INFLUENCE OF DEHYDRATION ON PHOTOSYNTHESIS 333 



Obviously, several independent phenomena are involved in the effect 

 of carbohydrates on photosynthesis. It depends on the species, light 

 intensity, temperature, and other factors. The form in which the 

 carbohydrates are accumulated must be of importance too — plants 

 capable of "immobihzing" excess carbohydrates in the form of starch 

 can be expected to be less affected than plants which produce only 

 soluble sugars. 



Because of the strong influence which the addition of sugars has on 

 respiration and fermentation, their effect on photosynthesis is closely 

 related to the coupling between these processes (as mentioned above in 

 connection with the results of Spoehr, van der Paauw and Gaffron). 

 We shall return to this problem in chapter 20, when dealing with the 

 relation between photosynthesis and respiration. 



D. Influence of Dehydration on Photosynthesis * 



Although water is a reactant in photosynthesis, the effect of water 

 content on the rate of photosynthesis cannot be treated as a "concentra- 

 tion effect" according to the law of mass action, since water is present 

 abundantly under all circumstances; the considerable influence exercised 

 by the removal of even a small part of it must be caused by changes in 

 the structure of the protoplasm (shrinking or swelling) rather than by 

 direct effects on the kinetic mechanism of photosynthesis. 



The water content of plant tissues can be changed either by drying 

 or by immersion into solutions of high osmotic pressure. Observations 

 on the effect of dehydration on photosynthesis were made, for example, 

 by Iljin (1923), Dastur (1924, 1925), Brilliant (1924, 1925), Meyer and 

 Plantefol (1926), Walter (1928, 1929), Wood (1929), Dastur and Desai 

 (1933), Alexeev (1935), Danilov (1935, 1936, 1937, 1940), Chrelashvili 

 (1940), and Brilliant and Chrelashvili (1941). As a rule, dehydration 

 decreases the rate of photosynthesis. However, Brilliant (1924) has 

 found that the photosynthesis of Hedera helix and Impatiens parviflora 

 rises to a maximum at a water deficiency of 5 to 15% (and drops to 

 almost zero at a deficiency of 41 to 63%). Similarly, Alexeev (1935) 

 found that the photosynthesis of apple leaves has a maximum at 28% 

 water deficiency; and analogous results were obtained by Chrelashvili 



(1940) with Allium, Primula, and Zea mais. Brilliant and Chrelashvili 



(1941) found that the stimulating effect of moderate dehydration can be 

 observed only at high light intensities, while the inhibiting effect of 

 strong dehydration is apparent in weak, as well as in intense, light. 



Dastur (1924, 1925) suggested that a decreasing water supply is responsible for the 

 loss in photosynthetic efficiency of aging leaves (an effect first described by Willstatter 

 and Stoll). He found that the maximum rate of assimilation of different leaves of one 



* Bibliography, page 348. 



