IMPACT OF UVB AND O-3 ON THE OXYGEN-FREE RADICAL SCAVENGING SYSTEM INARABIDOPSIS-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 er ecta (LER) and its transparent testa (tt5) mutant differing in UVB sen sitivity. Ultraviolet-B radiation decreased dry matter production of t t5, while the dry weight of LER remained unaltered. Ozone exposure dec reased 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 de creased the F-v/F-m ratio in both genotypes. Ultraviolet-B radiation e nhanced total ascorbic acid, total glutathione and their redox state a nd superoxide dismutase and glutathione reductase activities in both g enotypes and the increases were greater in tt5 compared to UVB-irradia ted LER. Although O, exposure enhanced total ascorbic acid and total g lutathione 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 biosynthe sis are sensitive to UVB in spite of their ability to maintain efficie nt oxygen free radical scavenging systems and (2) plants sensitive to UVB are comparatively tolerant of O, compared to UVB-insensitive plant s. The differential responses of plants are discussed with reference t o their ability to maintain high redox states of ascorbate and glutath ione.