Crafts et al. 



198 — 



Water in Plants 



the xylem and f fictional resistance to rapid flow plays a major role. These 

 factors continue to be dominant until the stomata are at least half closed ; 

 from then on, stomatal opening apparently determines water loss quite ac- 

 curately. 



Effect of Mineral Nutrients on Transpiration: — The presence of 

 osmotically active solutes reduces the diffusion pressure of water; other 

 things being equal, evaporation from a solution is a function of solute con- 

 centration. Some effects of the presence of salt in the soil solution are: 

 osmotic influences upon water uptake ; increased equilibrium concentration 

 within plant cells; and secondary effects, specific for the particular salt, 

 upon permeability, guard cell movement, leaf abscission, etc. 



The immediate effect of adding a salt to a soil of low salt content is to 

 increase the diffusion pressure deficit of soil water, reducing the gradient 

 between the soil solution and the root cells (Magistad, 1945). This re- 

 duces water absorption, less water is available to the leaves, and transpira- 

 tion is lowered. If, however, the added salt concentration is low and the 

 soil used is of low salt content, certain ions may act specifically causing an 

 increase or decrease in transpiration depending upon the nature of the ion 

 and not on its colligative properties (Harter, 1908; Reed, 1910; Bouyou- 

 cous, 1911). Similar results were obtained with the use of single salt 

 solutions (Hansteen-Cranner, 1914; Kisser, 1927). 



In short time studies on the addition of salt to low salt soils, Meyer 

 (1931) found a reduction in transpiration for all concentrations used. The 

 specific effects of low salt concentrations were not noted; apparently the 

 slight increases in the transpiration of plants in solutions of very low con- 

 centration were obscured by the concentration of salts naturally present in 

 the soil employed. The differences in retarding effect between different 

 salts could in this case be explained on the basis of their colligative proper- 

 ties. 



Secondary effects of salts were observed by Iljin (1922a), who found 

 sodium and potassium to stimulate opening of guard cells ; calcium checked 

 the opening. Sodium and calcium in high concentrations induced leaf ab- 

 scission. The water requirement decreased rapidly with increase in salt 

 concentration (Meyer, 1931), while CaCl2 caused, first, an increase, and 

 then a decrease. There was an increased succulence of tops where NaCl 

 and CaQ2 were added to the soil. 



Influence of Disease, Sprays, Dusts, and Waxes on Transpiration 



Rate: — Numerous workers have recently concerned themselves with the 



Table 47. — Water requirement in Zea as affected by nutrition 

 (data of Desai, 1937) : — 



