358 



David C. Fork 



an initial burst or spike which is followed by a rapid decrease to 

 a much lower steady-state O2 evolution in the light. Exposing the 

 chloroplasts with the same light again 3 minutes later results in 

 a smaller O2 spike than before. Aerobic chloroplasts also show 

 the 02-production spike but net O2 uptake is usually seen in the 

 steady state. An Oo gulp is also seen upon darkening. The 02- 

 production spike is also lower in the second aerobic exposure (15). 

 The decrease of the 02-production spike in chloroplasts without 

 added Hill oxidants is not due to a gradual deterioration of the 

 chloroplasts since a dark interval serves to regenerate the O2 

 spike again, de Kouchkovsky^l^) ^as shown that the recovery of 

 the O2 spike is more rapid in air than under N2. 



An even more rapid recovery of the Oo-production spike is 

 brought about by pre-illuminating the chloroplasts with far-red 

 light. All the O2 spikes shown in figure 7 were produced by the 





o 



D 



-o 



2 2 

 a 



IM 



-> 1 1 r 



T — ^ 1 1 r 



h-700in(j- 



b 



r 



Dark 

 control 



A_ 







4 6 8 

 Timc.min. 



10 



12 



Fig. 7. 02-production spikes produced by Swiss chard 

 chloroplasts upon exposure to white light (5.2 x 10^ ergs 

 cm-2 sec-1) filtered through 3 cm water and a Baltzers 

 heat-reflecting filter (Calf lex). The intensity of the 

 700 m^i light was 80 ergs cm"^ sec"^, Chloroplasts equi- 

 librated with the solution described in figure 7. Gas 

 phase, N2. 



same 1.3-second white light exposure. In the first 3-minute in- 

 terval between white flashes the chloroplasts were illuminated 

 with weak (80 ergs cm-2 sec"l) 700 mia light for 2.6 minutes. No 

 detectable O2 exchange resulted from this 700 mu exposure. How- 



