Impacts of ozone and CO 2 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 Â ambient CO 2 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 CO 2 and/or O 3 . Elevated CO 2 increased the size of chloroplasts and starch grains, number of mitochondria, P : N ratio, and contents of cell wall hemicellulose. Elevated CO 2 decreased the total leaf thickness, specific leaf area, concentrations of N, K, Cu, S and Fe, and contents of cell wall a-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 H 2 O 2 accumulation was found in older leaves. CO 2 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 CO 2 may predispose the trees to other biotic and abiotic stresses. Down-regulation and up-regulation of photosynthesis under elevated CO 2 through anatomical changes is discussed.

We studied the effects of elevated concentrations of carbon dioxide ([CO2]) and ozone ([O3]) on growth, biomass allocation and leaf area of field-grown O3-tolerant (Clone 4) and O3-sensitive clones (Clone 80) of European silver birch (Betula pendula Roth) trees during 1999-2001. Seven-year-old trees of Clones 4 and 80 growing outside in open-top chambers were exposed for 3 years to the following treatments: outside control (OC); chamber control (CC); 2 x ambient [CO2] (EC); 2 x ambient [O3] (EO); and 2 x ambient [CO2] + 2 x ambient [O3] (EC+EO). When the results for the two clones were analyzed together, elevated [CO2] increased tree growth and biomass, but had no effect on biomass allocation. Total leaf area increased and leaf abscission was delayed in response to elevated [CO2]. Elevated [O3] decreased dry mass of roots and branches and mean leaf size and induced earlier leaf abscission in the autumn; otherwise, the effects of elevated [O3] were small across the clones. However, there were significant interactions between elevated [CO2] and elevated [O3]. When results for the clones were analyzed separately, stem diameter, volume growth and total biomass of Clone 80 were increased by elevated [CO2] and the stimulatory effects of elevated [CO2] on stem volume growth and total leaf area increased during the 3-year study. Clone 80 was unaffected by elevated [O3]. In Clone 4, elevated [O3] decreased root and branch biomass by 38 and 29%, respectively, whereas this clone showed few responses to elevated [CO2]. Elevated [CO2] significantly increased total leaf area in Clone 80 only, which may partly explain the smaller growth responses to elevated [CO2] of Clone 4 compared with Clone 80. Although we observed responses to elevated [O3], the responses to the EC+EO and EC treatments were similar, indicating that the trees only responded to elevated [O3] under ambient [CO2] conditions, perhaps reflecting a greater quantity of carbohydrates available for detoxification and repair in elevated [CO2].

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Effects of elevated concentrations of carbon dioxide ([CO2]) and ozone ([O3]) on photosynthesis and related biochemistry of two European silver birch (Betula pendula Roth) clones were studied under field conditions during 1999-2001. Seven-year-old trees of Clones 4 and 80 were exposed for 3 years to the following treatments in an open-top chamber experiment: outside control (OC), chamber control (CC), 2x ambient [CO2] (EC), 2x ambient [O3] (EO) and 2x ambient [CO2] + 2x ambient [O3] (EC+EO). During the experiment, gas exchange, chlorophyll fluorescence, amount and activity of Rubisco, concentrations of chlorophyll, soluble protein, soluble sugars, starch, nitrogen (N) and carbon:nitrogen (C:N) ratio were determined in short- and long-shoot leaves. Elevated [CO2] increased photosynthetic rate by around 30% when measurements were made at the growth [CO2]. When measured at ambient [CO2], photosynthesis was around 15% lower in EC trees than in CC trees. This was related to a approximately 10% decrease in total leaf N, to 26 and 20% decreases in the amount and activity of Rubisco, respectively, and to a 49% increase in starch concentration in elevated [CO2]. Elevated [O3] had no significant effect on gas exchange parameters and its effect on biochemistry was small in both clones. However, elevated [O3] decreased the proportion of Rubisco in total soluble proteins and the apparent quantum yield of photosystem II (PSII) photochemistry in light and increased non-photochemical quenching in 2000. The interactive effect of CO2 and O3 was variable. Elevated [O3] decreased chlorophyll concentration only in EO trees, and the EC+EO treatment decreased the total activity of Rubisco and increased the C:N ratio more than the EO treatment alone. The small effect of elevated [O3] on photosynthesis indicates that these young silver birches were fairly tolerant to annual [O3] exposures that were 2-3 times higher than the AOT40 value of 10 ppm.h. set as a critical dose for forest trees.