IMPACT OF UVB AND O-3 ON THE OXYGEN-FREE RADICAL SCAVENGING SYSTEM IN ARABIDOPSIS-THALIANA GENOTYPES DIFFERING IN FLAVONOID BIOSYNTHESIS

The effect of UVB and ozone (O-3) on growth and ascorbate-glutathione cycle were investigated in Arabidopsis thaliana wild-type Landsberg erecta (LER) and its transparent testa (tt5) mutant differing in UVB sensitivity. Ultraviolet-B radiation decreased dry matter production of tt5, while the dry weight of LER remained unaltered. Ozone exposure decreased dry weight of both genotypes. Ultraviolet-B radiation decreased the F-v/F-m ratio in tt5 but not in LER plants, while O-3 exposure decreased the F-v/F-m ratio in both genotypes. Ultraviolet-B radiation enhanced total ascorbic acid, total glutathione and their redox state and superoxide dismutase and glutathione reductase activities in both genotypes and the increases were greater in tt5 compared to UVB-irradiated LER. Although O, exposure enhanced total ascorbic acid and total glutathione in both genotypes, the redox state was significantly higher in tt5. Ozone exposure enhanced superoxide dismutase and glutathione reductase activities in tt5 while there were no major changes in LER. These results suggested that (1) plants blocked in flavonoid biosynthesis are sensitive to UVB in spite of their ability to maintain efficient oxygen free radical scavenging systems and (2) plants sensitive to UVB are comparatively tolerant of O, compared to UVB-insensitive plants. The differential responses of plants are discussed with reference to their ability to maintain high redox states of ascorbate and glutathione.