PHOTAUTOXIDATION IN PRESENCE OF EXCESS OXYGEN 531 



cannot be attributed to a stimulation of the respiratory apparatus.) 

 Figure 58 shows that a high oxygen pressure also is required for phot- 

 autoxidation in vitro, if it is sensitized by dyesiuff-protein complexes (but 

 not by free dyestuff molecules, cf. page 513), as well as for the oxygen 

 inhibition of photosynthesis, according to Warburg (1919). An interpre- 

 tation of these relationships will be attempted on pages 544-548. 



Some preliminary results were obtained by Franck and French in the 

 study of the dependence of photautoxidation on light intensity. Photoxi- 

 dation continued to increase in the range > 30,000 lux, where the 

 photosynthesis of Hydrangea is saturated with light. Franck and French 

 showed (by comparing the rate of oxygen consumption in continuous 

 light with that in periodically interrupted light of the same average 

 intensity) that the oxygen consumption increased more slowly than 

 proportional to light intensity. Red and blue light were found to be 

 equally efficient in photautoxidation, thus proving that it is brought 

 about by chlorophyll, and not (or not exclusively) by the yellow leaf 

 pigments. 



3. Photautoxidation in the Presence of Excess Oxygen 

 and in Intense Light 



It was stated above that photoxidation can suppress photosynthesis 

 "autocatalytically," and thus make carbon dioxide starvation or the use 

 of poisons superfluous. The required stimulation of photautoxidation 

 can be achieved either by an increase in oxygen pressure or by a step-up 

 in the intensity of illumination. 



The inhibition of photosynthesis by excess oxygen was described in 

 chapter 13, which dealt with various chemical inhibitors. It was dis- 

 covered by Warburg in 1919. As shown by figure 32 (page 328), War- 

 burg found in Chlorella a 30% decrease in photosynthesis when the oxygen 

 pressure was increased from 15 to 760 mm. McAlister and Myers 

 (1940) found that, in the presence of 0.03% carbon dioxide and at high 

 light intensities (over 10,000 lux), the photosynthesis of wheat was 

 decreased by 20 or 30% when the oxygen content was increased from 

 0.5 to 20%. Wassink, Vermeulen, Reman, and Katz (1938), on the 

 other hand, found no difference between the rates of oxygen liberation 

 by Chlorella at 0% and 20% oxygen (cf. Fig. 33), but observed a strong 

 inhibition in pure ox3^gen, amounting to 20% in comparatively weak 

 light (4000 ergs/cm.Vsec, yellow sodium light), and as much as 50% 

 in strong hght (14,500 ergs/cm.Vsec). Qualitatively, the inhibition of 

 photos3^nthesis by oxygen shows the same dependence on oxygen con- 

 centration as does photautoxidation in vivo (cf. Fig. 58), but the loss 

 in oxygen production is ten or more times larger than the oxygen con- 

 sumption by photautoxidation (as observed by Franck and French at 



