440 H. GAFFRON 



assumptions. They themselves have pointed out that the appearance 

 and shape of the oxygen gushes depend on the oxidative metabolism 

 in the dark but are independent of carbon dioxide concentration. 

 Transferring our explanation of carbon dioxide transients to the 

 oxygen side we arrive at the following picture : The first sharp oxygen 

 spike occurs when the coenzymes, and consequently some intermedi- 

 ates, are being reduced. This spike may stand alone whenever the 

 photosynthetic carbon cycle is inhibited either for lack of carbon 

 dioxide or other reasons. The spike may disappear under strictly 

 anaerobic conditions, the coenzymes then being present already in 

 their reduced states and oxidized intermediates practically absent. 

 The dip in the rate of oxygen evolution following the first gush — an 

 observation made also by other investigators ((12), and Fig. 8 in ref. 

 3) — is brought about by two or three superimposed reactions: a mo- 

 mentary lack of hydrogen acceptors, a predominance of photosyn- 

 thetic phosphorylations needed to promote the carbon cycle, and 

 perhaps an increased rate of reoxidation. The third reaction should 

 reveal itself as an increase in the rate of respiration whenever the 

 latter was abnormally low ("starved algae") at the start of the 

 experiment. 



The aftereffects, i.e., the transients which we as well as Brackett 

 and Olson have seen when the light is turned off, are to be expected 

 when the reactions responsible for the induction are being reversed. 

 In comparing the slopes of the recorded curves at "on" and "off" we 

 have to keep in mind, however, that photosynthesis has in the mean- 

 time built up pools of intermediates which were not there before — 

 giving rise to the continued "pick up" of carbon dioxide, for instance — 

 and that the overall rate of respiration is generally slower than that 

 of the light-driven reactions. 



Discussion 



Frenkel : If j-ou speak of phosphorylation you have to consider that this might 

 i)e accompanied by pH changes. 



Gaffron : But these are inside the cells. 



Frenkel: But the pH inside the cells is likely to change. For instance, in the 

 case of a phosphorylation reaction, by phosphate uptake. 



Gaffron: You have to take all the different possibilities into account. The 

 explanation has to fit Olson and Brackett's experiment. It has to fit the Calvin 

 cycle. 



