AMELIORATION OF UV-B DAMAGE UNDER HIGH IRRADIANCE .1. ROLE OF PHOTOSYNTHESIS

Sensitivity to ultraviolet-B radiation (UV-B, 280-315 nm) is generally reduced when background irradiance is high. We tested the involvement of photosynthesis in the amelioration of UV-B damage by treating plants at high PAR (photosynthetically-active radiation. 400-700 nm; 1000 mumol m-2 s-1) with supplemental UV-B at double ambient levels of biologically-effective radiation (18 kJ m-2 d-1) and either ''ambient'' (450 mumol mol-1) or short term elevated (750 mumol mol-1) CO2 levels. Responses to UV-B were assessed by photosynthetic gas exchange, leaf expansion and production of UV-absorbing compounds (presumptive flavonoids) in cultivars of cucumber (Cucumis sativus L.) previously demonstrated to be relatively sensitive (cv. Poinsett) and insensitive (cv. Ashley) to UV-B. Except for marginal leaf interveinal chlorosis observed in Poinsett, both cultivars responded similarly. UV-B had little direct effect on leaf photosynthesis, but it did cause reductions in leaf area and corresponding increases in leaf dry matter per area. Increased CO2 stimulated plant growth, counteracting the effect of UV-B on leaf growth and indicating an important role for photosynthesis. In contrast, the accumulation of UV-absorbing flavonoid compounds was enhanced by UV-B exposure but was not affected by CO2 enrichment.