DIFFERENTIAL RESPONSE OF PHOTOSYNTHETIC PIGMENTS, RUBISCO ACTIVITY AND RUBISCO PROTEIN OF ARABIDOPSIS-THALIANA EXPOSED TO UVB AND OZONE

The effect of UVB (280-320 nm radiation) and ozone (O-3) on growth, ph otosynthetic pigments, ribulose bisphosphate carboxylase/oxygenase (ru bisco) activity and rubisco protein were investigated in Arabidopsis t haliana genotypes wild type Landsberg erecta (LER) and tt5, a flavonoi d-deficient mutant. The UVB exposure for 5 days decreased whole plant dry weight of only tt5 plants, while O-3 exposure decreased the whole plant dry weight of both genotypes. The UVB exposure enhanced chloroph ylls and carotenoids in both genotypes while O-3 exposure decreased ph otosynthetic pigments in both genotypes. Both UVB and O-3 exposure enh anced UV-absorbing compounds in LER but not in tt5. Ultraviolet-B expo sure decreased initial and total rubisco activities only in tt5 plants , which contained smaller amounts of UV-absorbing pigments. The effect of UVB was greater on initial rubisco activity resulting in decreased percent activatible rubisco. Ozone exposure decreased initial and tot al rubisco activities in both genotypes, and the magnitudes of decreas e were greater on total rubisco activity, resulting in enhanced levels of percent activatible rubisco, Immunoblot analysis performed with an tibodies raised against rubisco large subunit (LSU) and rubisco small subunit (SSU)showed no major changes in the levels of rubisco protein of either genotype irradiated with UVB. However, both rubisco LSU and SSU decreased in tt5 plants exposed to UVB for 7 days (70% of total le af area necrotic). In contrast, O-3 exposure of both the genotypes dec reased the levels of rubisco LSU and SSU before the appearance of visi ble symptoms of injury. These results suggested that UVB-induced limit ations of growth are independent of changes in rubisco protein while O -3-induced growth limitations appeared to be due to a significant redu ction in rubisco protein.