Effects of increasing inorganic carbon supply to roots on net nitrate uptake and assimilation in tomato seedlings

We investigated the influence of an increased inorganic carbon supply in the root medium on NO₃/^- uptake and assimilation in seedlings of Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were pre-grown for 4 to 7 days with 0 or 100 mM NaCl in hydroponic culture using 0.2 mM NO₃/^- (group A) or 0.2 mM NH₄/⁺ (group B) as nitrogen source. The nutrient solution for group A plants was aerated with air or with air containing 4 800 μmol mol^-1 CO₂. Nitrate uptake rate and root and leaf malate contents in these plants were determined. The plants of group B were subdivided into two sets. Plants of one set were transferred either to N-free solution containing 0 or 5 mM NaHCO₃, or to a medium containing 2 mM NO₃/^- and 5 mM NaHCO₃. Both sets of group B plants were grown for 12 h in darkness prior to 2 h of illumination, and were assayed for malate content and NO₃/^-uptake rate (only for plants grown in N-free solution). The second set of group B plants was labeled with ¹⁴C by a 1-h pulse of H¹⁴CO₃/^- which was added to a 5 mM NaHCO₃ solution containing 0 or 100 mM NaCl and 0 or 2 mM NO₃/^-, and ¹⁴C-assimilates were extracted and fractionated. The roots of group B plants growing in carbonated medium accumulated twice as much malate as did control plants. This malate was accumulated only when NO₃/^- was absent from the root medium. Both a high level of root malate and aeration with CO₂-enriched air stimulated NO₃/^- uptake. Analysis of ¹⁴C-assimilates indicated that with no NO₃/^- in the medium, the ¹⁴C was present mainly in organic acids, whereas with NO₃/^-, a large proportion of ¹⁴C was incorporated into amino acids. Transport of root-incorporated C to the shoot was enhanced by NO₃/^-, while the amino acid fraction was the major ¹⁴C-assimilates in the shoot. It is concluded that inorganic carbon fixed through phosphoenolpyruvate carboxylase (EC 4.1.1.31) in roots of tomato plants may have two fates: (a) as a carbon skeleton for amino acid synthesis; and (b) to accumulate, mainly as malate, in the roots, in the absence of a demand for the carbon skeleton. Inorganic carbon fixation in the root provides carbon skeletons for the assimilation of the NH₄/⁺ resulting from NO₃/^- reduction, and the subsequent removal of amino acids through the xylem. This 'removal' of NO₃/^- from the cytoplasm of the root cells may in turn increase NO₃/^- uptake.