Ameliorating effect of UV-B radiation on the response of Norway spruce andScots pine to ambient ozone concentrations

Elevated levels of both ozone and UV-B radiation are typical for high-altit ude sites. Few studies have investigated their possible interaction on plan ts. This study reports interactive effects of O-3 and UV-B radiation in fou r-year-old Norway spruce and Scots pine trees. The trees were cultivated in controlled environmental facilities under simulated climatic conditions re corded on Mt Wank, an Alpine mountain in Bavaria, and were exposed for one growing season to simulated ambient or twice-ambient ozone regimes at eithe r near ambient or near zero UV-B radiation levels. Chlorotic mottling and y ellowing of current year needles became obvious under twice-ambient O-3 in both species at the onset of a high ozone episode in July. Development of c hlorotic mottling in relation to accumulated ozone concentrations over a th reshold of 40 nL L-1 was more pronounced with near zero rather than ambient UV-B radiation levels. In Norway spruce, photosynthetic parameters at ambi ent CO2 concentration, measured at the end of the experiment, were reduced in trees cultivated under twice-ambient O-3, irrespective of the UV-B treat ment. Effects on photosynthetic capacity and carboxylation efficiency were restricted to trees exposed to near zero levels of UV-B radiation, and twic e-ambient O-3. The data indicate that UV-B radiation, applied together with O-3, ameliorates the detrimental effects of O-3. The data also demonstrate that foliar symptoms develop more rapidly in Scots pine than in Norway spr uce at higher accumulated ozone concentrations.