Charge Balance in NO3−-Fed Soybean

Soybeans (Glycine max L. Merr., cv Kingsoy) were grown on media containing NO(3) (-) or urea. The enrichments of shoots in K(+), NO(3) (-), and total reduced N (N(r)), relative to that in Ca(2+), were compared to the ratios K(+)/Ca(2+),NO(3) (-)/Ca(2+), and N(r)/Ca(2+) 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 NO(3) (-) 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 NO(3) (-) reduction, and N(r) was cycling through the plants at a high rate. Alkalinization of the medium by NO(3) (-)-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 NO(3) (-) 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 NO(3) (-) nutrition as compared to urea nutrition.