INFLUENCE OF NITRATE ON UPTAKE OF AMMONIUM BY NITROGEN-DEPLETED SOYBEAN - IS THE EFFECT LOCATED IN ROOTS OR SHOOTS

In non-nodulated soybean [Glycine max (L.) Merrill cv. Ransom] plants that were subjected to 15 d of nitrogen deprivation in flowing hydroponic culture, concentrations of nitrogen declined to 1.0 and 1.4 mmol N g(-1) dry weight in shoots and roots, respectively, and the concentration of soluble amino acids (determined as primary amines) declined to 40 mu mol g(-1) dry weight in both shoots and roots. In one experiment, nitrogen was resupplied for 10 d to one set of nitrogen-depleted plants as 1.0 mol m(-3) NH4+ to the whole root system, to a second set as 0.5 mol m(-3) NH4+ plus 0.5 mol m(-3) NO3- to the whole root system, and to a third set as 1.0 mol m(-3) NH4+ to one-half of a split-root system and 1.0 mol m(-3) NO3- to the other half. In a second experiment, 1.0 mol m(-3) of nitrogen was resupplied for 4 d to whole root systems in NH4+:NO3- ratios of 1:0, 9:1, and 1:1. Nutrient solutions were maintained at pH 6.0. When NH4+ was resupplied in combination with NO3- to the whole root system in Experiment I, cumulative uptake of NH4+ for the 10 d of resupply was about twice as great as when NH4+ was resupplied alone. Also, about twice as much NH4+ as NO3- was taken up when both ions were resupplied to the whole root system. When NH4+ and NO3- were resupplied to separate halves of a split-root system, however, cumulative uptake of NH4+ was about half that of NO3-. The uptake of NH4-, which is inhibited in nitrogen-depleted plants, thus is facilitated by the presence of exogenous NO3-, and the stimulating effect of NO3- on uptake of NH4+ appears to be confined to processes within root tissues. In Experiment II, resupply of nitrogen as both NH4+ and NO3- in a ratio of either 1:1 or 9:1 enhanced the uptake of NH4+. The enhancement of NH4+ uptake was 1.8-fold greater when the NH4+:NO3--resupply ratio was 1:1 than when it was 9:1; however, only 1.3 times as much NO3- was taken up by plants resupplied with the 1:1 exogenous ratio. The effect of NO3- on enhancement of uptake of NH4+ apparently involves more than net uptake of NO3- itself and perhaps entails an effect of NO3- uptake on maintenance of K+ availability within the plant. The concentration of K+ in plants declined slightly during nitrogen deprivation and continued to decline following resupply of nitrogen. The greatest decline in K+ concentration occurred when nitrogen was resupplied as NH4+ alone. It is proposed that decreased availability of K+ within the NH4+-resupplied plants inhibited NH4+ uptake through restricted transfer of amino acids from the root symplasm into the xylem.