Effects of chronic elevated ozone exposure on gas exchange responses of adult beech trees (Fagus sylvatica) as related to the within-canopy light gradient

The effects of elevated O-3 on photosynthetic properties in adult beech trees (Fagus sylvatica) were investigated in relation to leaf mass per area as a measure of the gradually changing, within-canopy light availability. Leaves under elevated O-3 showed decreased stomatal conductance at unchanged carboxylation capacity of Rubisco, which was consistent with enhanced delta C-13 of leaf organic matter, regardless of the light environment during growth. In parallel, increased energy demand for O-3 detoxification and repair was suggested under elevated O-3 owing to enhanced dark respiration. Only in shade-grown leaves, light-limited photosynthesis was reduced under elevated O-3, this effect being accompanied by lowered F-v/F-m. These results suggest that chronic O-3 exposure primarily caused stomatal closure to adult beech trees in the field regardless of the within-canopy light gradient. However, light limitation apparently raised the O-3 sensitivity of photosynthesis and accelerated senescence in shade leaves. (C) 2008 Elsevier Ltd. All rights reserved.