THE INFLUENCE OF PHOTOSYNTHETICALLY ACTIVE RADIATION ON THE EFFECTS OF ULTRAVIOLET-B RADIATION ON ARABIDOPSIS-THALIANA

Ultraviolet-B (UVB; 280-320 mn) radiation is a small. but biologically significant portion of the solar spectrum reaching the earth's surfac e. Research interests have been fostered because UVB has been increasi ng in recent years due to depletion of stratospheric ozone. Ultraviole t-B that penetrates into plant tissue may damage important cellular ma cromolecules. Although there has been considerable research on the eff ects of WE on plants, the influence of the level of photosynthetically active radiation (PAR; 400-700 mm) on effects of WE requires further definition as a prelude to studies of UVB sensitivity and defense mech anisms, Arabidopsis thaliana wildtype ecotype Landsberg erecta (LER), which is relatively insensitive to UVB, and the relatively sensitive L ER-based mutant transparent testa-5 (tt5), were grown under 100 or 250 mu mol m(-2) s(-1) PAR and then exposed to 0 or 7 kJ m(-2) day(-1) UV BBE under these PAR levels. Plants exposed to UVB had reduced dry weig ht and leaf area and higher levels of UV-absorbing compounds in leaf t issue. The level of PAR did influence the effects of UVB, with the hig her level of PAR prior to UVB exposure reducing sensitivity of LER to UVB. In contrast to other studies, higher PAR supplied simultaneously with UVB increased rather than decreased sensitivity of both genotypes to UVB. These results demonstrate the importance of controlling and c omparing PAR levels when undertaking studies of UVB sensitivity, as ef fects of UVB on plants are influenced by the PAR levels plants are gro wing under prior to and during exposure to UVB.