HYDROGEN PEROXIDE HYPOTHESIS 



285 



Table 11.11 



Inhibition of Photostnthesis of Scenedesmus nanus and of Catalasb 

 Activity bt Different Poisons (after Nakamura) 



Chlorella vulgaris is only half as efficient in photosynthesis as C. 

 pyrenoidosa, but ten times more efficient in peroxide decomposition. 

 Chlorella cells grown in iron-deficient solutions, and containing reduced 

 quantities of chlorophyll (cf. Vol. II, Chapter 32) differed markedly in 

 their photosynthetic efficiency, but had the same capacity for peroxide 

 decomposition. 



In contradiction to Yabusoe, the effects of temperature on catalase 

 activity and photosynthesis were also found to be different by Emerson 

 and Green. 



Van Hille (1938) observed that the decrease in the rate of photo- 

 synthesis of Chlorella with age was not accompanied by a similar decrease 

 in the capacity for hydrogen peroxide decomposition. Gaffron (1937) 

 found that the low cyanide sensitivity of the photosynthetic apparatus 

 of certain strains of Scenedesmus (cf. page 305) was not shared by the 

 mechanism decomposing hydrogen peroxide. 



All these experiments speak against the identification of the rate- 

 limiting catalytic reaction in photosynthesis with the decomposition of 

 hydrogen peroxide by catalase. However, in recent years it became 

 evident — mainly through the work of Franck, Gaffron and coworkers — 

 that not one but several dark catalytic reactions are involved in photo- 

 synthesis, and that the reaction which determines the maximum rate of 

 this process in strong light in most plants, including Chlorella, is not the 

 one which leads directly to the evolution of oxygen. Thus, arguments 

 against the hydrogen peroxide theory based on the comparison of the 

 maximum rates of photosynthesis and hydrogen peroxide decomposition 

 in nonpoisoned plants (Emerson and Green, van Hille) have become 

 invalid. Comparisons of the inhibition of photosynthesis and of catalase 

 activity by poisons may still be significant, but only if the poisons in 

 question affect specifically the oxygen-liberating enzyme in photosyn- 

 thesis. This is not true of cyanide, which affects primarily the carbon 

 dioxide fixation (c/. page 307). Thus, the similarity of the cyanide 



