Studies on the mechanism of the fluorescence decline induced by strong actinic light in PS II particles under different redox conditions

AbstractFluorescence changes induced by strong actinic light at room temperature were measured in isolated photosystem II (PS II) particles under different redox conditions. Comparative measurements of the absorption changes accompanying the light-induced fluorescence decay show that in the absence of Na2S2O4 the formation of a chlorophyll cation radical (probably together with oxidized carotenoid) causes a quenching while in the presence of Na2S2O4 photoaccumulation of the pheophytin anion radical (Pheo.−) takes place. The 695 nm band of the 77 K emission spectrum becomes specifically reduced if Pheo.− accumulates in the reaction center, whereas in the case of quenching by the cation radicals the ratio of the emission peaks at 685 nm and 695 nm remains constant. The present data favor the hypothesis that F-695 originates from the PS II reaction center [FEBS Lett. (1982) 147, 16–20]. If the primary plastoquinone acceptor (PQA) stayed oxidized in the dark before the onset of the illumination. Pheo.− photoaccumulation was not observed. This effect is explained by the existence of at least one further redox component which is able to accept electrons efficiently from Pheo.−. The proposed model also explains the differences in the redox titration curves of the electrochromic bandshift and the inital fluorescence, respectively