Photochemical and biochemical changes in wheat seedlings exposed to supplementary ultraviolet-B radiation

Abstract

The effect of UV-B radiation (312 nm; 1 mW cm(-2)) was studied on net photosynthesis, chlorophyll fluorescence and changes in flavonoid and carotenoid contents in wheat seedlings. Control plants (without UV-B treatment but identical light and temperature regime as for UV-B treatment) and UV-B treated plants were grown in two separate growth chambers for 15 days. The supplementary UV-B radiation caused a significant decrease in net photosynthesis which was much greater than could be explained by limitation of stomatal conductance. Initial fluorescence (F-o), F-w/F-m and photochemical quenching (q(p)) and non-photochemical quenching (q(N)) of chlorophyll fluorescence did not change due to 5 days of UV-B treatment, but longer treatment (up to 15 days) increased F-o while decreasing the F-w/F-m ratio. q(p) and q(N) also decreased after 15 days of UV-B exposure. Changes in UV-B absorbing phenolic compounds such as flavonol (kaempferol), caumarin and anthocyanin were observed, whereas cinnamic acid was only synthesised after 4 days of UV-B treatment. No zeaxanthin formation was observed while neoxanthin showed a decrease in longer UV-B treatment. Violaxanthin showed an increase in early treatment but decreased after 15 days of UV-B exposure. Our results suggest two types of damage, one within 4-5 days of UV-B exposure and another with longer than 5 days exposure.