Interactive effects of ultraviolet radiation and elevated CO concentration on photosynthetic characteristics of European beech saplings during the vegetation season.

To test the hypothesis that ultraviolet radiation (UV) modulates photosynthetic responses to elevated CO concentration ([CO]) in plants, saplings of European beech were grown for two vegetation seasons under ambient (400 ppm) and elevated (700 ppm) atmospheric [CO]. From April to November the saplings were exposed to (i) ambient UV radiation, (ii) excluded and (iii) enhanced UV (150% of ambient). Gas-exchange and chlorophyll fluorescence techniques were used throughout the second vegetation season together with biochemical analyses of the amount and activity of the Rubisco enzyme. We found support for the hypothesis that an impact of elevated [CO] on photosynthesis is substantially modulated by UV radiation. Moreover, we found that the [CO] × UV interaction is changing along the vegetation season: an enhanced UV radiation stimulated a positive effect of elevated [CO] on plant photosynthesis at the beginning of the vegetation season (short-term effect), whilst long-term cultivation reduced the stimulatory effect of elevated [CO] (a clear down-regulation of photosynthesis). Down-regulation was, however, not found in plants grown under the conditions of excluded UV radiation. We found evidence that the down-regulation of photosynthesis is associated with a complex acclimation at different hierarchical and functional levels, including an acclimation of primary photochemical reactions, carboxylation activity of Rubisco enzyme, and stomatal conductance.