332 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



The inhibiting effect of carbohydrates can be explained either by the 

 blocking of enzymes or "active surfaces" or by the diversion of light 

 energy to photoxidative processes (c/. Chapter 19) — in short, either by 

 an inhibition of photosynthesis, or by an acceleration of reverse reactions. 



However, the view than an accumulation of carbohydrates invariably 

 leads to a decrease in photosynthesis has not remained unchallenged. 

 Boysen-Jensen and Mtiller (1928), Chesnokov and Bazyrina (1930) and 

 Kjar (1937) asserted that there is no relation whatsoever between the 

 concentration of carbohydrates in leaves and the rate of photosynthesis, 

 especially insofar as the "midday depression" is concerned; Toshchevi- 

 kova (1936) observed that an excess in carbohydrates may cause both 

 an increase and a decrease in the rate of photosynthesis; and Spoehr 

 and McGee (1923) asserted positively— in direct contradiction to Kur- 

 sanov's conclusions— that plants starved in the dark for several hours 

 are less efficient in photosynthesis than those fed on glucose during the 

 dark period. Spoehr thought that the presence of carbohydrates 

 accelerates respiration, and the latter in its turn stimulates photo- 

 synthesis. Van der Paauw (1932), who observed an increase in photo- 

 synthesis of Hormidium (to the extent of .5-12% in strong light and 

 50-100% in weak light) by the addition of 0.7-1% glucose to the nutrient 

 solution, agreed with the Spoehr-McGee explanation, and saw in this 

 effect an example of the "indirect regulation" of photosynthesis held so 

 important by Kostychev (c/. Vol. II, Chapter 26). Van Hille (1938) 

 grew Chlorella in glucose-free solutions, starved them in the dark for 

 12 hours and then subjected them to prolonged illumination. After 25 

 hours, the starch content and the rate of photosynthesis continued to 

 increase steadily. Whether the increase in photosynthesis was a conse- 

 quence of the carbohydrate accumulation is not clear, but there is certainly 

 no evidence in these experiments that the accumulation of carbohydrates 

 was harmful to photosynthesis. Gaffron (unpublished) found no differ- 

 ence between the photosynthesis of Chlorella in water and in 1% glucose 

 (except under anaerobic conditions in which the presence of glucose 

 accentuated the inhibition of photosynthesis after a period of dark 

 anaerobiosis. According to the observations of Gaffron, sugar does not 

 interfere with algal photosynthesis as long as it is utilized for growth; 

 but if growth is inhibited, photosynthesis too, soon becomes affected. 

 In this connection, the experiments of Brown and Escombe (1905) may 

 also be recalled. They found that detached leaves of Catalpa bignonioides 

 assimilate, in 4-5 hours, from 50-120%. more carbon dioxide than similar 

 leaves attached to the plants, despite the disrupted translocation of the 

 carbohydrates. (Brown and Escombe ascribed this unexpected result to 

 the wider opening of stomata in detached leaves.) 



