386 DANIEL I. ARNON 



This interpretation of the difference between the non-cycHc electron 

 flow mechanism in chloroplasts and in photosynthetic bacteria is sup- 

 ported by recent evidence that it is experimentally possible to replace 

 water as the electron donor in non-cyclic photophosphorylation by 

 chloroplasts. 



As was already mentioned (Section 7), photosynthetic reactions of 

 chloroplasts in which oxygen is liberated require chloride, hence Reaction 4 

 could not proceed in the absence of chloride. As shown in Fig. 26, on 

 omitting chloride from the reaction mixture (a step that included purifica- 

 tion of those reagents that contained chloride impurities) TPN reduction 

 and oxygen evolution ceased and photophosphorylation was abolished 

 (Table XIII). 



TABLE XIII 



Effect of Chloride on Non-Cyclic Photophosphorylation by Isolated 



Chloroplasts 



(Bove, Bove, Whatley, and Arnon [103]) 



Micromoles ATP formed 

 Experiment Electron acceptor 



chloride + chloride 



3-3 

 3-7 

 3-6 



4-2 



These results indicated that chloroplasts deprived of chloride cannot 

 use water as the electron donor in Reaction 4. It was possible, however, 

 that they could use other electron donors that did not involve an oxidation 

 of water (i.e. OH ) and a resultant oxygen evolution. Vernon and Zaugg 

 [160] have found that chloroplasts which are incapable of photochemical 

 oxygen evolution, retain the capacity for photoreduction of TPN with 

 ascorbate (jointly with catalytic amounts of 2,6-dichlorophenol indophenol) 

 as the electron donor. 



Table XIV shows that using an ascorbate electron donor system, 

 chloroplasts carried out a "bacterial" type of non-cyclic photophos- 

 phorylation in which ATP formation and TPN reduction were not 

 accompanied by an evolution of oxygen. The participation of water (OH ~) 

 as an electron donor was prevented here by the omission of chloride and 

 also by the addition of dichlorophenyl dimethylurea as an inhibitor of 

 oxygen evolution (cf. [160]). 



These results support the view (cf. [121]) that the evolution of oxygen 

 in non-cyclic photophosphorylation by chloroplasts (and hence in photo- 

 synthesis of green plants) is not fundamental to the key photosynthetic 



