Chapter VII — 101 — Osmotic Quantities of Cells 



1928; Marsh, 1941). Fruit tree leaves exhibit a higher OP than the im- 

 mature fruits (Chandler, 1914). 



With respect to average osmotic pressures of entire plants, individuals 

 of unlike species growing under the same environmental conditions may 

 display significant differences in sap concentration (Harris, 1934). 



In general, a decrease in osmotic pressure is expected in the sequence 

 trees, shrubs, herbs (Harris and Lawrence, 1916). Winter annuals 

 show a decrease over perennial herbs. Succulents and hydrophytes are 

 characterized by rather low, xerophytes by high sap concentrations ; meso- 

 phytes He between (Maximov, 1929a, p. 274). 



Many studies have dealt with the effect of environmental factors on 

 osmotic pressure. Any change or condition leading to the development of 

 a water deficit in the plant is generally attended by an increase in sap con- 

 centration. The OP tends to vary directly with the concentration of the 

 soil solution, and is increased by those factors which cause the plant to 

 transpire at a greater rate. 



The cell sap concentration of leaves generally varies from a maximum 

 in the early afternoon to a minimum in the early morning hours ( Stoddart, 

 1935; Marsh, 1940). The rise during the daytime is due both to photo- 

 synthetic activity and to increasing water deficit within the leaves. Such 

 fluctuations are less pronounced in roots than in aerial organs. 



In addition to diurnal variation, plants show annual or seasonal fluctua- 

 tions in OP. In some — certain conifers for example — the sap concentra- 

 tion may be higher during the winter months (Gail, 1926; Meyer, 1928). 

 In others, a maximum is reached in the summer (Mallery, 1934). Ac- 

 cording to Marsh (1940), the osmotic pressure of lowland prairie species 

 which he examined increased during the growing season about one half over 

 that at early summer; for upland species the concentration doubled or 

 tripled. 



The variation in osmotic pressure as a result of extended water deficit 

 may be very great. Gasser (1942) used the limiting plasmolytic method 

 to determine the direction and extent of changes in Og of detached plant 

 organs (mainly leaves) placed under conditions which permitted slow 

 water loss. In an extreme instance, cells of the terrestrial alga Trentepohlia 

 aurea increased in Og from 123.4 atm. to 291.4 atm. in ZZ days. Out of 

 95 plant species, 69 indicated a rise of 15 to 223 per cent of the initial 

 value. The rest either showed no change or a slight decrease. Where a 

 condition of water excess was induced by placing the leaves in moist air, 

 72 out of 87 plants exhibited a decrease in Og, within a range of 5.3 to 

 66 per cent of the initial value. 



The changes in Og (the author uses Szg) are ascribed to processes em- 

 braced by the term osmotic regulation, wherein the solute concentration 

 varies due to hydrolysis or synthesis of polysaccharides, and by exosmosis 

 or endosmosis of solutes. The responses shown can possibly be attributed 

 in some degree to plastic stretching or shrinking of the walls which were 

 not conclusively shown to be inoperative. However, it is not unlikely that 

 other factors, as suggested by the author, are primarily responsible for the 

 changes observed. 



Diffusion Pressure Deficit. Methods of Measuring DPD : — The 



diffusion pressure deficit of the water in plant cells is a measure of the abil- 

 ity of those cells to absorb water. Its value may be measured directly, or it 

 may be obtained by difference if the osmotic and turgor pressures of the 

 cells are known. When TP = zero, as at limiting plasmolysis, OP = DPD, 



