15 



L.K.ll. Duyeens 



ximate proportionality of the initial rate of increase with in- 

 tensity and the progressive depression of the "steady state" at 

 increasing flash intensities are general phenomena. Apparently 

 this depression is caused either directly or indirectly by a 

 light reaction. In some experiments the fluorescence yield goes 

 through a maximum and decreases appreciably below the value 

 achieved within 10 usee after the starting of the flash: the 

 strongest depression appeared to occur after about 100 jisec. 

 Whether the depression is caused by a rapid reoxidation of QH 

 by P 700 »by another component of system 1, by a short cirr 

 cuit of Z to QH, or by another reaction is a subject of fur- 

 ther experimentation. 



Photo synthetic pho sphoryla tion 



We do not have experimental evidence on the sites of phos- 

 phorylation. As far as I know most experimental evidence from 

 other laboratories is consistent with the assumption that non- 

 cyclic photophosphorylation(46) occurs between QH and P, and 

 analogy with the respiratory system suggests that plastoquinone 

 and cytochrome may participate in the pho sphoryla ting reactions. 

 The possible sites of phosphorylation are given in the scheme 

 by the arrow with ADP. The fact that cyclic phosphorylation in 

 chloroplasts is, under certain conditions, not inhibited by 

 DCMU suggests then it occurs in dark reactions between XH and 

 P 700. The free energy loss in this redox reaction would be 

 sufficient to permit the production of 2 molecules of ATP per 

 2 electrons transported, but the measured quantum requirements 

 indicate a two times lower efficiencj(47) , 



SmOMARY 



A discussion is given of experimental evidence obtained in 

 our laboratory concerning the mechanism of hydrogen or electron 

 transport in photosynthesis. A scheme is given in which two dif- 

 ferent photochemical reactions operate in series. Unpublished 

 experiments are presented concerning the presumed primaiy pho- 

 toreductant of the system 2 (which system is closely connected 

 to the production of oxygen), and concerning the photooxidation 

 of cytochromes in purple bacteria at low temperatures. It is 

 shown that the initial rate of these photooxidations is de- 

 creased upon lowering the teciperature, and it is argued that 

 the cytochrome oxidation is a temperature dependent dark reac- 

 tion. However, at "normal" intensities the rate of this reaction 



