TRANSPIRATION AS INFLUENCED BY OSMOTIC CONCENTRATION AND CELL PERMEABILITY

IT is rather widely recognized that many plants which are resistant to drying conditions have a sap with a high osmotic concentration or a high colloid content or both. It is also recognized that a high content of solutes will reduce the vapor pressure of water, and that hydrophilic colloids have a high water holding capacity, and even in the ordinary "air dry" condition retain a high water content. When one realizes, however, that the vapor pressure of a 1 M solution is only about 1.8 per cent less than that of pure water and that the vapor pressure lowerings of the most concentrated leaf sap ever reported would not equal 10 per cent, it is obvious that this alone could not account for the resistance of these plants to water loss. Furthermore, although hydrophilic colloids when approaching the air dry condition have a very low vapor pressure, in the almost completely hydrated condition in which they occur in living cells, their vapor pressures are not greatly reduced but are more commonly within two to five per cent of that of pure water. Although the presence of solutes at high osmotic concentration or of a hydrophilic colloid lowers the vapor pressure of water to only a slight degree, Dr. 0. F. Curtis has observed that leafy shoots, if cut following a period of prolonged cloudy weather, will wither rather quickly even in a very humid atmosphere; whereas similar shoots, if cut following periods of clear weather, will wilt much less quickly even in a situation favoring more rapid transpiration. He has also found that the injection of a 10 per cent solution of sucrose into cut shoots enables the shoots to remain alive and fairly turgid in a greenhouse for several days when standing in a dry tumbler, while the leaves of similar shoots not so treated become dry and crisp in a day. These observations suggest that high concentrations of solutes or of hydrophilic colloids have an effect on water loss from living tissues much greater than can be calculated from the effects of these substances on the vapor pressure of water. He, therefore, suggested that I investigate this problem more thoroughly, to determine the degree to which differences in osmotic concentration alter transpiration and, if possible, why differences in concentration alter transpiration. There is very little literature on this problem. Aubert (1892), working on members of Crassulaceae, Mesembrianthemeae and Cactaceae, found a correlation between high concentrations of organic acids, e.g., malic acid and tartaric acid, and low rates of transpiration. His data gave transpiration rates in terms of loss of weight per gram fresh weight per hour. 1 Received for publication November, 2 1940. Acknowledgment: The author is( deeply grateful to Dr. 0. F. Curtis for suggesting the problems, for advice during the progress of the investigation and in the preparation of the manuscript. Fritsch and Haines (1923) in their studies of the moisture relations of terrestrial algae, stated, "It is plain that a high osmotic concentration of the sap, if it plays any role at all, is not a fundamental part of the mechanism (of water retention and drouth resistance). The gel-condition of the protoplast, with absence of all large vacuoles, indicates the possibility of imbibition phenomena being principally concerned." Shreve (1931) found that the evaporation from sucrose solutions across a parchment membrane in contact with the solution is much less than the evaporation across the membrane when in contact with water. If the rate of water loss across the membrane from the solution be designated as T, and that from water as E, then the changes in the rates T/E resemble very closely those changes in the rate T/E for desert perennials. It is a well known fact that a non-volatile solute lowers the vapor pressure of a solvent in a solution, but the lowering of vapor pressure by the presence of a solute is not very great, being only about 1.8 per cent for a solute at one molal concentration. It seems possible, however, that the retarding effect of a solute upon evaporation might be accentuated by the presence of a membrane as was indicated by the findings of Shreve ( 1 93 1 ). In order to get more complete data, therefore, and for purposes of comparison, evaporation rates were measured from open surfaces of water and sugar solutions, from similar solutions separated from the air by collodion membranes, as well as from living tissues made up of cells with varying osmotic concentrations. Thirty-two Petri dishes, sixteen containing 1 M glucose solution and sixteen distilled water, were so placed on a table that the dishes containing water alternated with those containing sugar. These were left on the table in the experimental room for two hours before weighing started. The loss of weight of these Petri dishes over a period of two hours was then measured. An electric fan was placed in front of a partly open door so as to blow some water vapor out of the room and circulate air over the dishes. Each Petri dish was then measured by means of inside calipers to determine its diameter from which the area was computed. Thus, the loss of weight by evaporation per square decimeter per hour could be obtained. The data are presented in table 1.