PHOTAUTOXIDATION IN PRESENCE OF EXCESS OXYGEN 535 



illumination. As long as oxygen consumption in light proceeds at a 

 constant rate, the cells suffer no irreversible injury. However, after about 

 two hours (at 38,000 f.-c), the rate of photoxidation begins to decrease; 

 and the cells show the first signs of bleaching. 



Similar observations were made by Fockler (1938) on Trichomanes radicans: at the 

 end of the first hour of intense illumination the oxygen consumption was equal to twice 

 the normal respiration; it increased to four times normal respiration after four hours, 

 and dechned afterwards. 



As in the case of inhibition by excess oxygen, the decline in photo- 

 synthesis caused by excessive illumination is many times stronger than 

 could be explained by a mere superposition of photautoxidation upon 

 normal photosynthesis. We are again led to the assumption that 

 photautoxidation inhibits photosynthesis — a conclusion which was also 

 reached by Fockler (1938) and Myers and Burr (1940). Besides the 

 tnagnitude of the effect, Myers and Burr pointed to its gradual onset 

 (contrasted with the immediate beginning of photoxidation in the 

 experiments of Franck and French) as a proof of the inhibition hypothesis. 

 The first 20 to 30 minutes of illumination (of. Fig. 60), during which the 

 slope decreases slowly to a constant final value, can be considered as the 

 period of inactivation of the photosynthetic apparatus. 



Similarly to the photautoxidation in C02-starved leaves, photautoxi- 

 dation in strong light is not a steady-state phenomenon, but involves a 

 progressive consumption of cellular reserve materials. Perhaps provision 

 of an organic oxidation substrate (e. g., glucose) could prevent irreversible 

 injury and permit the study of photoxidation in strong light under 

 steady conditions; but no experiments of this kind have yet been made. 

 However, a temporary steady state was reached in the central parts of 

 the time curves of Myers and Burr, where photautoxidation proceeded 

 for a while at a constant rate. Their slopes could be used for the analysis 

 of the dependence of photautoxidation on light intensity and other 

 factors. 



In figure 61, the rates of oxygen exchange in these periods of steady 

 photoxidation are plotted against light intensity. The shape of the 

 resulting "light curves" depends on the previous history of the cells. 

 Curve A, obtained with a suspension grown in strong light and ample 

 supply of carbon dioxide (5%), shows a broad "saturation plateau," 

 while curve B, obtained for a suspension grown in the same light but in 

 ordinary air, shows a sharp maximum in the region of 3,000 f.-c. im- 

 mediately followed by a decline. The difference is similar to that 

 between "sun plants" and "shade plants" according to Weis and 

 Lubimenko (which we have mentioned above). Curves similar to curve 

 B were also obtained by Myers and Burr with cells grown in darkness 



