AMELIORATION OF UV-B DAMAGE UNDER HIGH IRRADIANCE .2. ROLE OF BLUE-LIGHT PHOTORECEPTORS

Sensitivity of plants to UV-B radiation (280-315 nm) is often reduced at high background irradiance. Interpretation of plant responses to po tential increases in solar UV-B requires improved understanding of int eractions between UV-B and other environmental parameters. In this stu dy, photosynthetically active radiation (PAR, 400-700 nm) was kept app roximately 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). T wo lines of cucumber (cvs Ashley and Poinsett) with differential sensi tivity 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 sev ere inhibition of growth in a developing leaf in both cultivars. Growt h effects were detectable sooner and were accompanied by chlorotic les ions in the sensitive cultivar (cv Poinsett). Supplemental BL progress ively reduced symptoms, consistent with an important role for BL photo receptor(s) in prevention or repair of UV-B damage. Ultraviolet-induce d increases in UV-absorbing compounds on an area basis were significan t within 24 h of the start of the treatment but were independent of BL fluence over the range tested, suggesting that bulk accumulation of s creening pigments did not contribute to BL-dependent amelioration of U V damage. However, BL did stimulate net increases in extractable UV-ab sorbing compounds from an important role in overall UV-B protection no r do they rule out some more specific function for these compounds (e. g. antioxidants). Finally, BL effects on UV-B alteration of leaf growt h and accumulation of UV-absorbing compounds were not saturated under the conditions used here, suggesting that BL may contribute to interac tions between UV-B and natural levels of background irradiance. Cautio n is urged in the interpretation of data on UV-B effects obtained unde r conditions of low BL irradiance.