Responses of leaf traits of European beech ( Fagus sylvatica L.) saplings to supplemental UV-B radiation and UV-B exclusion

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This study describes the physiological responses of European beech saplings to varying levels of UV-B radiation under natural environmental conditions and in an outdoor pot experiment. In the field experiment, saplings at a forest site regenerating naturally in a clear-cut area were subjected to UV-B exclusion (UV-B exc). In the outdoor pot experiment saplings were subjected to both exclusion of UV-B and enhanced UV-B (UV-B enh). Three months after the start of the treatments, plants were monitored for photosynthetic pigment composition, chlorophyll fluorescence parameters, specific leaf mass (SLM, dry matter per unit leaf area) and UV-B absorbing pigments (flavonoids). In the forest site the leaves which developed under UV-B exclusion had significantly higher chlorophyll and water contents than under ambient UV-B (UV-B amb). Both the total pool size of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin: VAZ-pool) and the de-epoxidation state (DEEPS) of these pigments and the flavonoid content and specific leaf mass were significantly lower at midday under UV-B exc. UV-B exclusion moderated the changes in several leaf traits between morning and midday. In the pot experiment UV-B exc caused smaller, but significant, increase in chlorophyll content and decrease in flavonoid content as compared to UV-B amb than in forest site experiment. UV-B enh resulted in activation of photoprotective mechanisms (significant increases of total carotenoid content, VAZ-pool, DEEPS, flavonoid accumulation), but significantly lowered the concentration of chlorophylls and water content, while it slightly increased the Chl_a/b ratio as compared to the UV-B amb. Larger midday reductions of potential photochemical activity of PSII assessed as F v/ F m, ratio of variable chlorophyll fluorescence ( F v, difference between maximum ( F m) and ground fluorescence F 0) to maximum fluorescence ( F m) in dark-adapted leaves and leaf water content occurred under UV-B enh than under UV-B amb and UV-B exc. Altogether, greater differences were observed in leaf traits between UV-B amb and UV-B exc than between UV-B enh and UV-B amb. Chlorophyll fluorescence parameters of dark adapted leaves were not significantly affected by UV-B exc in either experiment. UV-B absorbing compounds responded most to the different UV-B levels, which may explain the practically unaltered photosynthetic activity as these compounds, due to their antioxidant properties, can reduce lipid peroxidation and damage to the photosynthetic apparatus.