THE ROLE OF D1-ASTERISK IN LIGHT-INDUCED D1 PROTEIN-TURNOVER IN LEAVES

Light-induced degradation of the D1 protein of photosystem II (PS II) was determined by radioactive pulse-chase labelling experiments in int act leaves of Schefflera polybotrya. PS II photochemical efficiency wa s monitored by measuring chlorophyll fluorescence. A significant and c onsistent decline in the F(v)/F(m) ratio was taken to indicate photoin hibition. The formation and degradation of a modified form of the D1 p rotein, D1, was different under photoinhibitory or non-photoinhibitor y light conditions. At photoinhibitory irradiance greater amounts of D 1 were formed relative to D1, and the degradation of D1* was slower w hen compared with non-photoinhibitory irradiance. The formation and de gradation of D1 were therefore shown to be at least partly light inte nsity dependent. Higher light intensities appeared to slow D1 degrada tion, which suggests a modification in PS II turnover properties.