| Abstract: |
Accelerated leaf senescence is one of the harmful effects of elevated tropospheric ozone concentrations ([O3]) on plants. The number of studies dealing with mature forest trees is scarce however. Therefore, five 66-year-old beech trees (Fagus sylvatica L.) have been exposed to twice-ambient (2×ambient) [O3] levels by means of a free-air canopy O3 exposure system. During the sixth year of exposure, the hypothesis of accelerated leaf senescence in 2×ambient [O3] compared with ambient [O3] trees was tested for both sun and shade leaves. Chlorophyll (chl) fluorescence was used to assess the photosynthetic quantum yield, and chl fluorescence images were processed to compare functional leaf homogeneity and the proportion of O3-injured leaf area (stipples) under ambient and 2×ambient [O3] regimes. Based on the analysis of chl fluorescence images, sun leaves of both ambient and 2×ambient [O3] trees had apparently developed typical necrotic O3 stipples during high O3 episodes in summer, while accelerated senescence was only observed with sun leaves of 2×ambient [O3] trees. This latter effect was indicated along with a faster decrease of photosynthetic quantum yield, but without evidence of changes in non-photochemical quenching. Overall, treatment effects were small and varied among trees. Therefore, compared with ambient [O3], the consequence of the observed O3-induced accelerated leaf senescence for the carbon budget is likely limited. |
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