AMELIORATION OF UV-B DAMAGE UNDER HIGH IRRADIANCE. II: ROLE OF BLUE LIGHT PHOTORECEPTORS

Abstract Sensitivity of plants to UV-B radiation (280–315 nm) is often reduced at high background irradiance. Interpretation of plant responses to potential increases in solar UV-B requires improved understanding of interactions between UV-B and other environmental parameters. In this study, photosynthetically active radiation (PAR, 400–700 nm) was kept approximately constant (38 mol m-² per day) while the daily blue light fluence (BL, 400–500 nm) was varied between 0.23 and 2.68 mol m-². Two lines of cucumber (cvs Ashley and Poinsett) with differential sensitivity to UV-B were compared. At low BL, 3 days of UV-B treatment (21 kJ m-² biologically effective radiation per 10 h per day) caused severe inhibition of growth in a developing leaf in both cultivars. Growth effects were detectable sooner and were accompanied by chlorotic lesions in the sensitive cultivar (cv Poinsett). Supplemental BL progressively reduced symptoms, consistent with an important role for BL photoreceptor(s) in prevention or repair of UV-B damage. Ultraviolet-induced increases in UV-absorbing compounds on an area basis were significant within 24 h of the start of the treatment but were independent of BL fluence over the range tested, suggesting that bulk accumulation of screening pigments did not contribute to BL-dependent amelioration of UV damage. However, BL did stimulate net increases in extractable UV-absorbing compounds on a total leaf busis, while high-performance liquid chromatography analysis indicated that BL and UV-B acted synergistically to increase specific components. Thus, the data do not necessarily exclude UV-absorbing compounds from an important role in overall UV-B protection nor do they rule out some more specific function for these compounds (e.g. antioxidants). Finally, BL effects on UV-B alteration of leaf growth and accumulation of UV-absorbing compounds were not saturated under the conditions used here, suggesting that BL may contribute to interactions between UV-B and natural levels of background irradiance. Caution is urged in the interpretation of data on UV-B effects obtained under conditions of low BL irradiance.