CHLOROPHYLL FLUORESCENCE AND PHOTOSYNTHESIS 823 



such as the binding of carbon dioxide by an "acceptor," to form a compound 

 designated as {CO2}. These reactions "prepare" the reactants for their 

 participation in the photochemical reaction proper. If one of the prepara- 

 tory reactions ceases to keep pace with the primary photochemical process 

 (equation 24.3), the conversion of the primary photoproduct HX-Chl-Z 

 back into the photosensitive form X- Chi -HZ will be retarded. If the 

 supply of the oxidant, {CO2}, is too small, while that of the reductant, 

 {H2O} (or of substitute reductants in bacterial metabohsm), is ample, 

 the chlorophyll-oxidant-reductant complex may accumulate in the re- 

 duced form, HX- Chl-HZ. If the supply of the oxidant is ample, but that 

 of the reductant is limited (a situation which can easily be realized in experi- 

 ments with bacteria), the complex will accumulate in the reduced form, 

 X • Chi • HZ . Both forms are stable, because they cannot be transformed into 

 the photosensitive form X- Chi- HZ by simple back reaction, and photostahle 

 because they cannot undergo the primary photochemical process (equation 

 24.3) . The accumulation of either of them is likely to enhance fluorescence. 

 It seems, in fact, that all factors (such as COo-starvation or cyanide poison- 

 ing) which limit severely the carboxylation reaction CO2 -^ [CO2] normally 

 increase the yield of fluorescence, and that the same is true of the factors 

 limiting the supply of the reductant (H2, H2S or thiosulfate) in purple bac- 

 teria. 



Considerations of this kind could explain why light saturation of 

 photosynthesis is accompanied by an increase in the yield of fluorescence 

 in some cases (namely, when saturation is caused by the limited velocity of 

 a "preparatory" reaction), and has no effect on the jdeld of fluorescence in 

 others (namely, when it is due to the limited rate of a "finishing" reaction). 



Closer consideration of the picture also could explain why, when changes 

 in the yield of photosynthesis are correlated with changes of fluorescence, 

 not only does the absolute extent of the latter vary within wide limits, but 

 sometimes, even the sign of the effect is reversed, the usual antiparallelism 

 being replaced by a parallel increase (or decrease) of the quantum yields of 

 both photosynthesis and fluorescence: 



Fluorescence and internal dissipation are affected in the same propor- 

 tion by a change in the rate of the primary photoprocess only if the factor 

 that caused this change does not affect the rate constant of internal dissipation. 

 There is no reason why this should always be true. The a priori prob- 

 ability (rate constant) of internal conversion may be quite different in 

 the complexes X- Chi -HZ, HX-Chl-Z, HX-Chl-HZ and X-Chl-Z. If 

 one of the "photostable" complexes, such as HX-Chl-HZ, dissipates 

 the excitation energy much more efficiently than the "photosensitive" 

 complex X- Chi -HZ, the fluorescence-quenching effect of its accumulation 

 ma}^ overcompensate the fluorescence-stimulating effect of the suppression 



