52 



Bessel Kok 



subsequent addition of indophenol dye and ascorbate, which 

 restores the photoreduction of TPN does not restore the 

 suppression of fluorescence. We may assume that the photo- 

 reduction of TPN in the presence of DCMU and reduced dye 

 requires only the long wave photoreaction (6). Although this 

 conversion can occur with a high quantum yield, (13) it is 

 not reflected in a suppression of fluorescence. 



DISCUSSION 



The following scheme seems to satisfy most of the presented 

 data. It assumes that the bulk of the light absorption is 

 carried out by one pigment assembly only. Some fluorescence 

 escapes at all times from the partners of this collector sys- 

 tem. Absorbed quanta drain preferentially in trapping mole- 

 cules II which occur in a concentration of 1 per 50 Chi. 

 (cf. Fig. 1). As long as these traps are unexcited (and kept 

 in the receptive state by a dark reaction) only the accidental 

 (fixed) fluorescence occurs. 



FIXED 



F650 F685 

 J 



Collector 

 Pigment 



TRAP3E - 



DCMU 



[H] ^ 



VARIABLE 



F685 



+ 

 I 



/ 



/ BYPASS 



1 



SPILLOVER 



QUENCHED AT 300° K 



F700 F730 



iChLai]"' 



^C700 



/ 



C730 



P700 



02 



After a trap II is photoconverted, however, the next 

 excitation will flow on to the long wave pigment C7OO ["spill 

 over" (16)] . Since the coupling with C700 is rather weak, 

 this transfer competes with an additional ("variable") fluo- 

 rescence emission of the collector system [cf . Franck (17)J . 

 If all absorbed quanta flow towards C7OO (as in the absence 



