Abstract
Illumination of isolated thylakoid membranes by UV-B light does not result in singlet oxygen production but induces free radicats, mainly hydroxyl and carbon-centered (methyl-like) ones. The absence of singlet oxygen demonstrates that the primary site of UV-B induced electron transport impariment in photosystem II is different from that of photoinhibition by excess photosynthetically active radiation. Membrane preparations from UV-B pre-illuminated leaves feature hydroxyl, carbon-centered and peroxyl radicals in the thylakoid enriched pellet and increased production of ascorbate radicals in the super-natant fraction. It is concluded that UV-B irradiation elicits multilevel oxidative stress. Besides resulting in immediate free radical production, observable in UV-B irradiated thylakoid membranes, it also initiates radical yielding reactions detectable in leaves even minutes after the cessation of the irradiation.
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