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Environmental and Stress Physiology

Charge Balance in NO₃^–-Fed Soybean

Estimation of K⁺ and Carboxylate Recirculation

Biochimie et Physiologie Végétales, Institut National de la Recherche Agronomique (CNRS, UA 573), Ecole Nationale Supérieure Agronomique, 34060 Montpellier Cedex, France

Soybeans (Glycine max L. Merr., cv Kingsoy) were grown on media containing NO₃^– or urea. The enrichments of shoots in K⁺, NO₃^–, and total reduced N (N_r), relative to that in Ca²⁺, were compared to the ratios K⁺/Ca²⁺,NO₃^–/Ca²⁺, and N_r/Ca²⁺ in the xylem saps, to estimate the cycling of K⁺, and N_r. 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 NO₃^– 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 NO₃^– reduction, and N_r was cycling through the plants at a high rate. Alkalinization of the medium by NO₃^–-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 NO₃^– medium originated from R^– produced in the shoots, and transported to the roots together with K⁺. As expected, cycling of K⁺ and reduced N was favoured by NO₃^– nutrition as compared to urea nutrition.

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