CARBON-14 PREILLUMINATION EXPERIMENTS 223 



action between R and O2, through which H2O2 is formed, will be sup- 

 pressed in the presence of high concentration of CO2, the existence 

 of catalase would have only a minor significance for the process 

 of photosynthesis occurring under the condition of high CO2 concen- 

 tration and low O2 tension. Even under such circumstances, however, 

 cyanide will inhibit photosynthesis, because it retards, besides in- 

 hibiting catalase, the reaction between R and CO2 and shifts the com- 

 petition between CO2 and O2 in favor of the latter, so that there 

 will occur the formation and accumulation of H2O2 which acts upon 

 R leading to a decrease of the rate of photosynthesis. In the photosyn- 

 thesis occurring under natural conditions, the concentration of CO2 

 available is usually suboptimal and moreover, there is a large amount 

 of O2 in the circumambient medium. Under such a condition, the com- 

 petition between CO2 and O2 for R will be more or less in favor of 

 O2, and, therefore, there will be ample possibility for the formation 

 of H2O2. According to our notion it is to protect the photosynthetic 

 machinery from such contingent accumulation of H2O2 that catalase 

 is playing a role in the mechanism of photosynthesis. 



Acknowledgment. This study was aided by grants from the Ministry of 

 Education and from the Rockefeller Foundation, both of which are grate- 

 fully acknowledged here. Thanks are due to Dr. A. H. Brown for his help 

 in the preparation of this paper and to Mr. Tetsuya Katoh for his assistance 

 during the experiment. 



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