Chapter X 



— 201 — 



Loss and Retention 



content of reducing sugars was found under these conditions also to be 

 high (GoLOViNA, 1939). Loss of 30 to 40% of the water from leaves de- 

 creases the synthetic and increases the hydrolytic power of phosphatase. 

 Monophosphatase and diphosphatase do not, however, respond identically 

 to the same water deficit (Sisakian and Kobiakova, 1940). 



The net rate of protein formation from amino acids decreases with re- 

 duced water content or in other words the net rate of hydrolyses increases 

 (Petrie and Wood, 1938). While the synthetic activity of invertase and 

 protease is particularly low in drought-susceptible varieties (Sisakian, 

 1939, 1940; Golovina, 1939; Kursanov, 1941). The periodicity in the 

 rhythm of invertase and protease activity is lost in non-drought-resistant 

 wheat on wilting (Sisakian and Kobiakova, 1941). Generally, then, the 

 hydrolytic processes in the cell are accelerated and the synthetic process de- 

 creased during drought. This usually results in injury. Sisakian believes 

 that such disturbances in the enzymic equilibrium are the principal causes 

 of the death of plants from drought. 



Co/7centrono/7 



Fig. 55. — The effect of DPD of CUmella cells upon photo- 

 synthesis. The cells were immersed in sucrose solutions of 

 the indicated molal concentrations. Photosynthetic activity 

 was greatly reduced as the concentration increased above 0.4 

 mol. Data of Greenfield (1942). 



The effect of desiccation on photosynthesis cannot be attributed directly 

 to the fact that water is a reactant in the process but must be due to the 

 physico-chemical changes which occur in protoplasm, the altered action 

 of enzymes, and the stomatal response which takes place with changing 

 water supply. Protoplasmic dehydration may be brought about either di- 

 rectly by drying or indirectly by the use of hypertonic solutions. The re- 

 sults of these two types of experiments are not always the same (Chre- 

 LASHVILI, 1941). 



Investigations using hypertonic solutions have shown that photosynthe- 

 sis is very sensitive to changes in the colloidal state of protoplasm ; perhaps 

 more so than respiration (Walter, 1928, 1929; Rabinowitch, 1945). 



Studies on the relation of water supply to photosynthetic activity show 

 that desiccation brought about by growing Chlorella cells in solutions of 



