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 .

Effects on sugar beet (Beta vulgaris L.) of current and elevated CO 2 and temperature alone and in combination and their interactions with abundant and deficient nitrogen supply (HN and LN, respectively) have been studied in three experiments in 1993, 1994 and 1995. Averaged over all experiments, elevated CO 2 (600 µmol mol -1 in 1993 and 700 µmol mol -1 in 1994 and 1995) increased total dry mass at final harvest by 21% (95% confidence interval (CI) = 21, 22) and 11% (CI = 6, 15) and root dry mass by 26% (CI = 19, 32) and 12% (CI = 6, 18) for HN and LN plants, respectively. Warmer temperature decreased total dry mass by 11% (CI = -15, -7) and 9% (CI = -15, -5) and root dry mass by 7% (CI = -12, -2) and 7% (CI = -10, 0) for HN and LN plants, respectively. There was no significant interaction between temperature and CO 2 on total or root dry mass. Neither elevated CO 2 nor temperature significantly affected sucrose concentration per unit root dry mass. Concentrations of glycinebetaine and of amino acids, measured as α-amino-N, decreased in elevated CO 2 in both N applications; glycinebetaine by 13% (CI = -21, -5) and 16% (CI = -24, -8) and α-amino-N by 24% (CI = -36, -11) and 16% (CI = -26, -5) for HN and LN, respectively. Warmer temperature increased α-amino-N, by 76% (CI = 50, 107) for HN and 21% (CI = 7, 36) for LN plants, but not glycinebetaine.Key-words: Beta vulgaris; sugar beet; biomass; CO 2 ; nitrogen; root quality; temperature; yield. INTRODUCTIONAtmospheric CO 2 concentration (C a ) is expected to increase to between 600 and 700 µmol mol -1 by the end of the next century. Global temperatures are also expected to increase, perhaps by 3°C. Because photosynthesis and biomass production of plants with C 3 photosynthetic metabolism are currently limited by C a , both increase as C a rises reaching a maximum at about 700-1000 µmol mol -1 CO 2 (Cure & Acock 1986;Lawlor & Mitchell 1991;Idso & Idso 1994): on average the increase in biomass is 33% with a CO 2 rise from about 350 to 700 µmol mol -1 (Kimball 1983). Elevated CO 2 increases photosynthesis and decreases photorespiration due to the characteristics of ribulose bisphosphate carboxylase/oxygenase (Rubisco). However, the effect of warmer temperatures on Rubisco is to increase photorespiration and decrease photosynthesis. Thus, CO 2 concentration will interact positively with temperature (Long 1991;Stitt 1991;Lawlor & Keys 1993).Growth characteristics of plants, particularly the duration of growth and accumulation of assimilates in storage organs (e.g. tubers, roots or seeds), together with respiration, affect the use of photosynthates. If this 'sink capacity' is restricted, compared to assimilate production, then the potential advantages of increasing CO 2 for biomass and yield production may be limited. Sink demand is affected by environmental conditions; for example, warmer temperature stimulates growth rate and respiration so that sink limitation would be less -and so the effects of CO 2 enrichment greater -at warmer than at cooler temperatures ...