334 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



and the same species is proportional to their water content. Dastur and Desai (1933) 

 observed that the relative photosynthetic efficiency of the species, Abutilon asiaticum, 

 Ricinus communis, and Helianthus annuus, is proportional to the average water content 

 of their leaves. 



One mechanism by which water deficiency affects photosynthesis 

 concerns the stomata. Kreusler had shown, in 1885, that the stomata 

 close in dry air. This indicates their primary function — regulation of 

 transpiration. The enormous leaf surface, developed as a device to 

 catch effectively both light and carbon dioxide, represents a danger as 

 an overefficient evaporation system. The stomata keep the evaporation 

 within reasonable limits, allowing at the same time for the gas exchange 

 required for photosynthesis and respiration. The stomata close during 

 the night, when no photosynthesis takes place, and are usually open 

 during the day. However they also close in daytime when evaporation 

 becomes too rapid {cf. Iljin 1923; StMfelt 1927, 1929, 1932; and Scarth 

 1932). The closure of stomata, induced by water deficiency, was 

 suggested by Iljin (1923) and Geiger (1927) as a possible cause of the 

 "midday depression." 



The operation of the stomata is based on equilibrium between starch 

 and sugars in the "guard cells." In these cells, water deficiency leads to 

 increased starch production (cf., however, Spoehr and Milner 1940); the 

 drop in sugar concentration causes a decrease of the turgor and a closure of 

 the slits. This effect is reversible, unless desiccation has injured perma- 

 nently the enzymatic mechanism of polymerization and depolymerization 

 of starch. If such an injury has occurred, the leaves may recover their 

 healthy aspect, but the response of the guard cells to changes in humidity 

 is lost and many stomata remain permanently closed. 



Stomata-free land plants (mosses, ferns, lichens) are much less sensitive 

 to desiccation than are higher plants. According to Spoehr (1926), 

 lichens and mosses can be dried to a powder and remain capable of 

 resuming photosynthesis upon moistening. However, closure of the 

 stomata is not the only way in which dehydration affects photosynthesis. 

 This is proved by the effects of water deficiency on the photosynthesis of 

 aquatic plants. Water can be extracted from these plants by increasing 

 the osmotic pressure of the medium. To avoid complications, the 

 solutes used for the adjustment of the osmotic pressure must not exercise 

 specific inhibiting or stimulating effects on photosynthesis. Treboux 

 (1903) found that the photosynthesis of algae decreases in an 0.1% 

 solution of potassium nitrate, as well as in similarly concentrated solutions 

 of other electrolytes, sucrose, and glycerol. Since the effects of isotonic 

 solutions were similar, Treboux interpreted them as purely osmotic in 

 origin. The inhibition was reversible, up to the point at which plasmol- 

 ysis set in. Walter (1928, 1929) conducted experiments on Elodea in 



