368 



Kouchkovsky et al 



ferricyanide are shown. As the slopes of the curves during photoabsorptions 

 indicate, light-induced uptake is lower when ferricyanide is present (Ch-^C, 

 Fig. 5). This may be explained by the existence of an equilibrium between 

 the photo reduction of ferricyanide and the reoxidation of ferrocyanide. When 

 the light is turned off, an extra-absorption appears vih ich has a first order 

 rate constant very close to that obtained without oxidant^"'. Analysis of the 

 curves, in the manner already described, made it possible to distinguish a 

 steady-state oxygen evolution (measured by Gj^/t, cf. Fig. 2) and a steady- 

 state oxygen absorption (measured by Fj^/t, cf. Fig. 2). The origin of this 

 oxygen absorption could be tentatively assigned to a Mehler reaction, which 

 would compete with the Hill reaction. It is interesting to note that, as in 

 chloroplasts without ferricyanide, hydrogen peroxide was detected and the 

 ratio H2O2/O2 was close to 1.5 (9). This suggests again the occurence of a 

 supplementary photo -oxidation. 



^^^^^ 



V/////////////////////A 



LIGHT 



DARK 



Fig. 5. Oxygen concentration changes in chloroplast suspensions 

 with (I) and without (II) Hill reagent (FeCy : IQ-^M). 



Chloroplasts : 315 a g chlorophyll/ml in TMS pH 6. 6 ; 

 -— -— : light ; /,,,,/ : dark ; C, Cr. E, F^. G^ : see text. 



