Impacts of elevated CO 2 and/or O 3 on leaf ultrastructure of aspen ( Populus tremuloides) and birch ( Betula papyrifera) in the Aspen FACE experiment

Abstract

Impacts of elevated atmospheric O 3 and/or CO 2 on three clones of aspen ( Populus tremuloides Michx.) and birch ( Betula papyrifera Marsh.) were studied to determine, whether or not elevated CO 2 ameliorates O 3-induced damage to leaf cells. The plants were exposed for 3 years at the Aspen FACE exposure site in Wisconsin (USA) prior to sampling for ultrastructural investigations on 19 June 1999. In the aspen clones, elevated CO 2 increased chloroplast cover index, leaf and spongy mesophyll layer thickness, intercellular air space volume in mesophyll, amount of starch in chloroplasts and cytoplasmic lipids but decreased the number of plastoglobuli in chloroplasts. In contrast, elevated O 3 decreased chloroplast cover index, starch content, and the proportion of cytoplasm and intercellular space in mesophyll, and increased the proportion of vacuoles, the amount of condensed vacuolar tannins and the number of plastoglobuli. Ozone also caused structural thylakoid injuries (dilation, distortion) and stromal condensation in chloroplasts, which was ameliorated by elevated CO 2 by 5–66% in aspen clones and by 2–10% in birch. Birch ultrastructure was less affected by elevated CO 2 or O 3 stress compared to aspen. In the most O 3-sensitive aspen clone, thinner leaves and cell walls, lower proportion of cell wall volume, and higher volume for vacuoles was found compared to more-tolerant clones.