Impacts of elevated CO2 and/or O-3 on leaf ultrastructure of aspen (Populus tremuloides) and birch (Betula papyrifera) in the Aspen FACE experiment

Impacts of elevated atmospheric O-3 and/or CO2 On three clones of aspen (Po pulus tremuloides Michx.) and birch (Betula papyrifera Marsh.) were studied to determine, whether or not elevated CO2 ameliorates O-3-induced damage t o leaf cells. The plants were exposed for 3 years at the Aspen FACE exposur e site in Wisconsin (USA) prior to sampling for ultrastructural investigati ons on 19 June 1999. In the aspen clones, elevated CO2 increased chloroplas t cover index, leaf and spongy mesophyll layer thickness, intercellular air space volume in mesophyll, amount of starch in chloroplasts and cytoplasmi c lipids but decreased the number of plastoglobuli in chloroplasts. In cont rast, elevated O-3 decreased chloroplast cover index, starch content, and t he proportion of cytoplasm and intercellular space in mesophyll, and increa sed the proportion of vacuoles, the amount of condensed vacuolar tannins an d the number of plastoglobuli. Ozone also caused structural thylakoid injur ies (dilation, distortion) and stromal condensation in chloroplasts, which was ameliorated by elevated CO2 by 5-66% in aspen clones and by 2-10% in bi rch. Birch ultrastructure was less affected by elevated CO2 or O-3 stress c ompared 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 va cuoles was found compared to more-tolerant clones. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.