O2-dependent electron flow,membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence

Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (q_P and q_N) by the saturation pulse method. This is part of an introductory talk at the Wageningen Meeting on The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of q_N. The role of O₂-dependent electron flow for pH- and q_N-formation is emphasized. Previous conclusions on the rate of pseudocyclic transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the q_N developed when CO₂-assimilation is limited. Dithiothreitol is shown to inhibit part of q_N-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 s luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of pH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.Abbreviations q_P coefficient of photochemical quenching - q_N coefficient of non-photochemical quenching - q_E coefficient of energy-dependent quenching - LED light emitting diode