INFLUENCE OF OXYGEN ON PHOTOSYNTHESIS 



329 



hand, found no difference between the rates in pure nitrogen and in air 

 (20% O2) but a considerable difference between the rates in air and pure 

 oxygen (Fig. 33). 



In attempting to explain the oxygen inhibition, one naturally thinks 

 of the fact that high oxygen pressure promotes photoxidation (c/. Chap- 

 ter 19), which could consume a part of the oxygen liberated by photo- 

 synthesis. However, it will be shown in chapter 19 that oxygen produc- 



50 100 ISO »I0* 



Intensity, ergs/cm./^ec. 



Fig. 33.— Inhibition of photosynthesis of Chlorella by excess oxygen (after Wassink, 

 Vermeulen, Reman, and Katz). No inhibition by 20% O2. A, O2; O, Nj; •, air. 



tion by photoxidation (which can be measured under the same partial 

 pressure of oxygen, in the absence of carbon dioxide, cf. Fig. 56) is 

 ten times smaller than the loss of oxygen production by photosynthesis. 

 Thus, oxygen inhibits photosynthesis and does not merely balance it by 

 photoxidation. Franck and French (1941) assumed that this effect is 

 caused by the photoxidation of an enzymatic component of the photo- 

 synthetic apparatus. The inhibition of photosynthesis by excess light, 

 {cf. Chapter 19, page 532) was assumed by Myers and Burr (1940) and 

 Franck and French (1941) to be brought about in a similar way, that is, 

 by photoxidation of one of the " photosynthetic enzymes." Specifically, 

 Franck and French suggested that the oxygen-sensitive catalyst is the 

 carboxylating enzyme, Ex. In very strong light, the supply of {CO2} 

 complexes, which is controlled by catalyst Ea, lags behind the velocity 



