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 carbon (DIC) in the growth medium of salinity-stressed plants are associated with carbon assimilation through phosphoenolpyruvate carboxylase (PEPc) activity in roots; and also in salinity-stressed tomato plants, enriched CO₂ in the rhizosphere increases NO₃^- uptake. In the present study, wild-type and nitrate reductase-deficient plants of barley (Hordeum vulgare L. cv. Steptoe) were used to determine whether the influence of enriched CO₂ on NO₃^-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 6 500 μmol mol^-1 CO₂. Nitrogen uptake and tissue concentrations of NO₃^- and NH₄⁺ were measured as well as activities of NR and PEPc. The uptake of NO₃^- by the wild-type was increased by increasing CO₂. This was associated with increased in vitro NR activity, but increased uptake of NO₃^- was found also in the NR-deficient genotype when exposed to high CO₂ concentrations; so that the influence of CO₂ on NO₃^- uptake was independent of the reduction of NO₃^- and assimilation into amino acids. The increase in uptake of NO₃^- 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 NH₄⁺ 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 DIC independently of anaplerotic carbon provision for amino acid synthesis, and CO₂ may directly affect the uptake of NO₃^- and NH₄⁺ in ways unrelated to the NR activity in the tissue.