TRANSIENTS FOR CARBON DIOXIDE 439 



evolution of car])on dioxide are evidently reduced in 1 or 2 seconds of 

 strong light and their reoxidation to the previous level takes a com- 

 j)aratively long time. The fact I hat (his first action of light does not 

 lead automatically to a subsequent, uptake of carbon flioxide may 

 be explained in two ways. Either the substances reduced immediately 

 at "light on" are not part of the photosyntheti(; carbon cycle or 

 another action of light is needed to move the precursor of the carbon 

 dioxide acceptor all the way aroiuid the cycle until carbon dioxide is 

 picked up to give new PGA, and this second reaction does not start 

 simultaneously with the first. Removing, for instance, respiratory 

 PGA by reduction to triose, will certainly prevent it from going into 

 pyruvate, etc. Now if more light is needed to drive the phosphoryl- 

 ation in the carboxylation cycle and the light flash was too short, this 

 triose will simply be reoxidized. 



Any deviation in the concentration of PGA from that prevailing 

 in the stationary light phase, as well as variations in the rate of photo- 

 synthetic phosphorylation, may well produce transients similar to 

 those observed. Furthermore, there is evidence for the appearance of 

 other kinds of reductive carboxylations in the light (8). The instan- 

 taneous nature of the break in the evolution of respiratory carbon 

 dioxide when the light is turned on calls, however, for a more specific 

 explanation. It can best be understood if the decarboxylation steps 

 in the respiratory cycle (Krebs cycle) were blocked. This would 

 happen if the pyridine nucleotides taking part as coenzymes became 

 reduced. 



Weigl and Calvin (9) showed that light prevents the entrance of 

 freshly assimilated carbon into the Krebs cycle. And Calvin (10) 

 later proposed a special " valve" action— assigned to thioctic acid — to 

 stop the decarboxylation of pyruvate. Plausible as Calvin's hy- 

 pothesis seems, it is not so serviceable for our purposes as the more 

 general assumption of the reduction of pyridine nucleotides. According 

 to Browai and Good (11), this reduction best explains the compensa- 

 tion by light of respiration in cyanide-poisoned Chlorella. There is no 

 doubt that a reduction of pyridine nucleotides will occur w^hen 

 conditions are right. 



Finally, we have to see whether such an explanation for our carbon 

 dioxide transients remains valid in view of Brackett and Olson's 

 observations on oxygen transients (see this book, pages 412-418). I be- 

 lieve the explanation also holds there without any straining or ad hoc 



