A THEORETICAL AND EXPERIMENTAL-ANALYSIS OF THE QP AND QN COEFFICIENTS OF CHLOROPHYLL FLUORESCENCE QUENCHING AND THEIR RELATION TO PHOTOCHEMICAL AND NONPHOTOCHEMICAL EVENTS

The initial (F(O)), maximal (F(M)) and steady-state (F(S)) levels of chlorophyll fluorescence emitted by intact pea leaves exposed to various light intensities and environmental conditions, were measured with a modulated fluorescence technique and were analysed in the context of a theory for the energy fluxes within the photochemical apparatus of photosynthesis. The theoretically derived expressions of the fluorescence signals contain only three terms, X = J2P2F/(1 - G), Y = T/(1 - G) and V, where V is the relative variable fluorescence, J2 is the light absorption flux in PS II, P2F is the probability of fluorescence from PS II, G and T are, respectively, the probabilities for energy transfer between PS II units and for energy cycling between the reaction center and the chlorophyll pool: F(O) = X, F(M) = X/(1 - Y) and F(S) = X(1 + (YV/(1 - Y))). It is demonstrated that the amplitudes of the previously defined coefficients of chlorophyll fluorescence quenching, q(P) and q(N), reflect, not just photochemical (q(P)) or nonphotochemical (q(N)) events as implied in the definitions, but both photochemical and nonphoto-chemical processes of PS II deactivation. The coefficient q(P) is a measure of the ratio between the actual macroscopic quantum yield of photochemistry in PS II (phi-(P) in a given light state and its maximal value measured when all PS II traps are open (phi-P(open) in that state, with phi-P(open) = F(M)-F(O))/F(M) and phi-P = 1 - (F(S)/F(M)). When the partial connection between PS II units is taken into consideration, 1 - q(P) is nonlinearily related to the fraction of closed reaction centers and is dependent on the rate constants of all (photochemical as well as nonphotochemical) exciton-consuming processes in PS II. On the other hand, 1 - q(N) equals the (normalized) ratio of the rate constant of photochemistry (k2b) to the combined rate constant (k(N) of all the nonphotochemical deactivation processes excluding the rate constant k22 of energy transfer between PS II units. It is demonstrated that additional (qualitative) information on the individual rate constants, k(N)-k22 and k2b, is provided by the fluorescence ratios 1/F(M) and (1/F(o)) - (1/F(M)), respectively. Although, in theory, phi-P(open) is determined by the value of both k2b and k(N)-k22, experimental results presented in this paper show that, under various environmental conditions, phi-P(open) is modulated largely through changes in k(N), confirming the idea that PS II quantum efficiency is dynamically regulated in vivo by nonphotochemical energy dissipation.