357 



David C, Fork 



chloroplast preparations. The effect can usually be detected 

 without undue difficulty in intact cells; however, it has various- 

 ly been reported to be present^^) or absent^^^) in chloroplast 

 preparations. Govindjee (this symposium) has outlined possible 

 explanations for failure to observe enhancement in chloroplasts. 



Evidence for the operation of two light reactions in chloro- 

 plasts has come from studies on absorption changes ^^^^. biochemi- 

 cal studiesd2,13) and measurements of enhancement^^* ^^\ 



The participation of two light reactions in the evolution of O2 

 by chloroplasts can be seen clearly in chloroplasts maintained in 

 buffered sucrose or NaCl solutions. While such a system may be 

 considered undefined, it may also be considered unchanged by the 

 addition of substances known to exert a profound effect on the 

 photosynthetic reactions of chloroplasts. Considerable insight 

 into the photosynthetic mechanism may be gained from a study of 

 chloroplasts lacking an added Hill oxidant since their limited 

 capacity for O2 evolution can be increased by pre-illumination or 

 by a period of darkness. 



Figure 6 shows time courses for O2 evolution upon illumination 

 of chloroplasts in the absence of an added Hill oxidant under an- 

 aerobic conditions. The time-course curve for O2 production shows 



1 1 1 1 1 1 1 1 1 1 1 I r 



Dark |« 647m)j »|« ^Dark »|4 647m;i »|< Dark 



J 60 



c 

 o 



t 40 



OJ 



o 



'o 20 



"5 



cc „ 



u^ 



J I L 



I I I I I L 



J L 



Time.min 



Fig. 6. Time-course curves of O2 evolution upon expo- 

 sure of whole Swiss chard chloroplasts to 647 mp light 

 (1500 ergs cm-2sec-l). The chloroplasts, held on a Tef- 

 lon-covered Pt electrode with dialysis membrane, were 

 equilibrated with 0.4 M Sucrose, 0.05 M K2HPO4 - KH2PO4 

 buffer, pH 7,0, and 0.01 M NaCl. Gas phase, N2. 



