TIEPENT RESULTS AT WAGENINGEN oOl 



Tamiya : What is the order of the dark roartion in your model? 



Kok : We cannot assign a definite order to the flash yield vs. dark time curve. 

 This is so because each measured point is obtained under a steady-state condition 

 of intermittent illumination. One has to postulate a certain reaction sequence, 

 set up and solve the corresponding equations, and compare the solutions witli the 

 experimental findings. 



Witt : Is the dark time dependence the same for the different flash times? 



Kok : We have measured flash yield vs. flash time curves with dark times be- 

 tween 0.02 and 2 seconds. The curves measured with very long dark times (suf- 

 ficient for all dark steps to run to completion) are easiest to interpret, but ex- 

 perimentally they are more difficult to ol)tain with sufficient accuracy. 



Rabinowitch: In connection with Kok's mechanism, I wonder how common 

 in enzyme kinetics is the assumption that a substrate can be transferred from 

 one enzyme to another by a bimolecular reaction? 



Gaffron : Kok presented it so nicely one would like to believe it is that simple. 

 But there are some questions one may ask. For example, it appears that one can 

 grow algae with "units" of different .size. Is there a difl'ei'ence in the chlorophyll 

 content? It is much easier to think that you can vary the number of enzyme 

 molecules than that you can vary the size of chlorophyll units. 



Rieke in our laboratory once showed that there are two different dark periods, 

 one which is .sensitive to cyanide, and another one — the Blackman-Emerson 

 period — which is not. I mention this as an example of the fact that we have to 

 reckon with a succession of enzymes, each of which might become limiting under 

 different conditions. 



Kok: Both the concentrations of U and E may vary, as expressed in relation to 

 chlorophyll concentration. Rieke's cyanide-sensitive enzyme may be identical 

 with our E. 



Rabinowitch : I don't think that the explanation of Rieke's results by Franck 

 (suggesting that a second dark reaction — a cyanide-sensitive and temperature- 

 dependent carboxylation — can become rate-limiting instead of the Emerson- 

 Arnold reaction) is sufficient to explain all the results. 



For example, according to Gilmour, you can get the same efl"ect of long dark 

 intervals also in the Hill reaction, in which carbon dioxide is not involved, and 

 which is not cyanide-sensitive. 



A second difficulty is that, in cells used in Tamiya's experiments, the maximum 

 steady rate of photosynthesis does not seem to have been significantly lower than 

 in Emerson and Arnold's algae. Therefore, one cannot assume that, in Tamiya's 

 case, another enzyme was. for some reason, deficient, and depressed the yield be- 

 low the limit fixed by the Emerson-Arnold enzyme. Rather, one would have to 

 make the unlikely assumption that the usual enzymatic "ceiling" was replaced by 

 another one having about the same height as the usual one. I do not deny that the 

 carboxylating enzyme can use longer dark periods — as Franck suggested — but its 

 effect must come on top of the complications we are now discussing. 



Gaflfron: On the matter of photoinhibition: at very low oxygen tensions, you 

 should not be bothered by photooxidation. Furthermore, photooxidation may be 

 proportional to the square of the light intensity. If, for example, photooxidation is 

 due to a peroxide, it is very likely that the formation of the latter will require two 



