PHOTOCHEMICAL AND BIOCHEMICAL-CHANGES IN WHEAT SEEDLINGS EXPOSED TO SUPPLEMENTARY ULTRAVIOLET-B RADIATION

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 t reatment but identical light and temperature regime as for UV-B treatm ent) and UV-B treated plants were grown in two separate growth chamber s for 15 days. The supplementary UV-B radiation caused a significant d ecrease in net photosynthesis which was much greater than could be exp lained by limitation of stomatal conductance. Initial fluorescence (F- o), F-w/F-m and photochemical quenching (q(p)) and non-photochemical q uenching (q(N)) of chlorophyll fluorescence did not change due to 5 da ys 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 af ter 15 days of UV-B exposure. Changes in UV-B absorbing phenolic compo unds such as flavonol (kaempferol), caumarin and anthocyanin were obse rved, whereas cinnamic acid was only synthesised after 4 days of UV-B treatment. No zeaxanthin formation was observed while neoxanthin showe d 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 expos ure and another with longer than 5 days exposure. (C) 1998 Elsevier Sc ience Ireland Ltd. All rights reserved.