Increased Levels of Monodehydroascorbate Radical in UV-B-Irradiated Broad Bean Leaves

Light-induced monodehydroascorbate (MDA) radical production was not detectable by EPR spectroscopy in untreated broad bean leaves, but it was observed after exposing the leaves to UV-B (280–320 nm) irradiation. After this pre-treatment, a low level of MDA radicals was also detectable without illumination. Light-induced MDA, MDA production in the dark, oxygen evolution, quantum yield of PSII as measured by Chi fluorescence, MDA producing and reducing enzyme activities were determined and compared in leaves after irradiation with various doses of UV-B. Our results suggest that UV-B irradiation disturbs the balance of MDA radical production and reduction, resulting in increased light induced MDA signal. The enhancement of ascorbate photooxidation at the UV-B damaged donor side of PSII appears as a major factor in this process.