Effects of CO₂ enrichment, leaf position and clone on stomatal index and epidermal cell density in poplar (Populus)

The effects of CO₂ , enrichment and leaf position on stomatal characteristics (stomatal density, stomatal index and stomatal pore length) and epidermal cell density were examined for two different Populus clones, Beaupre and Robusta, grown from cuttings in open-top chambers under ambient and elevated atmospheric CO₂ conditions. Both clones had amphistomatous leaves, and stomatal density was significantly larger on the abaxial leaf surface than on the adaxial. Significant interactions between CO₂ enrichment, leaf position and clone were observed for most stomatal and epidermal characteristics. A significant reduction of the number of stomata mm^-2 under elevated CO₂ was observed in expanding leaves near the upper portion of the plant for both leaf surface sides and in both clones. For the abaxial leaf side only, this reduction under elevated CO₂ , was accompanied by a similar reduction of the stomatal index in both clones. In mature leaves on the middle and lower portion of the plants, there was no significant effect of the CO₂ treatment on stomatal density. In young, expanding leaves near the upper part of the plant there were significant interactions between the CO₂ treatment and leaf surface side for epidermal cell density. The latter increased under elevated CO₂ at the abaxial leaf surface, but decreased at the adaxial surface on the upper part of the plant. Total epidermal cell numbers of mature, fully expanded leaves increased under elevated CO₂ , in both clones. The observation that interactions with leaf age and/or leaf position significantly confound the CO₂ , treatment effect on stomatal and epidermal cell densities, might contribute to the elucidation of the problem of the phenomenon of stomatal density reduction under elevated atmospheric CO₂ .