566 



PHOTOSYNTHESIS AND RESPIRATION 



CHAP. 20 



starvation (as in the experiments of van der Paauw 1932, and Franck 

 and French 1941), or by excess oxygen (as in the work of McAhster and 

 Myers 1940), or by excessive illumination (Myers and Burr 1940), the 



return to darkness always finds the 

 oxygen consumption enhanced (c/. 

 Fig. 62). It is improbable that this 

 increased absorption of oxygen is due 

 to an increase in normal respiration; 

 probably, photoxidation in the chloro- 

 plasts leaves a residue of "semioxi- 

 dized" intermediate products whose 

 oxidation can be achieved in the dark 

 without the participation of the respira- 

 tory enzymes. According to Franck 

 and French (1941) the "aftereffect" 

 of photoxidation of C02-starved leaves 

 is small, unless the leaves have been 

 supplied with glucose during the ex- 

 posure (c/. Table 20.11). 



40 60 



Time, minutes 



100 



3. Effect of Light on Respiration 



In chapter 19, we suggested that 

 the phenomena of photautoxidation 

 in starved or narcotized leaves in 

 the presence of excess oxygen or in 

 very intense light are brought about 

 by a cooperation of the primary 

 photochemical apparatus of photosynthesis with the action of heat- 

 resistant catalysts (cf. Gaffron 1939, 1940, and Franck and French 1941) 

 and has nothing in common with the enzymatic mechanism of ordinary 



Table 20.11 

 Aftereffects of Photoxidation (after Franck and French) 



Fig. 62. — Increased oxygen con- 

 sumption in the dark after a period of 

 photautoxidation (after Myers and 

 Burr 1940). The effect is shown by 

 the curvature in A and B. 



" After a first exposure for 25 minutes. 



'After two exposures of 25 minutes, each with 2 hours of darkness between them; final high rate 

 maintained for 90 minutes. 



' After the third exposure; rate maintained for 30 minutes. 



