203 



Daniel I. Arnon 



noncyclic photophosphorylation were found to be basically inde- 

 pendent of photoproduction of oxygen by chlorop lasts (12). To 

 demonstrate this independence it was necessary to use special ex- 

 perimental devices such as inhibitors of oxygen evolution, anaer- 

 obic conditions and, most recently, monochromatic light (12) at 

 a wavelength (663 m^) which is absorbed by both chlorophylls a 

 and b and at a wavelength (7O8 m^) which is absorbed by the 

 chlorophyll a pigment system but not by chlorophyll b (5^). 



The main results may be summarized as follows. At 7O8 mu* 

 i.e., at a wavelength at which light absorption by chlorophyll b 

 was excluded, isolated chloroplasts were unable to use water as 

 a hydrogen donor but retained the photoactivity which did not 

 depend on water as a hydrogen (electron) donor. Thus, little 

 oxygen evolution (Table 1 in ref . 12) was observed at 708 myx 

 (see also ref. 55), but at this wavelength chloroplasts were 

 able to photoreduce ferredoxin and sustain a ferredoxin-catalyzed 

 cyclic photophosphorylation. (Contrary to other reports (56) we 

 found no cyclic photophosphorylation at 708 m\i with phenazine 

 methosulfate.) At 7O8 m^, chloroplasts were also able to sustain 

 a noncyclic photophosphorylation coupled with TPN reduction but 

 only when ascorbate-DPIP couple was supplied to replace water as 

 the hydrogen donor system. The presence or absence of air had 

 no special effect on either cyclic or noncyclic photophosphory- 

 lation at 708 m\x. 



The photoactivity of isolated chloroplasts at 663 miji differed 

 from that at 7O8 mn but was essentially the same as in white 

 light. Both chlorophyll a and b were able to absorb light at 

 this wavelength; water served as the electron donor for the 

 photoproduction of ferredoxin, and the resulting reduction of 

 TPN and noncyclic photophosphorylation was accompanied by oxygen 

 evolution. 



At 663 mn, the presence or absence of oxygen had little effect 

 on noncyclic photophosphorylation but had a marked effect on 

 cyclic photophosphorylation catalyzed by ferredoxin. In the 

 absence of oxygen, ferredoxin-catalyzed cyclic photophosphoryla- 

 tion occurred only when electron transport from water was blocked 

 by the addition of CMU. No addition of CMU was required for a 

 ferredoxin-catalyzed cyclic photophosphorylation at 663 m^i in 

 air or, it will be recalled, under either aerobic or anaerobic 

 conditions at 708 m\x. Thus the presence of oxygen was necessary 

 for ferredoxin-catalyzed cyclic photophosphorylation only when 

 the flow of electrons from water remained open; no oxygen was 

 necessary when the electron flow from water was blocked, either 



