220 -N. K. TOIilSKFtT 



synthesis during the greening of an etiolated plant and the stepwise 

 acquisition of the ability to synthesize the photosynthetic organic 

 products. The induction period in photosynthesis by a fully grown 

 plant may be a measure of the rapidity with which a plant can build 

 up the pool size of carbon compounds provided all the enzymes are 

 present. The slow stepwise build-up of photosynthesis during green- 

 ing of an etiolated plant would appear to be limited by other proc- 

 esses such as synthesis of adequate amounts of enzymes to catalyze 

 some of these reactions. 



Discussion 



Rosenberg : Is there any evidence from your work, or from previous work in 

 the literature, that in the early stages of development of the photosynthesis ap- 

 paratus the ratio of CO2 to oxygen is higher than the steady-state value, as might 

 be the case if reduction to triose did not occur? 



Tolbert: No. Rather a literature comparison indicates that oxygen evolution 

 begins before CO2 fixation, which would make this ratio smaller than the steady- 

 state value. 



Gaffron : Did you say that oxygen development starts earlier than CO2 fixation? 



Tolbert: Unfortunately, all the investigators concerned have used different 

 plants. Dr. Thomas, in Utrecht, has measured oxygen evolution during the 

 greening process, and both he and Dr. Smith get oxygen evolution fairly soon 

 after chlorophyll formation begins. 



James Smith: It took pretty nearlj'^ an hour to get very much though. 



Tolbert : This would still be sooner than CO2 fixation, which did not begin for 4 

 hours. 



Gaffron : It is very interesting that the plant learns to do full photosynthesis by 

 steps. 



Fuller : I think that is a very important point. There are other ways that you 

 can control enzyme formation in photosynthesis. For instance, j^ou can divorce 

 enzyme formation from the greening process of the plant. We have grown Chlorella 

 variegata in the light on organic substrate, and it does not produce any active 

 carboxylation enzyme. If the endogenous organic substrate is removed then the 

 enzyme is rapidly formed. We have also found that by growing Euglena in the 

 presence of acetate in the light, where no chlorophyll is lost, carboxylation activity 

 is strongly suppressed. There are other ways of inactivating photosynthetic en- 

 zymes, so it is not the greening process itself but it is the pathways of metabolism, 

 as Dr. Tolbert points out, that control enz3ane formation. 



James Smith : I might say that steps in the oxygen evolution can be built up in 

 the dark. If you illuminate for 5 minutes and transform the protochlorophjdl to 

 chlorophyll, then put the leaf back in the dark for 2 hours, you get only a very 

 slight amount of oxygen evolution. But if you illuminate the leaf again in the air 

 and then measure the oxygen it just streams off. So there is a second photochemical 

 activation in this. In continuous light, this goes on all the time. But there is some- 



