Grafts et al. 



202 — 



Water in Plants 



varying osmotic concentration may affect the process through the dark or 

 enzymatic reaction rather than through the Hght reaction (Greenfield, 

 1941, 1942). Desiccation had Httle effect in subdued Hght but profoundly 

 affected photosynthesis in strong light where the dark reaction is limiting. 

 Figure 55 from Greenfield (1942) shows the effect of such physiological 

 desiccation on photosynthesis. 



Plasmolysis inhibits the production of chlorophyll, and soil moisture 

 content may affect pigment production (Furlinger, 1938; Beck, 1942). 



Slight dehydration may cause an increase in photosynthesis. Maximum 

 photosynthesis was found by Chrelashvili (1941) to take place in all 

 plants with slight dehydration but this action varied in different plants and 

 with different conditions. In Allimn, for instance, the photosynthetic ac- 

 tivity increases up to a certain sugar concentration ; in Primula no such 

 upper limit was observed under the experimental conditions. In Zea an 

 increase in carbohydrates caused a depression in photosynthesis and an 

 increase in respiration (Brilliant, 1924; Alexeev, 1935; Chrelash- 

 vili, 1941 ) . Here, again, the effect appears to be found only in relatively 

 high light intensities (Brilliant and Chrelashvili, 1941). With con- 

 siderable dehydration, photosynthetic activity decreases and finally ceases 

 in both strong and weak light (Dastur, 1924, 1925; Vassiliev, 1927; 

 Tumanov, 1929; Skvortzov, 1931 ; Dastur and Desai, 1933; Heinicke 

 and Childers, 1935 ; Brilliant and Chrelashvili, 1941 ; Loustalot, 

 1945). 



An excellent treatment of the carbohydrate changes which occur in 

 wheat during wilting can be found in the work of Vassiliev (1931) and 

 Vassiliev and Vassiliev (1936). The dynamics of sugar transformations 

 depend to a high degree on the water supply; hydrolysis predominates 

 during water deficit, condensation when the water supply improves. When 

 a wheat plant undergoes wilting, four stages can be noted : i ) a decrease in 

 the amount of monosaccharides and sucrose due to reduced photosynthesis 

 before wilting is evident; 2) on wilting of the lower leaves there is an in- 

 tense hydrolysis of insoluble carbohydrates and an accumulation of sucrose. 

 This process gradually spreads to the upper leaves, stem, and fruit; 3) be- 

 ginning again at the lower leaves, sucrose decreases and monosaccharides 

 accumulate; 4) finally, with the disappearance of sucrose and monosac- 

 charides, the plant is well on its way to complete desiccation. The lower 

 leaves are the first to lose sugars. Here the role of sugars in drought seems 

 to be the same as in freezing ; in both cases sugars protect the plants against 

 adverse desiccation. A similar trend in carbohydrates was observed in 

 lucerne by Henrici (1943, 1945) ; starch was the dominant reserve carbo- 

 hydrate instead of hemicellulose, as found in wheat. 



Investigations on the acceleration of starch dissolution due to a decrease 



Table 48. — The effect of decrease in water content on the amylolytic activity and 



starch dissolution of sunflozver leaves kept in the dark for 18 hours 



(data of Spoehr and Milner, 1939) : — 



