Ozone-induced H O accumulation in 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 , singly and in combination, from 1998 at the Aspen-FACE (free-air CO enrichment) site (Rhinelander, USA). •  The plants were studied for H O 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 O production was restricted to the apoplast, without any ultrastructural injuries. This was associated with ozone-induced proliferation of peroxisomes and increased transcript levels of Cat. In sensitive plants, ozone-induced H O 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 . In the most sensitive aspen clone, H O 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 O effects on leaf ultrastructure in sensitive trees exposed to elevated ozone. However, CO enrichment appears to alleviate chloroplastic oxidative stress.