LIGHT-INDUCED OXYGEN REACTIONS IN ISOLATED CHLOROPLASTS 

 Yaroslav de Kouchkovsky and Jean-Marie Briantais 



INTRODUCTION 



Studies of the kinetics of chlorophyll fluorescence (1) and of oxygen evolu- 

 tion ^2) in intact Chlorella cells have suggested a cyclic mechanism involving 

 three forms (A, B, C) of a "photocatalyst". Form A would accumulate in 

 darkness and be converted into form B. By a first dark reaction, B would be 

 converted to C with a concurrent evolution of oxygen. By a second dark 

 reaction, A would be regenerated from C. If the rate constant of the second 

 dark reaction is smaller than that of the first, the observed oxygen evolution 

 transients in Chlorella cells can be explained. Up to recently (^', similar 

 "induction" had not been found with chloroplasts(4). Now, however, our work 

 has shown that oxygen evolution in isolated chloroplasts is probably also 

 based on a cyclic mechanism similar to that suggested previously. Moreover, 

 our work has also shown that certain reactions associated with oxygen evolu- 

 tion by chloroplasts occur as well in the absence as in the presence of Hill 

 reagents . 



On the other hand, measurements of photosynthetic oxygen evolution are 

 sometimes uncertain because of the possibility of an influence of light on 

 respiration. For exannple, a measurement of quantum yield of photosynthes- 

 is may be in error if respiration, or other reactions of oxygen, have rates 

 dependent on illumination. 



These problems have led us to study the mechanism of photosynthetic 

 reactions of isolated chloroplasts and to compare our findings with those for 

 whole cells. 



MATERIALS AND METHODS 



Chloroplasts were isolated from Zea Mays. The methods of culture of 

 the corn and of the isolation of chloroplasts are given elsewhere (5). 



Hydrogen peroxide was detected colorimetrically using its reaction with 

 titanium sulfate in strongly acid solution. Concentrations of H2O2 as small 

 as 5 mumoles/ml could be measured (-> ). 



Photosynthetic activity was determined by following the oxygen concentra- 

 tion in an illuminated suspension. The oxygen concentration was measured 



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