EFFECT OF PHOTOSYNTHESIS ON RESPIRATION 563 



of photosynthesis. However, according to Gaffron (c/. Chapter 6) the 

 connection is only an indirect one. The primary effect of suppressed 

 respiration is fermentation (reversed Pasteur effect!); the latter brings 

 about the reduction of one or several components of the enzymatic 

 mechanism of photosynthesis, so that, upon illumination, no oxygen can 

 be released until these components have been regenerated. 



The photoxidative reactions which inhibit photosynthesis under high 

 oxygen pressures (Chapters 13, page 328, and 19, page 531) are distinct 

 from normal respiration, since their rate is affected by changes in oxygen 

 concentration between 10 and 100% (c/. Figs. 58 and 59). As described 

 in chapter 19, this type of sensitized autoxidation must be localized in 

 the chloroplasts, and can be attributed to reactions catalyzed by heat- 

 resistant catalysts of low molecular weight, rather than by true respi- 

 ratory enzymes. 



While the above experiments provide no evidence of a chemical 

 interaction between the catalysts or intermediates of respiration and the 

 photosynthetic apparatus, an indication that respiration may contribute 

 more than its end product— carbon dioxide — to photosynthesis can be 

 found in the observation of Warburg (1919) and van der Paauw (1932) 

 on cyanide inhibition of photosynthesis. According to these observers, 

 cyanide reduces photosjmthesis to the level of compensation, but does 

 not lead to a net consumption of oxygen and evolution of carbon dioxide. 

 The relevant experimental results and their interpretation were dis- 

 cussed in chapter 12 (pp. 302 et seq.). It was stated there that the 

 phenomenon requires renewed study, but that if its reality would be 

 confirmed it could be taken as an indication that carboxylic acids, formed 

 as intermediate products of respiration, can be utilized as oxidants in 

 photosynthesis, instead of the complexes {CO2}, thus avoiding the 

 cyanide-sensitive reaction by which {CO2} is formed from an acceptor 

 and free carbon dioxide. This is at present merely a conjecture, but it 

 is worth closer investigation; its plausibility is enhanced by the observa- 

 tion of Fan, Stauffer, and Umbreit (c/. Chapter 19, page 542) that other 

 organic carbonyl compounds also can be used as "substitute oxidants" 

 in photosynthesis. 



2. Effect of Photosynthesis (and Photoxidation) on Respiration 



Gaffron (1939) pointed out that photosynthesis can be completely 

 inhibited by hydroxylamine without an apparent change in respiration, 

 and concluded that respiration in green cells is not directly dependent 

 on the immediate products or intermediates of photosynthesis. This is 

 natural, since respiration occurs also in the nonphotosynthesizing cells 

 of multicellular plants. However, it does not prove that intermediates 



