Photosynthesis

Photosynthesis and transpiration are inseparably related functions of green plants.  Photosynthesis requires carbon dioxide and water as raw materials and takes place in chloroplasts which are usually inside the cells of leaves.  There is ordinarily a sufficient supply of carbon dioxide in the atmosphere, but in order for it to reach the chloroplasts it must first enter the leaf and then go into solution on the wet surface of the cell (Raven et. al. 1981).

There are two pathways by which gases may enter or exit a leaf: by diffusion through the epidermis or through small openings in the epidermis called stomata.  Terrestrial plant leaves have a layer of a wax-like substance called cutin which covers the epidermal layers.  The cuticle reduces diffusion through the epidermis to the extent that 90-95% of the gas exchange in leaves occurs through the stomata.

One of the most threatening problems faced by land plants is desiccation.  Several special adaptations have evolved that reduce water loss by transpiration, but each of these also reduces the supply of carbon dioxide.  Thus when the stomata are closed, water retention is high and the rate of photosynthesis is low; when the stomata are open, water retention is low and the rate of photosynthesis is high.  This relationship sometimes has a profound influence on plant survival in winter.

An assumption frequently made with regard to evergreen species is that they carry on photosynthesis throughout the year.  While this is probably true for tropical and subtropical plants, it is much less certain for temperate zone species.  It has been observed that many temperate zone coniferous species have distinct periods of winter dormancy characterized by a depression in photosynthesis (Tranquillini 1964).  Species differ with regard to the depth of the dormant condition and how far into the winter season photosynthesis occurs.  In a study of bristlecone pine (Pinus aristata) in the White Mountains of California, photosynthesis continued at a rate in excess of carbohydrate use in November.  However, for most coniferous species the net photosynthesis is zero in midwinter, then gradually increases from early spring until the summer rates are reached (Schulze et. al. 1967).

Although less information is available on temperate evergreen herbaceous plants, it is probable that during winter months they show the same type of photosynthesis depression as the coniferous species.  In a study of the herbaceous perennial Claytoni lanceolata, carbohydrates were measured at monthly intervals during winter.  As the season progressed, the levels of sugar and starch decreased steadily as might be expected if metabolism is supported by stored carbohydrates rather than active photosynthesis (Salisbury et. al. 1973).

Evergreenness among temperate zone plants, therefore, does not necessarily mean that net photosynthesis occurs throughout the year.  However, it probably does result in a longer period of net photosynthesis for evergreen species.  Another advantage of evergreenness, that may be more important in some areas than year-round photosynthesis, is conservation of mineral nutrients.  It has been demonstrated that the leaves of some evergreen species manufacture more photosynthate per unit of nitrogen and phosphorous than do the leaves of deciduous species, chiefly as the result of their longevity (Monk 1966, Small 1972).

Temperature and Water Stress

Plant species vary greatly with regard to their ability to withstand low temperatures.  When injury or death from chilling does occur, it is usually the result of (1) precipitation of protoplasmic proteins, (2) formation of intercellular ice and subsequent protoplast dehydration, or (3) rapid freezing with the formation of intracellular ice crystals.  Some winter hardy species can withstand temperatures as low as -62^oC without injury.  There is no evidence that the altitudinal or latitudinal plant limit is determined by low winter temperatures (Daubenmire 1974). 

Most terrestrial plants are periodically subjected to water stress throughout the year.  Water stress occurs when water loss by transpiration exceeds uptake by roots.  A common manifestation of this in summer is wilting of broad leaved species.  During winter, decreasing soil temperatures result in a lowered rate of water uptake.  It has been hypothesized that this is a consequence of reduced root cell membrane permeability and increased water viscosity (Goldstein et. al. 1985).  Thus, water stress in winter is most acute when the air temperature is higher than that of the soil.

Influence of Wind

The effects of wind and temperature on plant water relations are very complex.  It has long been known that during summer, plants exposed to winds of moderate velocity transpire more than plants in still air (Clements 1938, Oosting 1956).  In still air, a layer or "shell" of water vapor is formed around the transpiring leaf which reduces the vapor pressure gradient between leaf and air.  When there is a wind, this layer is swept away and a greater vapor pressure gradient is maintained.  However, under some circumstances the drying effect of even a low to moderate wind may cause leaf cells to lose turgor and the stomates to close, thus reducing transpiration (Bannister 1978, Daubenmire 1974).  When solar radiation is high the leaf may become warmer than the surrounding air.  Increasing wind speed in this instance may reduce transpiration by cooling the leaf and decreasing the upper pressure gradient (Mansfield and Davies, 1985).

The desiccating effects of winter wind has been offered as an explanation for such phenomena as parch blight, a type of winter injury to evergreens, the deformed vegetation called krummholz found at high altitudes, and the alpine tree line.  However, several studies have suggested that the effect of wind in these instances may have been overestimated.  There is a significant difference in summer and winter with regard to the way temperate zone plants lose water.  During winter when plants are in a state of dormancy, stomata are closed and water loss is mainly by cuticular transpiration.  One investigator (Marchand 1978) has presented evidence supporting the view that under these circumstances the net effect of wind would be to reduce rather than increase water loss.  Another investigator (Goldstein et. al. 1985) found no evidence of damage from winter desiccation to the leaves of white spruce (Picea glauca) growing at the tree line in the Brooks Range of Alaska.  On the basis of these studies it has been suggested that factors other than winter desiccation may be responsible for the position of tree line.

Water Content of Trees

Water from the soil enters the vascular system of most seed plants in two ways:  (1) by diffusion through the protoplasts of living cells from the root hairs to the xylem cells and (2) by movement through the micropores of the cell walls.  The principal pathway for water is through the cell walls while mineral nutrients follow the protoplasmic pathway.  Water moving through the micropores of the cell walls must cross the plasma membrane and protoplasts of a layer of cells called the endodermis before it reaches the xylem cells.

In trees the water moves upward in the xylem tissue.  When absorption is maximal, the vessels and/or tracheids of the xylem should be filled with water.  However, as a result of transpiration the water content of the stem tissue may vary.  A technique that has been used to study the water content of trees is to determine the relative water content (RWC) of twigs (Slatyer, 1967).  (See Water Content of Winter Twigs Activity)

In a study of several species during winter at the tree line on Mt. Washington, New Hampshire, it was found that RWC varied from 70-92% (Marchand and Chabot, 1978).  These fluctuations did not appear to be related to changes in air temperature, wind speed, or radiant energy.  Although the correlation was low, the greatest amount of variation in RWC was associated with the average maximum temperature for the two days prior to sampling.  Rapid midwinter increases in RWC are particularly difficult to explain since absorption by roots is usually lowest at this season.  It has been hypothesized that these increases might be explained by a redistribution of water within the plant.  One investigator has suggested that conifers in particular have a large amount of stored water (Marchand and Chabot, 1978).

Freezing Resistance in Trees

Many hardy tree species of the temperate zone are able to survive at extremely low temperatures.  A characteristic that makes this possible is the presence of cell sap that is capable of being supercooled without the formation of ice.  The cells of these species can often be supercooled to -40^oC and their geographic range usually does not include areas where winter temperatures drop below this figure (Salisbury and Ross, 1978).  The water in intercellular spaces and in the non-living cells of the xylem may freeze at temperatures that are considerably higher.

The relative freezing temperatures of the xylem water and the cell sap can be determined in the laboratory by using ethanol, dry ice, and a digital thermocouple inserted into the vascular cambium of twig (See Freezing Resistance in Plant Tissues Activity).  The xylem water often freezes when the ambient air temperature is -5^o to -10^oC, and the freezing will be reflected as a conspicuous exotherm.  Under field conditions a dendrometer attached to a tree would record this event as a pronounced expansion (See Tree Trunks and Telephone Poles Activity).

Although the freezing of xylem water may be complete, it is not lethal to the plant.  It has been demonstrated that this freezing does not reduce the movement of water through the stem.  There is evidence to support the suggestion that water continues to flow through the micropores of the cell walls (Havis, 1971).

If cooling of an experimental sample continues, a second exotherm at a much lower temperature is observed.  It is presumed this marks the freezing point of cell sap and micropore water.  Since ice formation inside the protoplast is almost always lethal, the second exotherm is the temperature at which tissue death occurs.

In the very hardy trees of the boreal forests (tiaga) of North American and Asia, the cell sap does not supercool.  Instead, extra-cellular ice crystals form, drawing water from the protoplasts until only the water of hydration or bound water remains.  Winter hardiness of these species appears to be their ability to withstand severe protoplast dehydration.  In a state of winter dormancy, these plants can survive temperatures as low as -196^oC, the temperature of liquid nitrogen (Salisbury and Ross, 1978).

An interesting application of the scientific method is possible by combining the activities "Tree Trunks and Telephone Poles" and "Water Content of Winter Twigs".  One can hypothesize that RWC of twigs is an approximation of the water content of the tree trunks on which they occur.  It is then possible to predict that the trunk of a tree with a high RWC contains more water than an equal sized trunk with a low RWC.  If on freezing the dendrometer expands more on the trunk with the high RWC, the hypothesis is supported.

Literature Cited

Bannister, P.  1976.  Introduction to Physiological Plant Ecology.  Blackwell Scientific Publications, London.

Daubenmire, R. F.  1974.  Plants and Environment, a Textbook of Plant Autecology (Third Edition).  John Wiley and Sons, New York.

Goldstein, G. H., L. B. Brubaker, and T. M. Hinckley.  1985.  Water relations of white spruce (Picea glauca) at tree line in north central Alaska.  Can. J. For. Res. 15: 1080-1087.

Havis, J. R.  1971.  Water movement in woody stems during freezing.  Cryobiology 8: 581-584.

Mansfield, T. A. and W. J. Davies.  1985.  Mechanisms for leaf control of Gas Exchange.  Bioscience 35: 158-164.

Marchand, P. J. and B. F. Chabot.  1978.  Winter water relations of tree-line plant species on Mt. Washington, New Hampshire.  Arctic and Alpine Research 10: 105-116.

Marchand, P. J.  1972.  Winter-exposed plants.  Rhodora 74:  528-531.

Monk, C. D.  1966.  An ecological significance of evergreenness.  Ecology 47: 504-505.

Oosting, H. J.  1956.  The Study of Plant Communities:  An Introduction to Plant Ecology.  W. H. Freeman and Co., San Francisco, CA.

Raven, P. H. and R. F. Evert.  1981.  Biology of Plants (Third Edition) Worth Publishers, Inc. New York, N. Y.

Salisbury, F. B. and C. W. Ross.  1978.  Plant Physiology. 3rd Ed.  Wadsworth Publishing Company, Belmont, CA.

Salisbury, F. B., S. L. Kimball, B. Bennett, P. Rosen, and M.  Weidner.  1973.  Active plant growth at freezing temperatures.  Space Life Science 4: 124-138.

Schulze, E. D., H. A. Mooney, and E. L. Dunn.  1967.  Wintertime photosynthesis of bristlecone pine (Pinus aristata) in the white mountains of California.  Ecology 48: 1044-1047.

Small, E.  1972.  Ecological significance of four critical elements in plants of raised sphagnum peat bogs.  Ecology 53: 498-503.

Tranquillini, W.  1964.  The physiology of plants at high altitude.  Ann. Rev. Plant Physiology 15: 345-362.

Weaver, J. E. and F. E. Clements.  1938.  Plant Ecology.  McGraw-Hill Book Co., Inc. New York, N.Y.