POSSIBLE PHOTOACOUSTIC DETECTION OF CYCLIC ELECTRON-TRANSPORT AROUND PHOTOSYSTEM-II IN PHOTOINHIBITED THYLAKOID PREPARATIONS

This study was undertaken to compare the kinetics of chlorophyll a fluorescence, of Photosystem II and Photosystem I electron transport activity, of the number of atrazine-binding sites, and of the photoacoustic signal during photoinhibition of rye (Secale cereale L.) thylakoids. Once all Photosystem II activity and all chlorophyll a variable fluorescence (F(v)) were lost, a substantial amount of energy was still stored in thylakoid membranes, suggesting photosynthetic activity. A significant fraction of atrazine-binding sites remained functional. Both the number of atrazine-binding sites and the amount of energy stored in the thylakoids showed a slow decrease once all Photosystem II activity was lost. The remaining energy storage is suggested to measure photosynthetic activity in Photosystem II beta centres, once all Photosystem II alpha centres are inactivated. These Photosystem II beta centres could store energy via a cyclic electron pathway involving cytochrome b-559.