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The effect of UVB (280-320 nm radiation) and ozone (0,) on growth, photosynthetic pigments, ribulose bisphosphate carboxylase/oxygenase (rubisco) activity and rubisco protein were investigated in Arabidopsis thaliana genotypes wild type Landsberg erecta (LER) and tt5, a flavonoid-deficient mutant. The UVB exposure for 5 days decreased whole plant dry weight of only tt5 plants, while O3 exposure decreased the whole plant dry weight of both genotypes. The UVB exposure enhanced chlorophylls and carotenoids in both genotypes while 0, exposure decreased photosynthetic pigments in both genotypes. Both UVB and 0, exposure enhanced UV-absorbing compounds in LER but not in tt5. Ultraviolet-B exposure 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 total rubisco activities in both genotypes, and the magnitudes of decrease were greater on total rubisco activity, resulting in enhanced levels of percent activatible rubisco. Immunoblot analysis performed with antibodies 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 leaf area necrotic). In contrast, 0, exposure of both the genotypes decreased the levels of rubisco LSU and SSU before the appearance of visible symptoms of injury. These results suggested that UVB-induced limitations of growth are independent of changes in rubisco protein while 0,-induced growth limitations appeared to be due to a significant reduction in rubisco protein 777

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