Ozone exposure decreases UVB sensitivity in a UVB-sensitive flavonoid mutant of Arabidopsis

The impact of sequential exposure to ozone (O3) and UVB (290-320 nm) was studied using two genotypes of Arabidopsis thaliana differing in UVB sensitivity. The negative impact of UVB on dry matter production and photosynthetic pigments was absent in the ecotype Landsberg erecta (LER), while the negative impact of UVB was more pronounced when LER plants pre-exposed to O3 were irradiated with UVB. However, the growth of tt5 plants (a mutant virtually incapable of synthesizing flavonoids) was significantly affected by the UVB exposures, while the impact of UVB was significantly counteracted when tt5 plants pre-exposed to O3 were irradiated with UVB. These results suggest that pre-exposure to O3 decreased sensitivity of tt5 but increased sensitivity of LER to UVB. Concentrations of UV-absorptive compounds were almost the same in plants exposed to UVB alone or sequentially to O3 and UVB. Exposures of LER and tt5 to UVB enhanced both ascorbic acid and glutathione as well as their redox state compared to control plants. Pre-exposure to O3 enhanced the total ascorbic acid and glutathione as well as the redox state of ascorbate and glutathione in tt5 but decreased the redox state in LER. Irradiation of plants pre-exposed to O3 with UVB enhanced the redox state of ascorbate and glutathione slightly in tt5 but decreased it further in LER. The high redox state of ascorbate and glutathione in tt5 pre-exposed to O3 would have protected plants from UVB and decreased their sensitivity to UVB in spite of their inability to synthesize flavonoids.(ABSTRACT TRUNCATED AT 250 WORDS)