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 NO−3uptake. 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 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 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 NO−3 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 NH+4 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.