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Assimilation of NH4+ results in H+ excretion which cannot be neutralized internally, and is excreted. In land plants, NH4+ assimilation takes place in the roots, and products transported to the shoots are such that no further large-scale H+ production takes place. Ammonium is better tolerated by some crop species (e.g. rice) than others. In flooded lowland rice soils, where the bulk of the soil is hypoxic or anaerobic, the main form of plant N available is NH4+. Rice exhibits a high capacity for NH4+ acquistion even at very low O2 concentrations (Kronzucker et al, 1998). In contrast, NO3- assimilation involves production of OH- which may be countered in roots by net efflux (or H+ influx). Alternatively, neutralization can result from carboxylation. Such carboxylation is the means of preventing pH imbalance during NO3- reduction in the shoot (Smith and Raven, 1979).
 Since nitrate assimilation causes alkalinization, this will tend to stimulate phosphoenolpyruvate carboxylase (PEPcase), resulting in the formation of malate. Ben Zioni et al (1971) developed a model, depicted above, relating nitrate reduction, malate synthesis and translocation, and preferential uptake of nitrate over K+ by roots (Ben Zioni et al, 1971). Plants growing in culture solutions take up a large excess of nitrate over potassium. This preferential anion uptake must be equilibrated with an endogenous anion; presumably HCO3-. The reduction of nitrate by nitrate reductase and nitrite reductase in the shoot is accompanied by stoichiometric synthesis of malate. Nitrate reduction by the shoot controls nitrate uptake in the root by the synthesis of malate which is used to generate the HCO3- employed as an exchanger for nitrate in the root.
Ben Zioni A, Vaadia Y, Lips SH 1971 Nitrate uptake by roots is regulated by nitrate reduction products of the shoot. Physiol. Plant. 34: 288-290. Kronzucker HJ, Kirk GJD, Siddiqi MY, Glass ADM 1998 Effects of hypoxia on 13NH4+ fluxes in rice roots. Plant Physiol. 116: 581-587. Smith FA, Raven JA 1979 Intracellular pH and its regulation. Annu. Rev. Plant Physiol. 30: 289-311.
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