Abstract:The isolated and purified photosystem Ⅱ (PS Ⅱ ) reaction center D1/D2/Cyt b559 complex was taken as the experimental system. It was observed that under anaerobic conditions, cytochrome b559 (Cyt b559) could be reduced by exposure to strong illumination, suggesting Cyt b559 could accept electrons directly from reduced pheophytin (Pheo-). And the photoreduction of Cyt b559 was irreversible. When the isolated D1/D2/Cyt b559 complex reconstituted with exogenous secondary electron acceptor 2,6-dimethyl-benzoquinone (DMBQ), the photoreduction of Cyt b559 was delayed in the function of illumination time. Meanwhile, the electrons transferred mainly through DMBQ and photoreduced Cyt b559 could be partially reoxidized in the dark incubation following illumination. It was concluded that the quinone-independent electron transfer via Cyt b559 was a new, secondary electron pathway, which represented one of the protective pathes for PS Ⅱ reaction center to dissipate excess excitation energy.