INTERPRETATION OF LIGHT CURVES OF FLUORESCENCE 1069 



The relationship between the yield of photosynthesis and the yield of 

 fluorescence has often been presented as a simple either-or; if this were 

 correct, then whenever P increases, F should decrease and vice versa. In 

 fact, however, (p is much too small (^^ 0.01) to be an effective competitor 

 of 7 (^:^ 1 for the primaiy process, although as many as 4 or 8 primary 

 processes may be needed to reduce one molecule of carbon dioxide). The 

 actual competition is between cp and 5, between the primary photochemical 

 reaction and the dissipation of energy. The yield of fluorescence can be 

 considered an index of the value to which these two much more efficient 

 processes together have reduced the life-time of the excited chlorophyll 

 molecules. In other words, we can write, in good approximation, instead 

 of (28.48) : 



(28.51) <p=^ kf/{k, + kd) and also y ^ kt/ikt + kd); 8 ^ kd/{kt + kd) 



Equation (28.51) shows that (p does not necessarily increase by the same 

 amount by which y decreases (or vice versa), since simultaneous changes 

 in 5 can change not only the absolute magnitude of the effect but even 

 its sign. If kt decreases — e. g., if the primary photochemical reaction be- 

 comes altogether impossible, while ka remains more or less unchanged, 

 fluorescence must of course increase; but if simultaneously with the de- 

 cline of ki, kd is strongly increased, in other words, the dissipation of light 

 energy is strongly accelerated by the change in the structure of the pig- 

 ment complex, the yield of fluorescence, cp, may decline parallel with the 

 jdeld of the photochemical transformation, 8. 



In discussing the theory of light curves of photosynthesis in section A7 

 of this chapter, we assumed that normally the photosensitive chlorophyll 

 complex has the composition X • Chi • HZ (or ACO2 • Chi • A'HR, if we postu- 

 late direct association of chlorophyll with bound carbon dioxide and the 

 reductant). In inten.se light or when one or both of the reactants, CO2 

 and HR, are absent, or when the preparatory catalysts are poisoned, the 

 chlorophyll complex may go over predominantly into a tautomerized or 

 chemically changed state. If the normal state is X- Chi -HZ, the likely 

 changed states are HX-Chl-Z (tautomeric), HX-Chl-HZ (reduced state) 

 and X-Chl-Z (oxidized state); if the normal state is AC02-Chl-A'RH, 

 the possibilities include, in addition to the tautomeric, oxidized and reduced 

 states, also three "starved" states, namely A-Chl-A'R (carbon dioxide- 

 starved), AC02-Chl-A' (reductant-starved) and A -Chi -A' (totally starv^ed 

 .state) . 



Furthermore, one can envisage states in which CO2 or RH are not merely 

 missing, but are replaced by other molecules — either also suitable to serve 

 as hydrogen acceptors or donors (such as oxygen, or the "substitute oxi- 



