126 



J. L. RosenlDerg azid Tevfik Bigat 



metastable state of chlorophyll a, and the combined energy losses 

 for fluorescence and metastable state formation would be too 

 excessive to allow for good quantum yields. 



(d) Existing fluorescence data need not he interpreted to 

 exclude the possibility that Q^ the substrate for Step (2) is. . 

 oxidized cytochrome f. This exclusion has been made by Buys ens '^ 

 and in somewhat different language by Kautsky et al.(7) on the 

 ground that the fluorescence is low after a dark period when 

 cytochrome f is known to be reduced. It must be emphasized that 

 oxidized Q is not the only quencher of chlorophyll a fluorescence. 

 It is well known that after a dark period metabolic oxidants 

 substitute for the normal primary photosynthetic oxidants during 

 the beginning of the induction period,^ ■' Even on steady irradia- 

 tion at intensities below compensation this type of substitution 

 occurs and is accompanied by lower than normal fluorescence 

 yields. (9) The lowering of fluorescence under such conditions 

 can be explained in terms of a postulated adsorption of these 

 substitute oxidants at the chlorophyll and their consequent 

 ability to use chlorophyll a singlet excitation directly in a 

 variant of Step (l ), thus quenching fluorescence . ^-'-^'' 



(e) Since the above model allows System 2 to sensitize both 

 photochemical steps, it is necessary for reasons of geometrical 

 economy about the bifunctional reaction center that there not be 

 too many different photochemical substrates that are part of the 

 built-in photosynthetic apparatus. For this reason, as well as 

 for others discussed elsewhere in this Symposium, ^ ^'' we propose 

 that Q, the normal quenching substrate for Step (2), is oxidized 

 cytochrome f and that the same cytochrome in the reduced form is 

 a substrate for Step (l). 



(f) In the absence of substitute reactions discussed in (d) 

 the chlorophyll a fluorescence yield in the light-limiting 

 region depends mainly on the ratio, r, of the rate of use of 

 System 2 for Step (l) to its use for Step (2). The shape of the 

 transients shown in Figs. 1 and 2 can be accounted for very well 

 in terms of the adjustment of the value of r to the illumination 

 regime, with the detailed model of Franck and Rosenberg predic- 

 ting that r may range from a value close to zero, immediately 

 after a period of System 1 light absorption and oxidized cyto- 

 chrome accumulation, to a value of unity during steady-state 

 absorption by System 2 alone. ^^j 



