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

Z. F. Gao, S. H. Lips

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    Abstract

    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.

    Original languageEnglish
    Pages (from-to)206-212
    Number of pages7
    JournalPhysiologia Plantarum
    Volume101
    Issue number1
    DOIs
    StatePublished - 1 Dec 1997

    Keywords

    • Bicarbonate
    • C incorporation
    • Lycopersicon esculentum
    • Nitrate uptake
    • Phosphoenolpyruvate carboxylase
    • Tomato

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