367 



Kouchkovsky et al. 



The existence of the different postulated reactions was supported also by 

 their different sensitivity to pH and temperature. The pH dependence of the 

 initial rate of oxygen evolution during the burst, and the pH dependence of the 

 steady-state rates of oxygen evolution and uptake during photoabsorption are 

 shown in Fig. 3. The optimum for the burst is at pH'^6, while that for oxy- 

 gen uptake is at pH/^ 8, similar to that for photophosphorylation ( ^ 1> 12)^ xhe 

 difference in maxima between the burst and the steady-state oxygen evolution 

 rests on the linaitation of the total rate by the slowest reaction. Measure- 

 ments as a f\inction of temperature permitted a similar discrimination of the 

 reactions '-^ '. 



Chloroplasts with Hill Reagents 



It was of interest to try to detect an induction in the Hill reaction, although 

 tlis has not been found previously ^^'. Fig. 4 shows that, with ferricyanide, 

 one can see a transient phenomenon very similar to the oxygen burst des- 

 cribed above. But here, of course, the burst is followed by continuous 

 oxygen evolution. On the other hand, with DPIP the steady-state is reached 

 immediately, i.e., without an induction period. 



Thus, different kinetics prevail in the Hill reaction with different oxidants. 

 One can suppose that the difference in kinetics could depend on the occurence 

 of only the photoreaction of "system 2" in the case of DPIP, and of both photo- 

 reactions ("systems 1 and 2") in the case of ferricyanide (cf. 7, 13). 



wm/mmm WMM 



FtCy DPIP 



Fig. 4. Oxygen concentration as a function of time in Hill reac- 

 tions with FeCy (lO'^^M) and DPIP (5x 10"^ M). 



Chloroplasts: 315 u g chlorophyll/ml in TMS pH6. 6. 

 T : light on. 



(9) 

 If one continues to observe the Hill reaction, with ferricyanide ' or with 



DPIp' ', one can note that, progressively, the rate of oxygen evolution 



decreases and finally attains a zero value. Afterwards, oxygen uptake occurs 



similarly to the photoabsorption already described in chloroplasts without 



Hill reagent. In Fig. 5, changes in oxygen concentration with and without 



