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Rising levels of atmospheric CO2 will have profound, direct effects on plant carbon metabolism. In this study we used gas exchange measurements, models describing the instantaneous response of leaf net CO2 assimilation rate (A) to intercellular CO2 partial pressure (Ci), in vitro enzyme activity assay, and carbohydrate assay in order to investigate the photosynthetic responses of wheat (Triticum aestivum L., cv. Wembley) to growth under elevated partial pressures of atmospheric CO2 (Ca). At flag leaf ligule emergence, the modelled, in vivo, maximum carboxylation velocity for RuBisCO was significantly lower in plants grown at elevated Ca than in plants grown at ambient Ca (70 Pa compared with 40 Pa). By 12 d after ligule emergence, no significant difference in this parameter was detectable. At ligule emergence, plants grown at elevated Ca exhibited reduced in vitro initial activities and activation states of RuBisCO. At their respective growth Ci values, the photosynthesis of 40-Pa-grown plants was sensitive to p(O2) and top(CO2), whereas that of 70-Pagrown plants was insensitive. Both sucrose and starch accumulated more rapidly in the leaves of plants grown at 70 Pa. At flag leaf ligule emergence, modelled non-photorespiratory respiration in the light (Rd) was significantly higher in 70-Pa-grown plants than in 40-Pa-grown plants. By 12 d after ligule emergence no significant differences in Rd were detectable. [ABSTRACT FROM AUTHOR]