The influence of dissolved inorganic carbon in the rhizosphere on carbon and nitrogen metabolism in salinity-treated tomato plants

The influence of variation in the concentration of dissolved inorganic carb on (DIC) in the form of CO2 and HCO3- in the root media on the C and N meta bolism of Lycopersicon esculentum cv. F144 was investigated under both sali ne and non-saline conditions. Tomato seedlings were grown in hydroponic cul ture (pH 6.5) with or without NaCl, and the root solution was aerated with either ambient CO2 (360 mol mol(-1)) or CO2-enriched air (5000 mu mol mol(- 1)). Nitrate uptake and root tissue NO3- concentrations were increased slig htly by elevated rhizosphere DIC concentrations in both control and salinit y-treated plants. This is associated with 46% higher nitrate reductase acti vity in the roots of control plants supplied with elevated DIC than in thos e supplied with ambient DIG. The activity of phosphoenolpyruvate carboxylas e (PEPc) in vitro in control and salinity-treated plants was unaffected by the supply of elevated rhizosphere DIC concentrations. However, PEPc activi ty in vitro was considerably higher than the rates of PEPc activity in vivo reported previously, indicating that PEPc activity was not in itself a lim itation on the provision of anaplerotic C. Therefore elevated DIC concentra tion in the rhizosphere stimulated the uptake of NO3- and provided alternat ive C skeletons for the assimilation of the NH4+ resulting from NO3- reduct ion into amino acids within the roots. Salinity stimulated root glutamine s ynthetase (GS) activity up to double that in control plants. Furthermore, e levated DIC caused an increase in leaf and root GS activity of control plan ts while inhibiting GS activity in the roots of salinity-treated plants. Gl utamine:2-oxoglutarate aminotransferase (GOGAT) activity of salinity-treate d plants was doubled by elevated rhizosphere DIC concentrations. These chan ges in GS and GOGAT activity must reflect changes in amino acid synthesis. Under saline conditions the xylem transport of NO3- is partly blocked and a larger root assimilation develops, requiring not only the transamination o f 2-oxoglutarate to glutamate but also that of oxaloacetate to aspartate an d the transamidation of aspartate to asparagine.