EFFECTS OF INCREASING INORGANIC CARBON SUPPLY TO ROOTS ON NET NITRATEUPTAKE 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 Lyc opersicon 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 nu trient solution for group A plants was aerated with air or with air co ntaining 4 800 mu mol mol(-1) CO2. Nitrate uptake rate and root and le af malate contents in these plants were determined. The plants of grou p 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 ass ayed 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 C -14 by a 1-h pulse of (HCO3-)-C-14 which was added to a 5 mM NaHCO3 so lution containing 0 or 100 mM NaCl and 0 or 2 mM NO3-, and C-14-assimi lates were extracted and fractionated. The roots of group B plants gro wing in carbonated medium accumulated twice as much malate as did cont rol plants. This malate was accumulated only when NO3- was absent from the root medium. Both a high level of root malate and aeration with C O2-enriched air stimulated NO3- uptake. Analysis of C-14-assimilates i ndicated that with no NO3- in the medium, the C-14 was present mainly in organic acids, whereas with NO3-, a large proportion of C-14 was in corporated into amino acids. Transport of root-incorporated C-14 to th e shoot was enhanced by NO3-, while the amino acid fraction was the ma jor C-14-assimilates in the shoot, It is concluded that inorganic carb on fixed through phosphoenolpyruvate carboxylase (EC 4.1.1.31) in root s of tomato plants may have two fates: (a) as a carbon skeleton for am ino acid synthesis; and (b) to accumulate, mainly as malate, in the ro ots, in the absence of a demand for the carbon skeleton. Inorganic car bon fixation in the root provides carbon skeletons for the assimilatio n of the NH4+ resulting from NO3- reduction, and the subsequent remova l of amino acids through the xylem, This 'removal' of NO3- from the cy toplasm of the root cells may in turn increase NO3- uptake.