INCREASED LEVELS OF MONODEHYDROASCORBATE RADICAL IN UV-B-IRRADIATED BROAD BEAN-LEAVES

Light-induced monodehydroascorbate (MDA) radical production was not de tectable by EPR spectroscopy in untreated broad bean leaves, but it wa s observed after exposing the leaves to UV-B (280-320 nm) irradiation. After this pre-treatment, a low level of MDA radicals was also detect able without illumination. Light-induced MDA, MDA production in the da rk, oxygen evolution, quantum yield of PSII as measured by Chi fluores cence, MDA producing and reducing enzyme activities were determined an d compared in leaves after irradiation with various doses of UVB. Our results suggest that UV-B irradiation disturbs the balance of MDA radi cal production and reduction, resulting in increased light induced MDA signal. The enhancement of ascorbate photooxidation at the UV-B damag ed donor side of PSII appears as a major factor in this process.