Influence of ambient ammonium on net potassium uptake by decapitated maize seedlings

Abstract

The effects of ambient ammonium on potassium uptake and accumulation in xylem exudate were examined in 6‐d‐old decapitated maize (Zea mays L., ev. Pioneer 3369a) seedlings, Net uptake of both K+ and NH+4 was measured by solution depletion during a 5 h treatment period. [86Rb]K+ influx during a 10 min uptake period was also measured for some treatment combinations over the 5 h period. The presence of ambient 0.2 mM NH+4initially resulted in net efflux of K+ to solution containing 0.2 mM K+, as well as to K+‐free solutions. A transition from net K+ efflux to net K+ uptake occurred after 1.25–1.5 h in solutions containing 0.2 mM K+ and 0–2 mM NH+4. After the transition, the rate of K+ uptake was ∼ 50% of that found in the absence of ambient NH+4 [86Rb]K+ influx from 0.2 mM K+ was inhibited by 50% in the presence of 0.2 mM NH+4. Increasing ambient NH+4 to 2.0 mM increased the initial net efflux of K+ and delayed the time at which transition to net uptake occurred. The combined data indicate that the inhibitory effect of ambient NH+4 on net K+ uptake resulted from an initial but temporary enhancement of K+ efflux, and a constant inhibition of K+ influx. A reciprocal inhibition of net NH+4 uptake by K+ was not observed. Net NH+4 uptake initially was associated with K+ efflux, after which a transition to net H+ occurred. In the presence of NH+4, the transition from net K+ efflux to net K+ uptake, and the associated onset of net H+ efflux took place without measurable alterations in the rate of K+ accumulation in the xylem fluid. These results suggest spatially separate compartments for K+ efflux and xylem deposition. Copyright © 1988, Wiley Blackwell. All rights reserved