THE CHLOROPLAST REACTION 209 



Whittingham : None at all. Cj'tochrome/ after extraction is in the reduced form ; 

 when added to chloroplasts no oxidation takes place. 



Clendenning: I would like to point out that in Dr. Thomas E. Brown's Ph.D. 

 thesis, the temperature relations of photosynthesis and of the Hill reaction in the 

 same cells are reported. This work was presented at the Paris meeting in July 

 1 954 which many in this audience attended. Very much higher rates of the Hill 

 reaction than of photosynthesis were observed at low temperatures, when using 

 either whole Chlorella or Nostoc cells. Such comparisons should be with the Hill 

 reaction in leaf chloroplasts. At 0°C., the Hill reaction rate in Chlorella was from 

 10 to 15 times higher than the full rate of photosynthesis, in aliquots of the same 

 cell suspension, determined either by the two-vessel method or with number 9 

 buffer. On raising the temperature to 10°C., the Hill reaction rate was about four 

 times higher. At 15°C., the Hill reaction rate was still above that of photosyn- 

 thesis. At 20° C, the Hill reaction rate may be either above or below that of photo- 

 synthesis depending upon the duration of the experiments. 



Rabinowitch : I think there is no doubt that, in strong light, the Hill reaction 

 with quinone follows the laws of enzymatic reactions. From the experiments of 

 Clendenning, it seems that there are two different temperature-dependent enzy- 

 matic reactions involved in photosynthesis, and that only one of them is left over 

 in Hill's reaction. Franck has always maintained that, at lower temperatures — 

 say to 10°C. — the reaction that determines the temperature dependence of photo- 

 sjmthesis is different from the one which causes this dependence at higher tem- 

 peratures — say 15° to 30°C. — and that the former reaction has something to do 

 with the fixation of carbon dioxide. 



At low temperature, the Hill reaction, which does not involve carbon dioxide, 

 can therefore proceed much faster than photosynthesis; while at higher tempera- 

 tures, a common reaction, having to do with the evolution of oxygen, limits both 

 to the same maximum speed. 



It is dangerous to draw conclusions from a comparison of the rates of the Hill 

 reaction in chloroplasts from one plant with that of photosynthesis in another one. 

 I don't see why cells of the same species as those from which the chloroplasts were 

 prepared should not be used for comparison. I believe that, in a good chloroplast 

 preparation, the maximal rate of oxygen evolution in bright light per unit chloro- 

 phyll amount will be found to be not slower than that of photosynthetic oxygen 

 production — not necessarily in Chlorella, but in the same species from which the 

 chloroplasts had been prepared. 



Whittingham : That is merely a question of opinion in the absence of experi- 

 mental data. I think if you have a suitable oxidant and a suitable system you may 

 get the Hill reaction much faster on a chlorophyll basis than you do get photo- 

 syTithesis. 



Rabinowitch : I think the comparison should be with the same type of cells and 

 not with Chlorella. 



Whittingham : There is not a good Hill reagent that we can use with Chlorella. 



Granick : With respect to the light curves for chloroplast and Chlorella how do 

 you compare them? 



Whittingham : Equal light absorption by chlorophyll. 



