THE INFLUENCE OF ENRICHED RHIZOSPHERE CO2 ON N UPTAKE AND METABOLISM IN WILD-TYPE AND NR-DEFICIENT BARLEY PLANTS

Positive influences of high concentrations of dissolved inorganic carb on (DIC) in the growth medium of salinity-stressed plants are associat ed with carbon assimilation through phosphoenolpyruvate carboxylase (P EPc) activity in roots; and also in salinity-stressed tomato plants, e nriched CO2 in the rhizosphere increases NO(3)(-)uptake. In the presen t study, wild-type and nitrate reductase-deficient plants of barley (H ordeum vulgare L. cv. Steptoe) were used to determine whether the infl uence of enriched CO2 on NO(3)(-)uptake and metabolism is dependent on the activity of nitrate reductase (NR) in the plant. Plants grown in NH4+ and aerated with ambient air, were transferred to either NO3- or NH4+ solutions and aerated with air containing between 0 and 6500 mu m ol mol(-1) CO2. Nitrogen uptake acid tissue concentrations of NO3- and NH4+ were measured as well as activities of NR and PEPc. The uptake o f NO3- by the wild-type was increased by increasing CO2. This was asso ciated with increased in vitro NR activity, but increased uptake of NO 3- was found also in the NR-deficient genotype when exposed to high CO 2 concentrations; so that the influence of CO2 on NO3- uptake was inde pendent of the reduction of NO3- and assimilation into amino acids. Th e increase in uptake of NO; in wild-type plants with enriched CO2 was the same at pH 7 as at pH 5, indicating that the relative abundance of HCO3- or CO2 in the medium did not influence NO3- uptake. Uptake of N H4+ was decreased by enriched CO2 in a pH (5 or 7) independent fashion . Thus NO3- and NH4+ uptakes are influenced by the CO2 component of DI C independently of anaplerotic carbon provision for amino acid synthes is, and CO2 may directly affect the uptake of NO3- and NH4+ in ways un related to the NR activity in the tissue.