Effects of elevated [CO2] and O3 on stomatal and surface wax characteristics in leaves of pubescent birch grown under field conditions

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The responses of pubescent birch (Betula pubescens Ehrh.) leaves to elevated levels of carbon dioxide and ozone were investigated. The micropropagated birch clones used in this research were divided into coastal (Kempele 64°N °25E) and inland (Ylikiiminki 65° N 26°E) clones according to the provenance of the parent tree. The seedlings were exposed to elevated levels of CO2 and CO2+O3 in open-top chambers over four growing seasons (1993–1996). The leaves used in this investigation were gathered in 1995 and 1996. Leaf characteristics seemed to be dependent on leaf age and weather conditions. Stomatal density was greater in plants grown in open-top chambers relative to unchambered controls. The CO2 and CO2+O3 treatments did not affect the stomatal density of the birches. However, the mean stomatal size of the coastal clone increased significantly (13%) in the CO2 treatment compared with the filtered and ambient controls in 1995. In 1996, the mean stomatal sizes of the inland clones were also significantly higher in the CO2 (10%), CO2+O3 (11%) and non-filtered (12%) treatments compared with the filtered control. The distribution of wax deposits on the abaxial leaf surface increased in the inland clone under elevated levels of CO2. The growth environment affected the development of the epicuticular waxes: non-filtered air, CO2 and CO2+O3 treatments seemed to stimulate the formation of wax deposits compared with the charcoal filtered control in which wax formation was the weakest. The combined effects of elevated CO2 and O3 levels on the stomatal size and epicuticular wax deposits were similar in comparison to the effects of elevated CO2 levels alone.