Potassium-Ammonium Uptake Interactions in Tobacco Seedlings

Abstract

Short-term (< 12 h) uptake experiments were conducted with 6–7-week-old tobacco (Nicotiana tabacum L. cv. Ky 14) seedlings to determine absorption interactions between K+ and NH4+. At equal solution concentrations (0.5 mol m−3) net K+ uptake was inhibited 30–35% by NH4+ and NH4+ uptake was decreased 9–24%. Removal of NH4+ resulted in complete recovery in K+ uptake rate, but NH4+ uptake rate did not recover when K+ was removed. In both cases, inhibition of the uptake rate of one cation saturated as the concentration of the other cation was increased up to 0.5 mol m−3. The relative effect of K+-NH4+ interactions was not altered when Cl- was replaced with SO42−, but the magnitudes of the uptake rates were less in the absence of Cl-. The Vmax for NH4+ uptake was reduced from 128 to 105 μmol g−1 dry wt. h−1 in the presence of 0.5 mol m−3 K+ and the Km for NH4+ doubled from 12 to 27 mmol m−3 in the presence of K+. The results of these K+-NH4+ experiments are interpreted as mixed-noncompetitive interactions. However, an enhanced efflux of K+ coupled to NH4+ influx via an antiporter cannot be ruled out as contributing to the decrease in net K+ uptake.