Quantum yields and rate constants of photochemical and nonphotochemical excitation quenching - Experiment and model

Sunflower (Helianthus annuus L.), cotton (Gossypium hirsutum L.), tobacco (Nicotiana tabacum L.), sorghum (Sorghum bicolor Moench.), amaranth (Amaranthus cruentus L.), and cytochrome b(6)f complex-deficient transgenic tobacco leaves were used to test the response of plants exposed to different light intensities and CO2 concentrations before and after photoinhibition at 4000 mu mol photons m(-2) s(-1) and to thermoinhibition up to 45 degrees C. Quantum yields of photochemical and nonphotochemical excitation quenching (Y-P and Y-N) and the corresponding relative rate constants for excitation capture from the antenna-primary radical pair equilibrium system (k(P)' and k(N)') were calculated from measured fluorescence parameters. The above treatments resulted in decreases in Y-P and k(P)', and in approximately complementary increases in Y-N and k(N)', under normal and inhibitory conditions. The results were reproduced by a mathematical model of electron/proton transport and O-2 evolution/CO2 assimilation in photosynthesis based on budget equations for the intermediates of photosynthesis. Quantitative differences between model predictions and experiments are explainable, assuming that electron transport is organized into domains that contain relatively complete electron and proton transport chains (e.g. thylakoids). With the complementation that occurs between the photochemical and nonphotochemical excitation quenching, the regulatory system can constantly maintain the shortest lifetime of excitation necessary to avoid the formation of chlorophyll triplet states and singlet oxygen.