Structural characteristics and chemical composition of birch (Betula pendula) leaves are modified by increasing CO2 and ozone

Impacts of ozone and CO2 enrichment, alone and in combination, on leaf anatomical and ultrastructural characteristics, nutrient status and cell wall chemistry in two European silver birch (Betula pendula Roth) clones were studied. The young soil-growing trees were exposed in open-top chambers over three growing seasons to 2 x ambient CO2 and/or ozone concentrations in central Finland. The trees were measured for changes in altogether 35 variables of leaf structure, nutrients and cell wall chemistry of green leaves, and 20 of the measured variables were affected by CO2 and/or O-3. Elevated CO2 increased the size of chloroplasts and starch grains, number of mitochondria, P : N ratio, and contents of cell wall hemicellulose. Elevated CO2 decreased the total leaf thickness, specific leaf area, concentrations of N, K, Cu, S and Fe, and contents of cell wall alpha-cellulose, uronic acids, acid-soluble lignin and acetone-soluble extractives. Elevated ozone led to thinner leaves, higher palisade to spongy ratio, increased number of peroxisomes and mitochondria, reduced content of Mn, Zn, Cu, hemicellulose and uronic acids, and lower Mn : N and Zn : N ratios. In the combined exposure, interactions were antagonistic. Ultrastructural changes became more evident towards the end of the exposure. Young leaves were tolerant against ozone-caused oxidative stress, whereas oxidative H2O2 accumulation was found in older leaves. CO2 enrichment improved ozone tolerance not only through increased photosynthesis rates, but also through changes in cell wall chemistry (hemicellulose, in particular). However, nutrient imbalances due to ozone and/or CO2 may predispose the trees to other biotic and abiotic stresses. Down-regulation and up-regulation of photosynthesis under elevated CO2 through anatomical changes is discussed.