field-grown aspen and birch is linked to foliar ultrastructure and peroxisomal activity

• Saplings of three aspen ( Populus tremuloides ) genotypes and seedlings of paper birch ( Betula papyrifera ) were exposed to elevated ozone (1.5 × ambient) and 560 p.p.m. CO 2 , singly and in combination, from 1998 at the Aspen-FACE (free-air CO 2 enrichment) site (Rhinelander, USA). • The plants were studied for H 2 O 2 accumulation within the leaf mesophyll, number of peroxisomes, level of gene expression for catalase ( Cat ), and changes in ultrastructure. • In tolerant clones, ozone-elicited excess H 2 O 2 production was restricted to the apoplast, without any ultrastructural injuries. This was associated with ozoneinduced proliferation of peroxisomes and increased transcript levels of Cat . In sensitive plants, ozone-induced H 2 O 2 accumulation continued from the cell wall to the plasma membrane, cytosol and chloroplasts, particularly in older leaves. However, chloroplastic precipitation was absent in the presence of elevated CO 2 . In the most sensitive aspen clone, H 2 O 2 accumulation was found in conjunction with chloroplast injuries, low number of peroxisomes and low cell wall volume, whereas in birch a simultaneous increase in cell wall thickness indicated defence activation. • Our results indicate that oxidative stress manifests as H 2 O 2 effects on leaf ultrastructure in sensitive trees exposed to elevated ozone. However, CO 2 enrichment appears to alleviate chloroplastic oxidative stress.