Sugar beet (Beta vulgaris L., cultivar Celt) plants were grown under simulated field conditions in pots and supplied with adequate or deficient nitrogen (HN and LN, respectively) combined with two C0 2 concentrations, ambient (c. 350/imol mol~1 C0 2-AC), or elevated CO 2 (c. 600 fimo\ mol~1 C0 2-HC). Chloroplast structure in mesophyll palisade cells of mature leaves (leaf number 19 in HN and 9 in LN), sampled at midday on 16 August 1993 was studied by transmission electron microscopy and quantified stereologically. The ultrastructure of palisade parenchyma chloroplasts was affected by the elevated CO 2 concentration and strikingly affected by nitrogen supply. Chloroplast diameter (cross-sectional length) was slightly, but not significantly, greater in HC than AC treatments within an N treatment, but was smaller in LN than HN; chloroplast cross-sectional area also increased with HC in both N treatments, but only significantly so in LN. Elevated C0 2 reduced the proportion of total thylakoids (significant at 5% and 0.1% in HN and LN, respectively) due to decreased granal thylakoids, but the proportion of inter-granal (stromal) thylakoid membranes was not affected compared to chloroplasts from plants grown with ambient CO 2. Chloroplast stroma increased as a proportion of chloroplast volume with elevated compared to ambient CO 2 with HN but not LN. Starch inclusions were not significantly different with elevated compared to ambient C0 2 at HN, but the proportion of starch increased considerably at elevated compared to ambient C0 2 at LN, indicating an overproduction of assimilates. Plastoglobuli in chloroplasts increased with deficient N, but decreased with elevated CO 2. Larger chloroplasts with a greater proportion of stroma, but a smaller proportion of granal thylakoids, suggest increased CO 2 assimilating capacity and decreased light harvesting/PSII capacity with elevated CO 2 .
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