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

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-Bexc). In the outdoor pot experiment saplings were subjected to both exclusion of UV-B and enhanced UV-B (UV-Benh). 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-Bamb). 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-Bexc. UV-B exclusion moderated the changes in several leaf traits between morning and midday. In the pot experiment UV-Bexc caused smaller, but significant, increase in chlorophyll content and decrease in flavonoid content as compared to UV-Bamb than in forest site experiment. UV-Benh 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-Bamb. Larger midday reductions of potential photochemical activity of PSII assessed as Fv/Fm, ratio of variable chlorophyll fluorescence (Fv, difference between maximum (Fm) and ground fluorescence F0) to maximum fluorescence (Fm) in dark-adapted leaves and leaf water content occurred under UV-Benh than under UV-Bamb and UV-Bexc. Altogether, greater differences were observed in leaf traits between UV-Bamb and UV-Bexc than between UV-Benh and UV-Bamb. Chlorophyll fluorescence parameters of dark adapted leaves were not significantly affected by UV-Bexc 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.