree of frost resistance may be killed
if frozen very suddenly. Apple buds that normally withstand −20° to −30°F. with
little injury, may be killed if frozen suddenly at 0°F. (3). Similarly, sudden
thawing may frequently cause more injury than gradual thawing. Alfalfa plants
frozen at −0.4°F (−18°C.) nearly all survived when thawed gradually, but most
were killed when thawed rapidly
.
(19).
To add to the complexity of the problem, it has been found that plants may
be fully frost-resistant in one climate yet may be killed by frost in a more
moderate climate. Thus, some clover varieties that are quite hard
i
y in Canada
were found to be winterkilled in England. It is even conceivable that some arctic
plants may not be able to survive the milder winters of other climates. This is
to be expected from the fact that even the most frost-resistant plants are quite
sensitive to frost at some stage in their development. Thus the Siberian pea
tree (
Caragana arborescens
) is capable of surviving temperatures as low as −80°
F. in its native habitat, yet when new growth has just been produced in spring it
is readily killed by freezing at
−
^
+
^
24°F. Similarly, it has
[:
]
been found that
perfectly hardy shrubs may suffer severe injury if growing near street lamps (11).
Neither of these cases of injury could occur under natural conditions, yet
they point to a fact that is of both practical and theoretical importance, namely,
that plants must undergo a hardening-off process before they can withstand
[:
]
freezing. Newly formed tissues have not developed this hardiness and are, as a
rule, sensitive. Similarly, many woody plants cannot develop hardiness unless
subjected to a “short day” (i.e., a daylight period of less than 12 to 14 hours)
for a period of some weeks. This is the reason for the injurious effect of street
EA-PS Levitt: Frost resistance
lamps mentioned above. These daylight requirements vary with the plant. Vari–
eties from southern climates may need the short day earlier in the season than
varieties from northern climates (17).
Not only must plants develop frost hardiness, they must also retain it until
danger of frost is past. This retention depends partly on internal plant charact–
eristics that are not understood. Thus some trees (e.g.,
^
the
^
blue or Colorado spruce,
Picea pungens
) fail to grow at any time during winter even if transferred to
spring or summer temperatures. Others (e.g., Indian bean,
Catalpa hybrida
) will
sprout as early as January if given the opportunity. The former have a longer
“rest period” and therefore stay hardy for the length of the winter. The latter
will begin to grow in an unseasonably warm period during winter and will then
lose much of their frost resistance. A subsequent cold snap will destroy a
tree like the Indian bean but will fail to injure the blue spruce.
This may explain why certain varieties of plants hardy in Canada fail to sur–
vive the more moderate English climate. If the winter remains severe, they re–
tain their hardiness, but if moderate temperatures occur during winter, they
lose their hardiness. This is apparently true even of different parts of the
same plant. Thus injury to the south side of trees may well be due to the fact
that the tissues on this side lose their hardiness on sunny days when their tem–
perature rises far above that of the north side. However, this kind of injury
may also be caused by rapid freezing about sunset and thawing after sunrise.
It might be questioned why the term “frost resistance” is used rather than
“cold resistance.” There is good reason for the choice. At the beginning of
the twentieth century, it was clearly shown that the cold itself may be completely
innocuous, for if a frost-sensitive plant is kept absolutely quite during the
temperature fall it supercools and on rewarming shows no sign of injury (16;27).
EA-PS Levitt: Frost resistance
If, however, owing to shaking or some other factor,
f
ice forms in the tissues,
the same plant is killed almost instantly. This kind of plant is therefore cold–
resistant but frost-sensitive. An extreme example is afforded by the seeds of even
the most frost-sensitive plants, which can be cooled to as near absolute zero as has
been attained without suffering injury (14). Such seeds do not contain any free
water and cannot freeze. If, however, they are allowed to imbibe water beforehand,
they will readily freeze at temperatures below the freezing point of water and in
many cases will be killed. It is true that some plants are cold-sensitive and may
be injured or even killed at temperatures above the freezing point, but these are
all native to tropical or subtropical regions.
It is possible that supercooling may be of some importance in the case of
certain arctic plants. In the less severe climate of the state of Washington,
pine and f
u
^
i
^
r needles do not appear to freeze, judging by their pliability at low
temperatures (4). Further
o
more, even very frost-sensitive plants can be plunged
into liquid air without injury if certain precautions are taken that prevent ice
formation in their tissues (15). One of these precautions is to use a layer of
tissue only one cell thick in order to get the extremely rapid temperature fall
necessary to avoid ice formation. Another requires plunging the liquid-air-cooled
tissue into a salt solution to prevent ice formation during the rise in temperature.
It is, perhaps, possible that some arctic plants may be capable of supercooling,
but they would have to remain supercooled all winter. Otherwise, this adaptation
would do more harm than good, for if a supercooled plant f
^
r
^
eezes, the ice formation
in its tissues occurs rapidly and is more likely to do damage than if the plant
froze gradually without supercooling. On the basis of observations that have been
made,
we must conclude that all or nearly all plants do freeze when exposed to temperatures
a few degrees below the freezing point of their sap. Thus all or near
^
l
^
y all the native
EA-PS Levitt: Frost resistance
trees have ice in their tissues on a normally cold winter day in the state of New
York (28). It seems certain, therefore, that all or nearly all overwintering
arctic plants must possess frost resistance in order to survive.
An understanding of frost resistance is possible only if we know why frost
causes injury. One of the earliest ideas, still current among some practical men
(as well as some scientists), is that ice formation in plants ruptures the cells.
This explanation was long ago given for frost splits of trees (5). It was also sug–
gested that frost-resistant plants survive owing to the ability of their cells to
expand without rupture. Like many other theories based largely on speculation,
this
^
one
^
was exploded by the very first careful observations. Instead of expanding
on freezing, plant tissues usually contract (2;8). It is this contraction of the
outer part of a tree trunk without much contraction in the center that seems to
cause the frost splitting (2).
The most conclusive evidence against the cell
[:
]
-rupture theory was produced
by the painstaking observations of Göppert in 1830 (7), which were later confirmed
by other workers. He examined thousands of cells of frost-injured plants and failed
to find any cell
[:
]
rupture. The reason for this and for the contraction that
actually does occur is now clear. Ice does not form inside the cell, but arises
in the intercellular spaces (18). As the temperature drops, these intercellular ice
crystals grow at the expense of water drawn from the cells. Consequently, in frozen
plants the cells are actually severely contracted instead of being expanded. The
outer cutinized or suberized layer surrounding a leaf or twig contains no water
and therefore contracts as all normal solids do when their temperature drops. It
seems that this outer layer acts like a tightening belt around the plant organ, and
squeezes air out of the intercellular spaces, which gradually become filled with ice
formed from the cell’s water.
EA-PS Levitt: Frost resistance
Under artificial conditions it is possible to induce ice formation inside
the cells, for instance by supercooling the tissues well below their freezing point
and then inoculating with an ice crystal (25). The subsequent ice
^
^
formation is so rapid
t
j
^
h
^
at t
j
^
h
^
ere
o
^
i
^
s
[:
]
insufficient time for the water to come out of the cells and solidify
on the
[:
]
intercellular ice crystals. Consequently, instead of relatively few,
large intercellular crystals, a vast number of minute ice cr
u
^
y
^
stals form inside the
[:
]
cells. In almost all cases, such ice formation inside the cells is fatal, even
if the tissues are frost-hardy. This may well be the reason for the fact that
rapid freezing is more injurious than slow freezing.
Plants that have just thawed frequently have water-soaked, flaccid leaves.
This was originally thought to be due to the supposed cell
[:
]
rupture, but the true
explanation is now known to be the melting of the intercellular ice crystals. If
the cells are dead, they cannot absorb this intercellular water and the flaccid
leaves soon dry up. If the cells are alive, they gradually reabsorb it, regain–
ing their turgor and permitting air to flow back into the intercellular spaces.
It is an interesting fact that very frost-resistant tissues, such as the buds of
native trees and shrubs, do not show any such water-soaked appearance on thawing.
Instead, they reabsorb the intercellular water just as soon as the ice melts(28).
It is quite possible that the injury occurring during rapid thawing may be re–
lated to this formation of intercellular water.
The fact that ice formation normall
^
y
^
occurs outside the cells leads to the
conclusion that frozen tissues are just as truly dehydrated as are tissues of
plants wilted during a drought. In both cases water is removed from the cells and
as a result the cells collapse. It is therefore not surpris
e
ing that frost and
drought resistance frequently go hand in hand. The normal way to harden off a
plant against frost injury is to expose it to low temperatures (above freezing)
EA-PS Levitt: Frost resistance
for several days. But it is also possible to increase frost resistance by allowing
the plant to dry out for the same period at normal growing temperatures. Furthermore
when cells are frost-resistant, they are also resistant to other forms of dehydra–
tion injury (20;21). In fact, one of the best indirect methods of measuring frost
resistance is to determine how severely the cells can be dehydrated by “plasmoly–
sis” without in
f
^
j
^
ury. This is done by placing sections of the tissue in a series of
sugar solutions of different strengths, and finding the weakest solution that de–
hydrates the cells to the point from which they are unable to regain their turgor
on transfer to pure water (24).
Such evidence leads to the conclusion that frost injury is really a dehydration
injury. Unfor
u
tunately, the reason why dehydration in general causes injury is
not any better understood than the reason why frost itself causes injury; but
recent experimental evidence has at least led to the proposal of a working hypoth–
esis (9;10). According to this concept, it is the physical effects of the cell
collapse and the subsequent cell expansion that
[:
]
injures the protoplasm.
The properties of frost-resistant cells explain how they avoid such
injuries
injury. Frost-resistant cells are more permeable to water than frost-sensitive cells
(13). This may protect them from injury in two ways: (
1
) it may permit the water
to leave the cells rapidly enough during freezing to prevent the fatal ice forma–
tion inside the cells; and (
2
) it may permit sufficiently rapid reabsorption of
intercellular water during thawing to prevent thawing injury.
It has also been shown that other physical properties of the protoplasm are
markedly altered during frost hardening. If the protoplasm of frost-sensitive cells
is dehydrated by placing them in a strong sugar solution, it becomes brittle and
snaps readily. In the same sugar solution, the protoplasm of hardy cells can still
be stretched and appears to retain its normal fluidity (24).
EA-PS Levitt: Frost resistance
These and many other tests indicate that the protoplasm of frost-resistant
cells can retain considerable amounts of water in the presence
^
^
of dehydrating for–
ces that remove nearly all the water from the protoplasm of frost-sensitive cells.
Recently it has been possible to show that such frost-resistant cells have a
much larger fraction of water-soluble protein than do the frost-sensitive cells (23).
This leads to the conclusion that the ability of frost-resist
na
^
an
^
t protoplasm to hold
onto more water is due to the hydrophilic (water-loving) nature of its proteins.
This may also explain the higher permeability of frost-resistant cells to water.
Plant breeders have succeeded in proving that more than one factor is involved
in frost resistance; so in spite of the importance of protein hydration there must
be some other factor as well. Physiologists long ago succeeded in recognizing another
factor — the sugar content of the plant. As a general rule, the more frost-resistant
a plant is, the greater will be the concentration of sugars in its cells. It has,
in fact, been possible to arrange several varieties of wheat in decreasing order
of frost resistance simply by arranging them in order of decreasing sugar content
(1). Similarly, the sugar content of the cortical cells of trees rises from a
minimum at the time of greatest sensitivity to frost in spring to a maximum (which
may be five times the minimum) during midwinter when frost resistance is highest
(13). But this relationship does not always hold. Some plants with high sugar
content are completely lacking in frost resistance (e.g., sugar cane); and there
are cells that survive severe freezing with little or no sugar (e.g., xylem paren–
chyma of many trees). Moreover, some varieties fail to show the relationship
mentioned above.
From all these results we can safely conclude that sugar content is only a
secondary factor in frost resistance. It enhances the frost resistance of cells
having moderately hydrophilic protoplasm, but it fails to affect the resistance
EA-PS Levitt: Frost resistance
either of protoplasm that is very readily dehydrated or of protoplasm that is ex–
tremely hydrophilic.
In order to understand the interaction of these two factors, we have to consider
quantititat
a
ively what happens during freezing. The formation of ice in the intercel–
lular spaces dehydrates both the protoplasm of a cell and the vacuole enclosed by
it.
As a result, the cell collapses and is exposed to mechanical stresses (9;10). At a
certain degree of dehydration (and therefore at the subzero temperature just suffi–
cient to produce it), the protoplasm becomes sufficiently brittle to be injured by
these stresses. The injury therefore depends on two factors: (
1
) the degree of brittle–
ness of the dehydrated protoplasm; and (
2
) the degree of contraction of the cell,
which sets up the stresses.
Increasing the hydrophily of the protoplasm will result in a lack of brittleness
even in severe frosts which set up strong dehydrating forces; on the other hand,
increasing the sugar content of the vacuole increases the osmotic pressure of the
cell sap. The degree of contraction of a cell on freezing is inversely proportion
[:
]
^
al
^
to this osmotic pressure (3). Theoretically, one might expect this to be true only
down to the eutectic point of the sugars — i.e., the temperature at which the sugars
crystallize out. But sugars do not possess a definite eutectic point, and, on concen–
tration, they form a thick sirup from which they fail to crystallize out, even at
−7.6°F. (−22°C.) (3). Thus, if one cell has twice the sugar
[:
]
content of a sec–
ond cell, it will have nearly twice the osmotic pressure, and at any one freezing
temperature the second cell will contract approximately twice as severely as the
first. Consequently, protein hydration and sugar content may act as two independent
factors that have an additive effect on the frost resistance of the cell.
It may be not only the sugars that have this secondary effect on frost resistance.
Cortical cells of frost-resistant trees also have an accumulation of colloidal mate–
rial in their vacuoles. As a result, it is impossible to contract them to less than
EA-PS Levitt: Frost resistance
40% of their normal volume, even if all the water be removed (13).
The problem of frost resistance is thus a comple
s
^
x
^
one; but much is now
known, and the way appears to be open to a fuller understanding of the mechanism.
Jacob Levitt
EA-PS Levitt: Frost Resistance
BIBLIOGRAPHY
1.
A
^
Å
^
kerman, A.
Studien uber den Kältetod und die K
a
^
ä
^
lteresistenz der
Pflanzen
. Lund, 1927.
2. Caspary, Robert. “Bewirkt die Sonne Risse in Rinde und Holz der Bäume?”
Botanische Zeit
. vol.15, pp.153-56, 329-35, 345-50, 361-71, 1857.
3. Chandler, W.H. “The killing of plant tissue by low temperature,” Missouri
Agric. Exp. Sta.
Res. Bull
. no.8, 1913.
4. Clements, Harry F. “Mechanisms of freezing resistance in the needles of
Pinus ponderosa
and
Pseudotsuga mucronata
,” Washington. State
College.
Res. Stud
. vol.6, no. 1, pp.3-46, 1938.
5. Du Hamel et de Buffon. “Observations des diff
e
^
é
^
rents effects que produisent
sur les
[:
]
V
e
^
é
^
g
e
^
é
^
taux les grandes gel
e
^
é
^
es d’Hiver et les petites
gel
e
^
é
^
es du Printemps.” Histoire de l’Acad
e
^
é
^
mie Royale des Sciences
avec les M
e
^
é
^
moires de Mathematique et de Physique, pp.273-298, 1740.
6. Eggert, R. “Cambium temperatures of peach and apple trees in winter,”
Amer. Soc. Hort. Sci. Proc. vol. 45, pp.33-36, 1944.
7. Göppert, H.R.
Über die Wärme-Entwickelung in den Pflanzen deren Gefrieren
und die Schutzmittel gegen Dasselbe
. Breslau, 1830.
8. Hoffmann, Hermann.
Witterung
[:
]
^
u
^
nd Wachsthum, oder Grundzüge der Pflanzenkli
–
matologie
. Leipzig, 1857, pp.312-34.
9. Iljin, W.S. Über den Kältetod der Pflanzen und seine Ur
as
^
sa
^
chen,”
Protoplasma
,
vol.20, pp.105-24, 1933.
10. ----. “Die Ursache der Resistenz von Pflanzenzellen gegen Austrocknung,”
Ibid
. vol.10, pp.379-414, 1930.
11. Kramer, P.J. “Photoperiodic stimulation of growth by artificial light as
a cause of winter killing,”
Plant Physiol
. vol.12, pp.881-83, 1937.
12. Levitt, J.
Frost Killing and Hardiness of Plants
. Minneapolis, Burgess, 1941.
13. ----, and Scarth, G.W. “Frost-hardening studies with living cells,”
Canad. J. Res
. Ser.C, vol.14, pp.267-305, 1936.
14. Lipman, C.B. “Normal viability of seeds and bacterial spores after exposure
to temperatures near the absolute zero,”
Plant Physiol
. vol.11,
pp.201-5, 1936.
15. Luyet, B.J. “The vitrification of organic colloids and of protoplasm,”
Biodynamica
, vol.29, pp.1-14, 1937.
EA-PS Levitt: Frost Resistance
16. Molisch, Hans.
Untersuchungen über das Erfrieren der Pflanzen
. Jena, 1897.
17. Moschkov, B.S. “Photoperiodismus and Frosthärte ausdauernder Gewächse,”
Planta
, vol.23, pp.774-803, 1935.
18. Müller-Thurgau, H. “Uber das Gefrieren und Erfrieren der Pflanzen,”
Landwirtschaftl
.
^
Jahrb.,
^
vol.9, pp.133-89, 1880.
19. Peltier, G.L., and Tysdal, H.M. “Hardiness studies with 2-year-old alfalfa
plants,”
J. Agric. Res
. vol.43, pp.931-55, 1931.
20. Scarth, G.W. “Cell physiological studies of frost resistance,”
New Phytol
.
vol.43, pp.1-12, 1944.
21. Scarth, G.W. “Dehydration injury and resistance,”
Plant Physiol
. vol.16,
pp.171-
[:
]
79, 1941.
22. Schübler, G. “Beobachtungen über die Temperatur der Vegetabilien und
einige damit verwandte Gegenstände,”
Annalen Phys
., Leipzing, vol.
10, pp.581-92, 1827.
23. Siminovitch, D., and Briggs, D.R.
Arch. Biochem
. Vol. 23,pp.
[:
]
8-17, 1949.
24. Siminovitch, D., and Levitt, J. “The relation between frost resistance
and the physical state of protoplasm. II,”
Canad. J. Res
.
Ser. C, vol.19, pp.9-20, 1941.
25. Siminovitch, D., and Scarth, G.W. “A study of the mechanism of frost
injury to plants,” Canad.
J. Res. Ser
.C, vol.16, pp.467-81, 1938.
26. Solom
e
^
é
^
. “Observations sur la temp
e
^
é
^
rature interne des v
e
^
é
^
g
e
^
é
^
taux, compar
e
^
é
^
e
a
^
á
^
celle de l’atmosph
e
^
è
^
re,”
Annales Chim
. (Phys.) vol.40, pp.113-22,
1803.
27. Voigtlander, H. “Unterkühlung und Kältetod der Pflanzen,”
Beitrage z. Biol.
Pfg
. vol.9, pp.359-414, 1909.
28. Wiegand, K.M. “The occurrence of ice in plant tissue,”
Plant World
, vol.9,
pp.25-39, 107, 1906.
J. Levitt
Thermoperiodism, Vernalization, and Photoperiodism in the Arctic
EA-Plant Sciences
(A. Lang)
THERMOPERIODISM, VERNALIZATION, AND PHOTOPERIODISM IN THE ARCTIC
CONTENTS
|
|
Page
|
|
Introduction
|
1
|
|
Thermoperiodicity
|
3
|
|
Vernalization
|
4
|
|
Photoperiodism
|
9
|
|
Ecological Aspects
|
18
|
|
Adaptive Significance
|
20
|
|
Significance for Arctic Agriculture and Horticulture
|
24
|
|
Bibliography
|
26
|
EA-Plant Sciences
(A. Lang)
FIGURES
With the manuscript of this article, the author submitted 5 figures
(in duplicate). Because of the high cost of reproducing them, one set
will accompany the first copy of the manuscript and the other held at
The Stefansson Library.
EA-Plant Sciences
(A. Lang)
THERMOPERIODISM, VERNALIZATION, AND PHOTOPERIODISM IN THE ARCTIC
Introduction
It is generally agreed that plants for their growth and development
require a definite set or constellation of environmental conditions, among
which the most important are the conditions of temperature and light.
[By growth, we mean the quantitative increase of a plant in size, leaf
number, etc., under development qualitative changes, as the transition
of the dormant seed to germination or of a vegetative plant to flower
formation.] In some cases, this growth and development requirement can
be expressed in a very simple manner. For example, some plants (pears and
other fruit trees, etc.) will reach a definite stage in their annual cycle
of development, when the sum of the average daily temperatures, which
surpass a certain level, reaches a definite value, the “temperature sum”
characteristic of that stage. Let us assume, for example, that, for some
plant, temperatures of more than 5°C. are effective and that the tempera–
ture sum required for flowering is 1000°. Then this plant will flower after
50 days at 25° <formula>(1000/(25 – 5)) = 50</formula> or after 100 days at 15°
<formula>(1000/(15- 5)) = 100</formula>
A similar situation is found in certain cases in relation to light duration.
Both light and temperature conditions in such cases have to be adequate
throughout the entire life of the plant. If a green plant, at any point in
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
its development, is exposed to an inadequate temperature or is deprived
of sufficient light, it sooner or later will cease growing and may eventually
die. This however represents only the simplest instance of relationship
between plant and environment. In the course of the last three decades
it has become increasingly apparent that the relationships of plant life
to environment are in numerous cases much less simple and straightforward.
Many plants exhibit certain specific environmental requirements, and unless
these are met they may stop developing even though the general temperature
and light conditions, etc., seem optimal. A particularly important feature
of these requirements is that they frequently can be satisfied in a rather
short time. To pass through its entire life cycle a plant always will
require favorable temperature and light, but for a certain time it may need
low temperature or a certain number of dark hours per day. During the action
of these specific conditions visible changes may not take place at all, but
the subsequent development may be profoundly different depending on whether
or not those conditions were met. Such type of action may be called
inductive
.
We know at present of three instances of specific environmental require–
ments in flowering plants, which are of general occurrence and which therefore
play an important part in determining plant life under natural conditions:
(1) Very many
[:
]
plants require
different day and night temperatures
and will develop optimally only if the temperature is subject to diurnal
fluctuation: This is thermoperiodicity.
(2) Many plants, for passing their life cycle, particularly for forma–
tion of flowers, require a certain period of
low temperatures:
This is
vernalization.
(3) Many plants for flower formation require a definite
day-length
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
range: This is photoperiodism.
It is obvious that in the Arctic, where temperature changes enormously
both in the course of the day and of the year and where day-length also
undergoes tremendous seasonal variations, all three of these phenomena will
be of the utmost significance for plant life. Investigations devoted to
this question are, however, remarkable by their fewness, due no doubt
largely to the lack of appropriate experimental facilities. This situation
seems to be improving; nevertheless, all that is possible at present is a
brief outline of our knowledge of the phenomena enumerated and an indication
of some general ideas about their bearing for Arctic plant life.
Thermoperiodicity
It had been known for some time, particularly to practical horticulture,
that many plants thrive best if night temperatures are comparatively low.
Thus, the following plants show optimal development at the temperatures indicated
(in degrees Centigrade)*:
|
|
Day
|
Night
|
|
Violets
|
8.5-14
|
4.5-1.0
|
|
Snapdragon
|
14-16
|
7-9
|
|
Sweet peas
|
13-15.5
|
7-10
|
|
Roses
|
21-23
|
14.5-16.5
|
1
F. W. Went showed, from 1944 on, that this is a widespread, perhaps general,
phenomenon and explained at least some of its physiological nature. In
tomatoes actual stem growth takes place exclusively during night, and the
optimal temperature is comparatively low. The value changes during development;
in seedlings it may be as high as 30°, in older plants it may drop to as low
as 8°. The levels seem to be different in different varieties; this indicates
EA-PS. Labg: Thermoperiodism, Vernalization, and Photoperiodism
genetic control. On the other hand, growth also depends on the con
[:
]
itions
to which the plant is exposed during the daytime, and here the optimal
temperature is considerably higher (26°). Therefore, if day and night
temperatures are equal, growth is inferior; certain other combinations
(low day and high night temperatures) may even cause pronounced damage.
Apart from stem growth, flowering and in particular fruit-set are also
affected. In all species from temperate zones and higher latitudes which
have so far been investigated basically
f
^
t
^
he same situation was found. The
physiological cause of thermoperiodicity is probably that those processes
on which growth depends during day and during night, have somewhat different
temperature optime. Also, in some cases excessive night temperature may
induce excessive respiration which will cause a general depletion of
material required for growth.
Vernalization
Low temperature requirement has become best known in cereals. All
small-grain cereals (wheat, barley, rye, oats) include both summer (spring)
and winter (fall) varieties. The former are sown in spring and harvested
in summer and fall, while the latter have to be planted in the fall of the
preceding
^
y
^
ear; summer varieties thus pass their development within one
growing season, winter varieties require (parts of) two. If a pronounced
winter variety is planted in spring, it will give, that summer, excellent
vegetative growth, producing abundant leaves and side shoots (tillers), but
will neither head nor flower and fruit. If plants of the same variety are
artificially exposed to low temperature for a certain time, they will
behave as plants of a summer variety. This change, of course, is individual and
not hereditary; seeds harvested from
[:
]
plants of a winter variety cold treated
and grown as a summer crop will give rise again to typical winter plants.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
That a specific “cold requirement” is the physiological basis of the
winter habit of cereals and other plants was first conclusively demonstrated
in 1918 by the German plant scientist G. Gassner. Later, Russian authors
introduced the term
yarovizaciya
(from
yar
= summer). Which was translated
into English as “vernalization.” This term is used to cover both the
phenomena of cold requirement as such and the actual cold treatment, either
experimental or by the low temperatures of the season from fall through
early spring. For the cold treatment, the term “thermal induction” also
is used.
We can distinguish two groups of cold-requiring plants. In the one,
the
winter annuals
(e.g., winter cereals, white and black mustard,
Vicia
villosa
), cold requirement has a quantitative or facultative character,
i.e., cold treatment hastens flowering, but if the growing season is arti–
ficially extended, by greenhouse culture with supplementary light, the plants
will ultimately proceed to flowering even if continuously grown at tempera–
tures of 20°C. or above. In the second group, the true
biennials
(e.g.,
beets, turnips, many cabbage varieties, carrots, and many other
Umbelliferae
,
the biennial variety of the henbane
Hyoscyamus niger
), cold requirement is
qualitative or obligatory; without thermal induction the plants will remain
vegetative indefinitely. Most biennial plants have a tuber and a pronounced
leaf rosette.
Plants not exhibiting any cold requirement are called summer annuals
(summer cereals, cotton, tobacco, annual henbane, etc.). One and the same
species may comprise both summer and winter annuals, or summer annuals and
biennials; in several cases (certain cereal varieties, beets, henbane) the
genetic basis of the difference is simple (1 or 2 genes). Moreover,
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
particularly in cereals, there are all intermediates between summer and
pronounced winter annuals, between varieties that usually behave as
summer crops but may to some extent be accelerated in flowering by low
temperatures, and such that without thermal induction would never conclude
development if planted under natural conditions in spring. Usually, the
less pronounced winter types require shorter induction times and are
favored by comparatively higher induction temperatures (sometimes up to
−
^
+
^
15°), whereas typical ones as well as biennials require longer periods
(in general, around 6 to 8 weeks, but sometimes several months) and lower
temperatures (
−
^
+
^
1° to
−
^
+
^
10°, mostly closer to the lower limit).
Temperatures below 0° have at most only a slight vernalizing action,
but if vernalization has been started at a temperature above freezing point,
it may be concluded at temperatures below. The low temperatures act on the
growing region (growing points) themselves, that is on the regions which
later produce the flowers; whether other organs or tissues of the plant
are also exposed to cold or not is of no importance. On the other hand,
low temperature need not act continuously, but may be interrupted daily,
by some (although not too many) warm hours; therefore, vernalization is
completed under natural conditions even if temperatures rise during the
day periods to levels which, in themselves, would be too high.
The purpose of this review as well as space preclude a detailed
discussion of our understanding of the physiological basis of vernalization,
which, incidentally, is still far from complete. Quite
[:
]
generally, the
situation may be as follows: For flowering to set in, a certain condition
(III) must be reached in the plant; it is attained starting from an initial
condition (I) and passes through an intermediary one (II). However,
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
can also be converted along another pathway, leading to IV, which is
inactive as to flower formation:
[Figure]
Each one of the changes involved would, as usual for physiological processes,
be accelerated with increasing temperature; but the degree of this increase
is different, so that at high temperatures only IV is reached, at lower ones
(also) III. Temperature, thus appears to act by adjusting the balance of
these changes. The nature of the changes is yet unknown. Presumably they
are chemical reactions which lead to the formation of definite substances
(II IV may also be the destruction or inactivation of a compound). In
further consequence of III, a substance seems to
^
be
^
produced which causes flower
formation and which can move within the plant (a “flowering hormone”). As
was first shown (in 1936/37) by G. Melchers in Germany, by grafting a cold–
treated biennial henbane plant, or an annual one, on a non-cold-treated
biennial this latter is caused to flower itself. This means that some sub–
stance (or substances) move from the plant which is ready to flower (the
“donor”) to the plant which, by itself, would not be able to do so (the
“receptor”). Similar results have been obtained in beets, cabbage, mustard,
and carrots. In grasses (cereals) grafts are not possible, but it is rather
certain that formation of substances inducing flowering is a general phenomenon
in plants, biennial and winter-annual plants being capable of forming these
substances only after exposure to low temperature, summer annuals without.
Donor and receptor in the graft experiments may belong to different species
and even genera; the flower-forming substance therefore must be identical
in different plants.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
One physiological trait of vernalization remains to be stressed. This
is the fact that in vernalization the inductive type of action is particu–
larly strongly pronounced. The cold treatment results in an acceleration
of flowering particularly if the plant afterwards is exposed to higher
temperatures, namely, those temperatures which constitute the temperature
range ‘generally’ adequate for plant growth and development in the climate
given. If, for example, a winter annual plant with an optimal vernalization
temperature of 5° were kept at this temperature permanently, it would grow
very slowly and flower - if at all - probably much later than an individual
which was kept permanently at 20 to 25°, a temperature which is ineffective
in vernalization. But if such a plant is kept at the low temperature for the
period necessary to complete thermal induction, and then transferred to the
higher one, it will proceed to flowering much faster than the nonvernalized
individual. Under natural conditions, too, the actual flowering of cold–
requiring plants usually takes place when the temperature is considerably
elevated as compared with the vernalization temperature. On the other hand,
the low temperatures are already effective at amazingly early stages of the
development of the plant, at least in winter-annuals. Usually, winter cereals
and other winter annuals undergo the cold treatment in the seedling stage,
but it can applied also to seed which barely has started germination and
even to seed which is developing on the mother plant (though not to the dry
ripe seed). Flowering, on the other hand, never occurs before the plant has
made a certain amount of vegetative growth. Throughout this period of
development the effect of the cold treatment, then, remains “latent”, i.e.,
is carried in the plant without becoming visible; as long as actual flower
formation has not set in, a winter-annual plant grown from vernalized seed
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
cannot be distinguished from another grown from nonvernalized seed. Thus,
the effect of the low temperature of flowering in winter-annual and biennial
plants becomes manifest only under elevated temperatures and in winter-annuals
frequently only after a pronounced latency period. Biennials, as a rule,
have to make some vegetative growth before they become responsive to ver–
nalization.
Of the different stages within the flowering process, it is particularly
the
onset
of flowering which is hastened by vernalization. In the process
of flower formation, several steps can be distinguished. The first is the
formation of microscopic flower initials of flower
primordia
. In numerous
plants, including cereals, beets, etc., it is usually accompanied by stem
elongation (“heading” or “bolting”). Subsequent steps are the growth of
the primordia to visible flower buds, the opening of the buds to flowers
proper ready for pollination (anthesis), and, after fertilization had been
accomplished, fruit development and seed ripening. In a not cold-treated
winter-annual plant, flower initiation is delayed as compared with a vernal–
ized individual. But if this nonvernalized plant is allowed, by greenhouse
culture, to attain this stage at all, the later stages of flowering will be
passed at much the same rates as in vernalized individuals, or at any rate
the differences will be less pronounced than in flower initiation.
Photoperiodism
The discovery that day
length
, i.e., the relative duration of day and
night, is a factor controlling development in a great many plants, was made
in 1920 by the American plant physiologists W. W. Garner and H. A. Allard.
Garner and Allard established the two types of day-length-dependent plants
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
which are recognized today;
long-day plants
and
short-day plants
. Both
long- and short-day plants possess a
critical day length
. In long-day
plants, in day lengths below the critical one, flowering is either delayed
or does not occur at all; in day lengths above it is promoted. For example,
the long-day plant henbane will not flower if given 10 hours of light per
day or less. However, if plants grown on 10-hour days are transferred
to longer days, microscopic flower buds will be formed in the following
periods, while visible buds will appear some 2 or 3 weeks and open flowers
some 6 weeks later (all this at a temperature of approximately 22°):
|
11-hour days
|
about 40-45 days
|
|
12-hour days
|
about 15 days
|
|
13-hour days
|
about 12 days
|
|
14-hour days
|
about 10 days
|
|
15-hour days
|
about 9 days
|
|
18-hour days
|
about 8-9 days
|
|
20-hour days
|
about 8-9 days
|
|
24-hour days (continuous
light)
|
about 8-9 days
|
Short-day plants, on the contrary, will flower in day lengths
below
their
critical day
l
-length value and will fail to flower above; or will be more
or less delayed in flowering. For example, a fall chrysanthemum variety
for the formation of visible flower buds
|
in 14-hour days
|
about 65 days
|
|
in 13-hour days
|
about 35 days
|
|
in 12-hour days
|
about 25 days
|
|
in 10-hour days
|
about 25 days
|
|
in 8-hour days
|
about 25 days
|
It will not flower in 15-hour days. As can be seen from these figures,
differences in day length have a particularly great influence close to the
critical day-length value. If day length is increased or decreased beyond
a certain limit, flowering no longer is much affected. If henbane, this
limit is reached at approximately 15 or 16 hours of light, in chrysanthemum
at 12 hours.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
It is not possible — although it is often done, even in textbooks —
to give a generally valid demarcation line of the long- and the short-day
range. The critical day length may be different in different representatives
of both photoperiodic types. It is a specific or vertical character, and
the critical day-length values of long-and short-day plants overlap.
Therefore, many short-day plants will still flower more or less readily in
day lengths in which many long-day plants, in their turn, are already capable
of flowering, too. Thus, henbane and chrysanthemum will both flower in day
lengths of 11 to 14 hours. The short-day
[:
]
plant cocklebur (
Xanthium
spp.)
flowers even in a 15 1/2-hour day, whereas some stonecrop (
Sedum
) species —
all long-day plants — flower only if the day length exceeds 14 hours.
Therefore, if the plants are grown on a 13-hour day, one might easily be led
to consider the cocklebur as the long-day and
Sedum
as the short-day plant.
The essential thing is that in henbane and stonecrop flowering will be
promoted if the plants are transferred to comparatively longer days, in
chrysanthemum and cocklebur it will be delayed or suppressed.
In an over-all fashion, however, one may say that for most day-length
dependent plants, both of the long- and the short-day type, the limit between
the flower promoting and flower inhibiting day lengths lies in the range
from 11 to 14 hours of light per day. Examples of long-day plants, besides
those already quoted are: most small-grain cereals, lettuce, radish, spinach,
China aster,
Rudbeckia
,
Stenophragma thalianum,
etc. Short-day plants are:
rice, sorghum, millet, teosinte, poinsettias, Maryland-Mammoth tobacco, many
Chenopodiaceae
and
Amerantaceae
, etc. Plants in which flowering is independent
of day lengths are called indeterminate or
day-neutral
plants; some examples
are corn, tomato, chili pepper, cucumber, snapdragon. One and the same species
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
may include day-neutral and long-
or
short-day strains. In side-oats
grama (
Bouteloua curtipendula
) both long- and short-day strains have
been described; but this seems to be rather an exceptional situation.
The differences long-day vs. day-neutral habit and short-day vs. day-neutral
habit may be caused by a single gene.
Only a brief outline can be given of the physiological basis of
photoperio
^
d
^
ism. Those features which have ecological significance will
be particularly stressed. The foremost feature of this sort is the induc–
tive nature of day-lengths action. As with low temperature in vernalization,
the appropriate day length need not act continuously, until visible response
occurs, but only for a comparatively short time, after which the plant will
respond even if it is then transferred to a day length in which flowering
would never have occurred. This is known as photoperiodic aftereffect or
photoperiodic induction.
Day lengths favorable of flower formation, therefore,
are frequently referred to as “inductive,” those unfavorable as “noninductive.”
Day length
s
itself is often called “photoperiod,” a photoperiod of, e.g.,
15 hours meaning 15 hours of light
−
^
+
^
9 hours of darkness. The time necessary
for photoperiodic induction is generally much shorter than that for thermal
induction in vernalization. In particularly sensitive plants, as cocklebur,
one single inductive photoperiod may be sufficient (although it will not give
maximal response). In contrast to vernalization photoperiodic induction is
not permanent in most cases. With but a few exceptions, most notably again
with the cocklebur,
g
the plant returns to vegetative growth after having been
a longer or shorter time under noninductive conditions.
It must be emphasized that photoperiodic responses actually involve a
periodic
element. One might assume — and this has indeed been done — that,
EA-PS. Lang: Thermoperio
i
^
d
^
ism, Vernalization, and Photoperiodism
for flowering, long-day plants require a definite amount of light and
short-dayplants similarly a certain amount of darkness. Thus the dark
periods in the former and the light periods in the latter would cause
only an interruption of the favorable condition and thus delay the response.
This however, is not true, as can be shown by experiments in which the
cycle length
is changed. For instance, henbane flowers in cycles of 16
hours light
−
^
+
^
32 hours darkness, but not in 8
−
^
+
^
16 hours. Soybean behaves
in the opposite way. In all cases the total amounts of light and darkness
are equal. Thus, both light and dark conditions are somehow involved in
the outcome, and the response depends not only on the total duration, but
also on the
timing
of light and darkness.
Wherever light exerts as effect on an organism, it must be absorbed by
some pigment. The pigment responsible for the adsorption of the photoperiodi–
cally effective light has recently been shown to be not identical with any
of the pigments already known in flowering plants. Its chemical nature has
not yet been identified, but it seems to be identical in long- and short-day
plants. In the light of this discovery another interesting and significant
feature of photoperiodism can probably be explained. If a long-day plant or
a short-day plant is grown in short-day conditions, but illuminated with
supplementary light, very low intensities are sufficient to promote flowering
in the former and to inhibit or to delay it in the latter. Even the light
of the full moon may cause some photoperiodic response. Increasing the
intensity of the supplementary light beyond a certain limit has no further
photoperiodic effect. This limit is still very low, far below the light
intensities which are required to maintain plant life. Thus the photoperiodic
responses are caused by comparatively small quantities of light, and the
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
reason for this is probably that the pigment responsible is present only
in small quantities and therefore able to utilize, at a time, only small
amounts of light.
The interaction of light and darkness in long-day plants may be
visualized perhaps as follows: In the light, some substance or substances
are produced which serve as precursor for a specific substance required
for flower formation. In darkness, this substance is destroyed or inactivated;
this latter process seems to be localized in the leaves:
[Figure]
If the light periods are short and dark periods long, i.e., in short
days, all of the “precursor” is subject to destruction or is inactivated;
if the relation is reverse, i.e., in long days, the flower-forming substance
can be made.
This explanation is based, mainly, on two observations: (a) If certain
long-day plants (henbane), which are able to stand such treatment, are
deprived of all leaves, they produce flowers independently of day lengths,
that is, in long as well as short days. (b) As the temperature is decreased,
long-day plants become capable of flowering in progressively shorter photo–
periods. For example, henbane, as mentioned, has a critical day length
of about 10 hours. This is true, however, only at a temperature of about
22°; at 15.5° the plants will form flowers even in 9-hour photoperiods,
whereas at 28.5° they require at least 12 hours of light per day. The
effect of temperature is particularly exerted during the dark period. This
means that the processes which are going on in the dark phases and which
are inhibitive to flowering, are accelerated with higher temperature and thus
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
become effective in shorter times. The explanation also accounts for the
fact that long-day plants flower fastest in continuous light, i.e., need
no dark periods whatever for flower formation, whereas they fail to flower
if the dark periods are too long. One may say that the photoperiodic
response of long-day plants is not flower formation is long days, but its
inhibition in long nights. In the action of the light most probably two
separate effects have to be assumed. One action of the light consists in
supplying, assimilates by photosynthesis. The assimilates are the material
on which all processes which are going on in the plant are dependent. This
light action, therefore, is not specific for photoperiodism. The second
light action is involved either in the production of the special flowering
substance precursor or it consists in the prevention of its
[:
]
destruction
or inactivation. It is thus the specific photoperiodic light effect; it is
probably carried out by the light absorbed by the “new” pigment. As either
light effect, however, is ultimately directed toward promotion of flowering,
the twofold character of light action in long-day plants is difficult to
see and to separate.
In short-day plants the situation is different. These plants require
an
alternation
of light and dark periods of appropriate length. Soybeans,
for example, will flower under regimes in which light periods of 3 or 4
hours to 12 hours are combined with 10 1/2 to 14 hours of darkness; if
either of the periods is longer or shorter, flowering will be greatly delayed
or suppressed. In short-day plants it is therefore evident that light has
two different actions in regard to flower formation. These two effects are,
respectively, (a) a promoting one, which accounts for the need of light
periods, and (b) an inhibitive one, which accounts for the need of dark periods.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
As in long-day plants, the promoting actions seems to consist in the supply
of assimilates by photosynthesis, whereas the inhibitive effect again seems
to be tied in with the specific photoperiodic light action perceived by the
special pigment. Long- and short-day plants, then, differ primarily in
that the specific light effect in the former is promoting, in the latter
inhibitive with regard to flower formation. Nevertheless it seems in either
case to be mediated by one and the same pigment.
As the final outcome of photoperiodic induction, both in long- and
short-day plants, a special flower-forming substance appears to be formed.
This material moves from the leaves in which it is generated to the growing
points and causes these to change into flowers. The possible existence of
such a “flowering hormone” was first suggested to early workers by the fact
that the photoperiod effect is perceived by a part of the plant which is
separated from the part where the response occurs. In vernalization, both
perception of the cold stimulus and the response to this stimulus are
carried out by the growing points themselves. In photoperiodism, the
effect of the light is perceived
by the leaves
. It is of no consequence
whether the growing points of the plant are also exposed to the inductive
photoperiod or not. The flowers, however, are naturally formed by the
growing points. A mediator of the flowering response must therefore
pass through the plant.
This was borne out by grafting experiments, similar to those done with
cold-requiring plants (first by J. Kuijper and L. K. Wiersum in Holland,
and by M. H. Chilahian and B. S. Moshkov in Russia, in 1936 and 1937).
If a short-day plant is induced to flower by short-day treatment and is
then grafted to an individual which has been kept on long day, the latter
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
individual, too, will produce flowers. The same can be done using a
day-neutral or long-day plant as conor. Vice versa, a long-day plant
maintained in short-day conditions can be caused to flower by graft union
with a short-day plant. As in the grafts with cold-requiring plants, donor
and receptor may belong to different species or genera (provided they can
be grafted together satisfactorily).
Since short- and long-day plants are thus capable of mutual flower
induction and since the taxonomic relationship of the grafted plants is of
no importance for the effect on flowering, the flowering substances of all
photoperiodically sensitive plants must be identical or at least very closely
related. The flowering substance demonstrated in grafts with cold-requiring
plants may be different. Even names for the two “flowering hormones” have
been introduced: “florigen” for the one suggested by the experiments with
photoperiodically sensitive plants, “vernalin” for the one suggested by
those with cold-requiring plants. However, so far neither of these substances
has been isolated and identified, and therefore this question, and the whole
question, whether specific flowering hormones indeed exist, is not yet
finally settled.
Flowering although the most spectacular, is not the sole photoperiodic
response in nature. Among the animals, aphids, snails, birds, mammals show
photoperiodic responses. In the plant world photoperiodic phenomena other
than flower induction are also known. Further steps in the process of
flower formation itself have a different day-length dependence. For example,
the formation
(
flower primordia may be promoted by one day-length condition,
their further development until anthesis by another day-length condition
Furthermore, the fertility and, in monoecious plants (plants having separate
EA-PS. Lang: Thermoperiodism, Vernalization, and Phtoperiodism
male and female flowers on the same individual), the sex of the flowers
may depend on day length. In grasses with a tendency to vivipary, as
Deschampsia caespitosa,
this tendency becomes much more pronounced in
short days. The production of cleistogamous and chasmogenmous flowers
in violets is also regulated by day-length, although frequently in close
intersection with temperature effects.
In cleistogemous flowers pollination takes place in the bud, and the
corolla does not open at all; chasmogamous flowers are the ones developing
normally. Thus,
Viola hirta
and
V. silvestria
, if grown in photoperiods of
12 to 14 hours at elevated temperatures, from exclusively cleistogamous
flowers; in 8-hour photoperiods at such temperatures they do not flower
at all; but
^
by
^
a low-temperature treatment are caused to form chasmogamous
flowers.
V. odorata
forms chasmogemous flowers occasionally also without
cold treatment.
In addition, various vegetative morphological responses to photoperiod
have been observed. Among such cases are:
1
. Tuber formation is a
short-day response in many species as in certain wild potatoes, in yams,
arrowroot, Jerusalem artichoke, etc. The flowering response of the plant
may be the opposite.
2
. Bulb formation in onion is a long-day response.
3
. In numerous strawberry species and varieties, in hawksbeard (
Hieracium
pilosella
), and in other plants runners are formed in long days.
4
. In
various succulent plants (species of
Bryophyllum
,
Sedum
, etc.) the habit
may be deeply modified by day length, short-day causing more pronounced
succulence.
5
. In numerous other plants the general habit, too, is modified
by day length, although the differences are not very pronounced and therefore
are frequently overlooked.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
As a rule, long-day grown specimens exhibit one or more of the follow–
ing features: greater stem-elongation, longer petioles, leaves narrow and
more erect; short-day specimens are less tall and have shorter petioles
and broader and more horizontally arranged leaves. Quite generally, plants
grown in long-day conditions are more vigorous than individuals grown in
short-day conditions, although here the total amount of light and not only
the day length may be of significance. Finally, in some cases day length
has been found to effect physiological characters of a plant without changing
its morphology. Thus, in locust trees (Robinia pseudoacacia) of southern
origin, but grown in higher latitudes, short days increase frost and drought
resistence.
Ecological Aspects
The information summarized in the preceding sections has been obtained
nearly exclusively under experimental conditions. What implications do the
results have for plant life in the natural environment, and most particularly
in the Arctic? These implications can be divided into two groups. The first
comprises the consequences of changes of the temperature and day length regimes
for the development of a given plant growing in higher latitudes as compared
with lower ones. Evidence on this point, as mentioned above, isstill very
scarce.
There are, however, a few observations which indicate two things:
(a) Winter-annual plants, when moved farther to the North, frequently behave
as summer annuals. (b) Plants which do not possess a pronounced cold re–
quirement, frequently
shorten
their development time at higher latitudes.
An example of the first phenomenon is constituted by certain Russian wheats
which in temperate or southern regions of the country are
i
^
u
^
sed as winter crops,
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
but which in the Russian Subarctic can be sown in the spring. Apparently,
the cold requirement of these varieties can be satisfied by the low tempera–
tures both during the early and the late parts of the growing season, the
former acting on the seedlings, the latter on the developing seed. It may be
that, at least in some cases, the second effect is the essential one; if seeds
harvested in the North are planted in the South the plants still be have as
summer annuals, and only their offspring again exhibits the winter habit.
Similar observations have been made in mountain regions which, as far as
the temperature factor is concerned, can be compared with the North. Wheats
which in Switzerland are used as summer varieties behave in southern Germany
as winter crops. In the opposite way, some Italian wheat and oats as well
as turnips, which grow as winter annuals or as biennials at an elevation of
160 m. can be treated as summer crops at 1,600 m. It is very significant in
this connection that vernalization can be achieved with intermittently low
temperatures. Likewise, it becomes significant that vernalization, as explained,
primarily accelerates the
onset of flower in
t
^
i
^
tiation
and has no comparable
effect on the further stages of flowering, which, in addition, ordinarily
takes place at already rather elevated temperatures. It is, therefore, not
surprising that it is the heading and earing which are particularly promoted
in cereals in the North; the subsequent stages from anthesis to seed ripen–
ing may even be somewhat delayed.
In the acceleration of the development of summer-annual plants in the
North two factors may be operative: (1) longer days, and (2) higher daytime
temperatures, possibly in combination with lower night temperatures. Many
plants which grow both in moderate and in subarctic to arctic regions are of
long-day character, and since the day length in summer increases with latitude,
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
they may find conditions more favorable for flowering in the higher latitudes.
It is of importance in this connection that photoperiodic responses are
caused by very small amounts of light. It has been shown, by the Swedish
scientists Kjellman and Thore Fries, that arctic plants are capable of carry–
ing out photosynthesis and growth in the light of arctic nights. In photo–
periodic responses considerably smaller light quantities are already effective
than those required for phytosynthesis. As far as photoperiodism is concerned,
the “White Nights” of the North are continuous light, which is the optimal
light regime for flowering in long-day plants.
The temperature conditions of the Arctic may favor development in two
ways, thermoperiodically and photoperiodically. Summer day temperatures in
the North may reach values considerably above the average temperatures of
lower latitudes, and such a temperature periodicity will be favorable to
plants requiring comparatively high day and low night temperatures. Low night
temperatures, on the other hand, may act by improving the photoperiodic
conditions, since the photoperiodic response of long-day plants is favored
by low dark-phase temperatures.
Adaptive Significance
The possibility of transferring plants from lower to higher latitudes
(and altitudes) and vice versa, discussed in the preceding section, still
is a limited one. A plant which requires elevated temperatures in all its
stages of development will not grow in the Arctic, even though the day
temperatures in summer may be more favorable than at a lower latitude. This
failure is due to the fact that the temperature average through the entire
growing season (the “temperature sum”) will be too low. An Arctic or
subarctic plant may fine, farther south, a better average temperature, but
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
its low temperature and day-length requirements may remain unsatisfied.
Plants can grow and reproduce only under those environmental conditions to
which they are adapted.
Adaptation is a
genetical
phenomenon. Natural selection, acting on a
variety of hereditarytypes, chooses that one best suited for the particular
habitat. Frequently, if not generally, adaptation is adjusted to rather
specific conditions, and even within a comparatively limited area there
may be several types of a plant species, each adapted to “its own” environment.
Whether a plant is capable of inhabiting a smaller or greater area, or
areas with less or more diversified conditions, depends on how wide its
potential range of hereditary variation is. One may say, if a given plant
is to be able to succeed in a given area, it must be able to produce
hereditary types suited for the conditions of this area. This is evident
from the situation as encountered in nature. Species with wide or environ–
mentally diversified distribution areas usually comprise numerous different
strains, varieties, etc., which may differ only physiologically or both
morphologically and physiologically. Sometimes, a plant which appears
farily uniform in its natural habitat, if transferred to new conditions,
“breaks up” into numerous genetically different stains. This is easy to
visualize. Let us assume we are dealing with a long-day plant, different
strains of which have different critical day lengths, ranging between 14 and
16 hours. If this plant is grown at Montreal, where in summer day length
exceeds 16 hours, it will develop uniformly; if it is grown in Texas, where
the summer days never are longer than 15 hours, some individuals will flower,
others not. On the other hand, species with uniform and rigidly fixed
environmental requirements inhabit either limited areas, or
[:
]
at least
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
areas with uniform environmental conditions.
Vernalization, thermoperiodism, and photoperiodism have a strictly
adaptive character. For example, in northern region a plant without
specific cold and day-length requirements (provided its seeds have no
dormancy period) might germinate shortly after fruit bearing, some time
in summer, and its development might fall into the increasingly unfavorable
conditions of autumn. Both cold and long-day requirement will result in
delaying the development until the favorable season sets in. This, then,
is the second aspect of the significance of the phenomena discussed for
plant life. Vernalization, photoperiodism, and thermoperiodism, all three,
have an outstanding importance in the adaptation of plants to environment,
and thus are operative in determining and limiting the distribution of
plants. For such a role, these “specific” environmental requirements are,
actually, far better suited than the “general” ones of rather elevated
temperature and adequate light. These are operative permanently and by
the virtue
s
of this will have no specific regulative value.
Specific requirements, particularly those with the inductive type
of action, constitute, on the contrary, blocks in the course of development
which have to be released if the development is to proceed. This significance
of the specific environmental requirements is again evident from the geo–
graphical situation. Plants from higher latitudes have low night temperature
optima, and are frequently winter-annual or biennial and long-day
[:
]
plants.
Plants form low latitudes prefer high night temperatures, never possess low
temperature requirements, and are either short-day or day-neutral plants.
Within one single species, there may be a fine gradation. For example,
strains of
Bouteloua curtipendula
from southern Texas and Arizona proved
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
to be short-day types flowering on photoperiods of not over 13 hours,
whereas a strain from North Dakota was a long-day plant flowering only
on photoperiods of 14 hours or more.
This role of our phenomena in the adaptation of plants to environment
is, in the long run, much more deep seated that the effects on the life
of the individual plant. It may even appear that for most arctic plants
vernalization, thermo- and photoperiodism are no longer of direct importance,
because these plants are so well adapted to the conditions of their habitat
that their temperature and day-length requirements are always satisfied.
This, however, is only the consequence of the fact, that all plants which
were not capable of such adaptation have long been eliminated by selection.
While the general significance of vernalization, photo- and thermo–
periodism in adaptation is thus unquestionable, their individual shares are
sometimes hard to evaluate in a particular case. The three factors are
related by close interactions. Thus all, or nearly all, winter-annual and
biennial plants also have the long-day habit. They require, for normal
development, first, exposure to low temperature, second exposure to long days.
Long-day treatment without preceding cold treatment is without effect on
flowering as is cold treatment without subsequent long-day treatment
(both requirements may be qualitative or quantitative). It is, however,
impossible to tell by inspection which inductive factor or factors are
lacking in a nonflowering specimen of such a plant. Between photo- and
thermoperiodism there may be likewise an interrelation. The adverse effect
of excessively high night temperatures in tomatoes may be compensated for
to a certain extent by reducing the photoperiod. In many individual instances,
an experimental investigation is the sole way to establish the condition or
conditions operative.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
Significance for Arctic Agriculture and Horticulture
The question of application of our phenomena to practical problems can
be discussed in the same manner as, and with close reference to, that of
their bearing on wild plant life. There are certain direct applications,
i.e., the possibility of regulating the individual development of some
crops. In the early thirties of this century, great hopes were held for
vernalization in this respect. In the Soviet Union, in 1937, 10,000,000
acres were planted with winter cereals vernalized as soaked seed. The
reason underlying these attempts is that vernalized winter varieties in
yield and sometimes also in quality are superior to summer varieties. At
present, however, these attempts appear to have been entirely abandoned.
The reason for this probably is that large-scale cold treatment of seed is
rather difficult. Direct application of vernalization will be limited to
special cases, as in breeding work with winter varieties or seed production
of biennials. Photoperiodic treatment is used in floriculture and to some
extent may prove useful in horticulture, with crops as lettuce, which can
be prevented from premature flowering (“bolting”) by short-day treatment,
or strawberries in which short-day treatment extends flowering time.
Thermoperiodic measures are also used in floriculture, and as a matter of
fact were so used long before the significance of the phenomenon was
discovered. They are also useful for some horticultural crops,
By far the grea
^test^ significance is, however, as with wild plants,
or a genetical nature. As in wild plants, genetically fixed thermo- and
photoperiodic and low temperature requirements or operative in determining
adaptation to a given region. Before the significance of these requirements
was realized, it was sometimes unaccountable why certain crops, introduced
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
to certain areas, yielded only complete failure. After this realization
has been made, breeding can be attempted along adequate lines. One also
will have some indications where to look for suitable material. For
instance, for introduction to subarctic or arctic regions one will preferably
try varieties grown in mountain regions rather than varieties adapted,
through long periods of time, to the higher and more uniform temperature
conditions of plants. As cold and day-length requirements were discovered
but approximately 30 years ago, and thermoperiodicity even more recently,
it is understandable that at present the application of our knowledge to
plant breeding still has a chance and casual character. Nevertheless, in
several crops (cereals, sugar beet, onion, soybeans, forage plants) it has
already been marked with considerable success. Of the northern countries,
Russia and more recently also Finland have been doing pioneer work in this
field, particularly in their extreme northern parts, up to the 70
th
parallel.
EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism
BIBLIOGRAPHY
1. Fries, Th. C.E. Abisko Naturwetensk. Stat., Meddel.1, Stockholm,
pp.20, 1918.
2. Kivinen, E. Medd. Norske Myrselsk. Vol.46, p.1, 1948.
3. Kjellmann, F.R. Ur polarvexternas liv. Stockholm, 1884.
4. Lang, A. Chapter on “Developmental Physiology” in “Fortschritte der
Botanik,” vol.11, p.268, 1944, and vol.12, p.340, 1949.
Berlin-Heidelberg-Göttingen: Springer.
5. Lona, F. Nuovo Giorn. bot ital. vol.56, p.516, 1949.
6. Malyshev, A.A. Acad.Sci.USSR, Doklady, vol.59, p.771-791, 1948;
vol.60, p.153, 1948.
7. Melchers, G., and Lang, A. Die Physiologie der Blutenbildung.
Biolog.Zentralbl, vol.67, p.105, 1948.
8. Olmsted, Ch.E. Bot. Gaz.
106
, p.46 and 382, 1944/45.
9. Smirnov, L.A. Bot. Zhurn. (Russia) vol.33, p.559, 1948.
10. Went, F.W. Thermoperiodicity. In “Vernalization and Photoperiodism”
(A.E. Murneek and R.O. Whyte, edit.), Lotsya vol.1, p.145,
Waltham, Mass.: Chronica Botonica.
11. Whyte, R.O. Crop production and environment. London: Faber, 1948.
A. Lang