EFFECT OF CARBOHYDRATES ON PHOTOSYNTHESIS 331 



is followed by a sharp decline. Livingston and Franck (1940) found 

 that detached leaves of Hydrangea otaksa are strongly, but not com- 

 pletely, inhibited by 20% carbon dioxide; young leaves and leaves rich 

 in sugar are more tolerant than old or starved leaves. Leaves can be 

 "conditioned" to high carbon dioxide concentrations by increasing the 

 concentration in small increments; "conditioned" leaves retain one-half 

 their maximum capacity for photosynthesis in 20% carbon dioxide and 

 one-sixth in 50% carbon dioxide, but are less eflQcient than before at the 

 lower concentrations. 



One clue to the behavior of plants in excess carbon dioxide is furnished 

 by the observation of Chapman, Cook, and Thompson (1924) that a 

 high pressure of carbon dioxide induces closure of the stomata. It is 

 therefore advisable to make quantitative experiments on carbon dioxide 

 inhibition with stomata-free mosses and algae. The observation of 

 Spoehr (1939) that a high concentration (> 10%) of carbon dioxide 

 stops the dissolution of starch in the chloroplasts may explain the closure 

 of the stomata (cf. page 47). 



According to Ballard (1941) the effect of excess carbon dioxide on 

 photosynthesis is strongly dependent on temperature. In Ligustrum, 

 the depression can be observed at 6" C, beginning with 2-2.5% carbon 

 dioxide, while at 16" no effect is noticeable even at 5%. The effect is 

 stronger in intense light than in weak light, showing that excess carbon 

 dioxide obstructs an enzymatic reaction and does not merely displace 

 intermediates from the association with chlorophyll (as one could suggest 

 on the basis of scheme 7.VA). 



C. Effect of Carbohydrates on Photosynthesis * 



Accumulated assimilation products have long been assumed to 

 exercise an important influence on photosynthesis. As early as 1868, 

 Boussingault suggested that this effect was responsible for the decline of 

 photosynthesis in detached leaves. Sapozhnikov (1893) found that the 

 photosynthesis of detached leaves of Vitis vinifera stops when the 

 carbohydrate concentration had risen to 23-29% of the dry weight. 

 Warburg (1919), Henrici (1921), Kursanov (1933), von Guttenberg and 

 Buhr (1935), and Monch (1937) were of the opinion that the accumulation 

 of carbohydrates is at least partially responsible for the decline in photo- 

 synthesis after prolonged illumination, as well as for the so-called "mid- 

 day depression" (Vol. II, Chapter 26). Kursanov (1933) made labora- 

 tory experiments with leaves and algae, some of which were starved while 

 others were fed on 1% glucose solution in the dark. Upon illumination, 

 starved leaves showed a higher rate of photosynthesis and a less pro- 

 nounced midday minimum than the leaves supplied with sugar. 



* Bibliography, page 348. 



