1994 journal article
INFLUENCE OF NITRATE ON UPTAKE OF AMMONIUM BY NITROGEN-DEPLETED SOYBEAN - IS THE EFFECT LOCATED IN ROOTS OR SHOOTS
JOURNAL OF EXPERIMENTAL BOTANY, 45(280), 1575β1584.
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 1.4mmol Ngβ1 dry weight in shoots and roots, respectively, and the concentration of soluble amino acids (determined as primary amines) declined to 40ΞΌmol gβ1 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 mβ3 NH4+ to the whole root system, to a second set as 0.5 mol mβ3 NH4+ plus 0.5 mol mβ3 NO3β to the whole root system, and to a third set as 1.0 mol mβ3 NH4+ to one-half of a split-root system and 1.0 mol mβ3 NO3β to the other half. In a second experiment, 1.0 mol mβ3 of nitrogen was resupplied for 4 d to whole root systems in NH4+ : NO3β ratios of 1:0, 9:1, and 1:1. Nutrient solutions were maintained at pH 6.0. When NH4+ was resupplied in combination with NO3β 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 NO3β was taken up when both ions were resupplied to the whole root system. When NH4+ and NO3β were resupplied to separate halves of a split-root system, however, cumulative uptake of NH4+ was about half that of NO3β. The uptake of NH4+, which is inhibited in nitrogen-depleted plants, thus is facilitated by the presence of exogenous NO3β, and the stimulating effect of NO3β on uptake of NH4+ appears to be confined to processes within root tissues. In Experiment II, resupply of nitrogen as both NH4+ and NO3β 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+: NO3β-resupply ratio was 1:1 than when it was 9:1; however, only 1.3 times as much NO3β was taken up by plants resupplied with the 1 :1 exogenous ratio. The effect of NO3β on enhancement of uptake of NH4+ apparently involves more than net uptake of NO3β itself and perhaps entails an effect of NO3β 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+-resup-plied plants inhibited NH4+ uptake through restricted transfer of amino acids from the root symplasm into the xylem.