Pheophytin reduction and fluorescence quenching in reaction centres of Photosystem II

Accumulation of reduced pheophytin (Pheo-) in photosystem II under illumination at low redox potential is known to be accompanied by a pronounced decrease of the chlorophyll fluorescence yield. Therefore, reduced pheophytin has been proposed to be an efficient fluorescence quencher. Simultaneous measurement of the fluorescence quenching and absorbance changes in photosystem II reaction centres in the presence of dithionite showed different temperature dependence of both events. While the fluorescence quenching is blocked below 200 K, Pheo- is accumulated and re-oxidised throughout the used temperature range (77 - 280 K). At 77 K, the fluorescence is quenched only in RC where Pheo- has been accumulated at room temperature before sample freezing. This shows that the fluorescence quenching and Pheo reduction are likely two separate processes. The fluorescence quenching above 200 K could results from a conformational change in the reaction centre which is initiated by pheophytin reduction. At lower temperatures the conformational change is blocked and the quenching is not observed.