The role of d 1* in light-induced d 1 protein turnover in leaves

Light-induced degradation of the D 1 protein of photosystem II (PS II) was determined by radioactive pulse-chase labelling experiments in intact leaves of Schefflera polybotrya. PS II photochemical efficiency was monitored by measuring chlorophyll fluorescence. A significant and consistent decline in the F J F m ratio was taken to indicate photoinhibition. The formation and degradation of a modified form of the D 1 protein, D 1, was different under photoinhibitory or non-photoinhibitory light conditions. At photoinhibitory irradiance greater amounts of D 1 were formed relative to D , and the degradation of D 1 was slower when compared with non-photoinhibitory irradiance. The formation and degradation of D 1 were therefore shown to be at least partly light intensity dependent. Higher light intensities appeared to slow D 1 degradation, which suggests a modification in PS II turnover properties