UV-B INDUCED FREE-RADICAL PRODUCTION IN PLANT-LEAVES AND ISOLATED THYLAKOID MEMBRANES

Illumination of isolated thylakoid membranes by UV-B light does not re sult in singlet oxygen production but induces free radicals, mainly hy droxyl and carbon-centered (methyl-like) ones. The absence of singlet oxygen demonstrates that the primary site of UV-B induced electron tra nsport impairment in photosystem II is different from that of photoinh ibition by excess photosynthetically active radiation. Membrane prepar ations from UV-B pre-illuminated leaves feature hydroxyl, carbon-cente red and peroxyl radicals in the thylakoid enriched pellet and increase d production of ascorbate radicals in the supernatant fraction, It is concluded that UV-B irradiation elicits multilevel oxidative stress. B esides resulting in immediate free radical production, observable in W -B irradiated thylakoid membranes, it also initiates radical yielding reactions detectable in leaves even minutes after the cessation of the irradiation.