37 



Bessel Kok 



normal reduction by the second photoact is impeded by the 

 poison) . 



Table 2 shows that essentially the same happens in long 

 wave light: In this experiment, performed like the one shown 

 in Table 1, we simultaneously measured the amount of dye re- 

 duced in each flash — whether reacting back or remaining in 

 the reduced form — and the net amount of DCPIPH accumulated by 

 all flashes. 



If N flashes of 674 rau light are given, an amount of DCPIPH 

 accumulates which nearly equals N times the amount reduced 

 per flash. However, if 725 mu flashes are given instead, only 

 1/30 of this amount accumulates, 97% of the reduced dye is 

 re-oxidized in the dark periods by P700 . The long wave drop 

 of the quantum yield of dye reduction (5) therefore is due to 

 a lack of the normal reductant generated by photosystem II 

 which reacts with P700+ much faster than DCPIPH. Actually, 

 if one corrects for the low fractional absorption of 725 mu 

 light, the data of Table 2 show that in this wavelength area 

 dye is reduced with twice the quantum yield observed in 67^ 

 light (analogous to the reduction of TPN in expt. Fig, 1). 



We may conclude that in these discussed experiments DCPIP 

 is reduced at the same locus as TPN: by the primary reductant 

 X~ made in the first photoact. It is clear then that in the 

 absence of the second photoact (DCMU or long wave light), 

 indophenol dye mediates a vigorous cyclic electron transport. 

 Trebst and Eck (6) showed that this cycle is coupled to ATP 

 formation and therefore must include the site of photophos- 

 phorylation. 



We agree with Vi/itt, et. al. (7) that at least in fresh 

 chloroplasts DCPIPH does not reduce P700+ directly but via 

 another intermediate. It remains to be proven that this 

 intermediate is cytochrome b^ or plastoquinone such as would 

 be required by hypotheses which correlate one of these com- 

 ponents with photophosphorylation. The hi^h normal potential 

 of the dye and the considerable concentration of the oxidized 

 form which can result from its photo-oxidation (even in the 

 presence of ascorbate cf. k Table 1) argue against this pos- 

 sibility. A more likely site for reduced dye to re-enter the 

 cycle is cyt. f. From an energetic viewpoint, this would 

 exclude ATP formation at this locus (cf. k) , 



