THE EFFECT OF GROWTH AT ELEVATED CO2 CONCENTRATIONS ON PHOTOSYNTHESISIN WHEAT

Rising levels of atmospheric CO2 will have profound, direct effects on plant carbon metabolism. In this study we used gas exchange measureme nts, models describing the instantaneous response of leaf net CO2 assi milation rate (A) to intercellular CO2 partial pressure (C-i), in vitr o enzyme activity assay, and carbohydrate assay in order to investigat e the photosynthetic responses of wheat (Triticum aestivum L., cv. Wem bley) to growth under elevated partial pressures of atmospheric CO2 (C -a). At flag leaf ligule emergence, the modelled, in vivo, maximum car boxylation velocity for RuBisCO was significantly lower in plants grow n at elevated C-a than in plants grown at ambient C-a (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 a t elevated C-a exhibited reduced in vitro initial activities and activ ation states of RuBisCO. At their respective growth C-i values, the ph otosynthesis of 40-Pa-grown plants was sensitive to p(O-2) and to p(CO 2), whereas that of 70-Pa-grown plants was insensitive. Both sucrose a nd starch accumulated more rapidly in the leaves of plants grown at 70 Pa. At flag leaf ligule emergence, modelled non-photorespiratory resp iration in the light (R(d)) was significantly higher in 70-Pa-grown pl ants than in 40-Pa-grown plants. By 12 d after ligule emergence no sig nificant differences in R(d) were detectable.