Charge Balance in NO₃⁻-Fed Soybean

Abstract

Soybeans (Glycine max L. Merr., cv Kingsoy) were grown on media containing N03-or urea. The enrichments of shoots in K@, N03-, and total reduced N (Nr), relative to that in Ca2", were compared to the ratios K+/Ca2+,NO3j/Ca2+, and Nr/Ca2+ in the xylem saps, to estimate the cycling of K+, and Nr. The net production of carboxylates (R-) was estimated from the difference between the sums of the main cations and inorganic anions. The estimate for shoots was compared to the theoretical production of R-associated with N03-assimilation in these organs, and the difference was attributed to export of R-to roots. The net exchange rates of H+ and OH-between the medium and roots were monitored. The shoots were the site of more than 90% of total N03-reduction, and Nr was cycling through the plants at a high rate. Alkalinization of the medium by N03-fed plants was interrupted by stem girdling, and not restored by glucose addition to the medium. It was concluded that the majority of the base excreted in N03-medium originated from Rproduced in the shoots, and transported to the roots together with K+.As expected, cycling of K+ and reduced N was favoured by N03-nutrition as compared to urea nutrition. Plant roots absorb ions at different rates, so that cation and anion uptakes may not be in equilibrium. (12).Nitrogen contents of plant tissues are greater than those of other mineral elements. This implies that the ionic imbalance of absorption depends on whether N is present in the medium as a cation or as an anion (23). OH-generated by reduction of N03-in the roots may be released in the medium. This kind of cytoplasmic detoxification is not possible in shoots, due to the small volume ofleafapoplast, and to the limited ability ofphloem to transport OH-and HCO3-(1, 22). OH-generated in aerial organs is converted in ionized carboxylic groups, but the calculated accumulation of organic anions is often smaller than the NO3 reduction in shoots. A model was proposed by Ben Zioni and co-workers (4) to explain this discrepancy. According to this model, malate synthesized in the leaves is loaded in the phloem and transported together with K+ into the roots where it is decarboxylated. The HCO3-produced is exchanged for NO3-, which is transported into the shoots together with K+. Thus, the transport of K+ carboxylates by phloem, and K+ cycling are means by which NO3-reduction in shoots controls NO3-uptake by roots.The validity of the Ben Zioni model has been questioned for barley (6), corn (16), tomato (17), and inoculated soybean (cv Ransom) (15), which were reported to accumulate carboxylates in amounts approximately equivalent to N03 reduction in shoots. Castor oil plants excrete more than 50% of the OH-they produced by assimilating N and S (1, 18, 29). In these plants, shoots were responsible for approximately 40 to 60% NO3-assimilation and they returned to the medium approximately 15 to 20% (29) to 60% (1) of the OH-they produced.In these experiments, the majority of excreted OH-originated from roots. Evidence for the cy...

Effect of Phloem-Translocated Malate on NO3− Uptake by Roots of Intact Soybean Plants

“…The shoot was the site of more than 90% of total NO3-reduction, and most of the OH-equivalents that should have been produced during NO3-assimilation were excreted. Moreover, the excretion rate of OH-equivalents predicted by this calculation was equal to the monitored alkalinization rate of the nutrient solution, and it balanced well the difference between the uptake rates of anions and cations (21). Furthermore, the OH-equivalents excreted by the roots originated from the carboxylates produced in the shoot, which implies that the negative charge released by N03-reduction in the shoot was transported to the roots through the phloem.…”

“…The close relationship between NO3-uptake by roots and alkalinization of the rhizosphere is well documented (5,15,17,18,21 …”

“…1,13,14,16,23,24). In a previous paper (21), we showed that the cycling scheme occurs in NO3--fed soybean (Glycine max L. Merr. cv Kingsoy) during the vegetative growth phase.…”

Fumaric acid: an overlooked form of fixed carbon in Arabidopsis and other plant species

“…Alternatively, the primary function of fumaric acid present in phloem exudates could be in nitrate transport. Phloem-localized malate has been proposed to function in carbon transport (Hamilton and Davies 1988) and in transport of nitrate from roots to shoots (Ben Zioni et al 1971;Touraine et al 1988;Touraine et al 1992). …”

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

“…The extent to which the movement of organic anions to the roots, with metabolism and OH" excretion, occurs in plants reducing NO3" in their shoots is variable (e.g. Armstrong & Kirkby, 1979;Van Beusichem et al, 1985;Allen & Raven, 1987;Van Beusichem, Kirkby & Baas, 1988;Touraine, Grignon & Grignon, 1988).…”