Charge Balance in NO3−-Fed Soybean

Abstract

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 (Nr), relative to that in Ca2+, were compared to the ratios K+/Ca2+,NO3-/Ca2+, and N4/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 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 Nr 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.