Simultaneous influx of ammonium and potassium into maize roots: kinetics and interactions

Abstract

The interaction between ammonium and potassium during influx was examined in roots of dark-grown decapitated corn seedlings (Zea mays L., cv. Pioneer 3369A). Influx was measured during a 10-min exposure to either ((15)NH4)2SO4 ranging from 10 to 200 μM NH 4 (+) with and without 200 μM K((86)Rb)Cl or to K((86)Rb)Cl ranging from 10 to 200 μM K(+) with and without 200 μM NH 4 (+) as ((15)NH4)2SO4. The simple Michaelis-Menten model described the data well only for potassium influx in the presence of ambient ammonium. For the other three instances, the data were improved by assuming that a second influx mechanism became operative as the low-concentration phase approached saturation. Two distinct mechanisms are thus indicated for both ammonium and potassium influx within the range of 10 to 200 μM.The influx mechanism operating at low concentrations showed greater affinity for potassium than for ammonium, even though the capacity for ammonium transport was twice as large as that for potassium. It is suggested that this phase involved a common transport system for the two ions and that localized low acidity next to the internal surface, following H(+) extrusion, favored ammonium deprotonation and dissociation from the transport system-ammonium complex. Parallel decreases in V max and increases in Km of the low-concentration saturable phase occurred for ammonium influx when ambient potassium was present and for potassium influx when ambient ammonium was present. The data support a mixed-type inhibition in each case. Simultaneous measurement of potassium and ammonium influx showed that they were highly negatively correlated at the lower concentrations, indicating that the extent to which influx of the inhibited ion was restricted was associated with influx of the inhibitor ion. Presence of ambient ammonium eliminated the second phase of potassium influx. In contrast, the presence of ambient potassium decreased the concentration at which the second phase of ammonium influx was initiated but did not restrict the rate.

Regulation of K+ Transport in Tomato Roots by the TSS1 Locus. Implications in Salt Tolerance

“…Evidence for an inhibitory effect of NH 4 ϩ on K ϩ transport has been reported in other plant species on the basis of short-term radiometric studies (Scherer et al, 1984;Vale et al, 1987Vale et al, , 1988Wang et al, 1996;Spalding et al, 1999;Santa-María et al, 2000). In medium containing NH 4 ϩ , increasing K ϩ concentrations also evoked rapid membrane depolarizations in both wild-type and tss1 seedlings (Fig.…”

Rapid Ammonia Gas Transport Accounts for Futile Transmembrane Cycling under NH3/NH4 + Toxicity in Plant Roots

“…1, A-C). While previous tracer studies have also demonstrated that NH 3 /NH 4 + fluxes exceed those of K + at equimolar concentrations (Scherer et al, 1984;Vale et al, 1988;Wang et al, 1996), none have provided parallel membrane potential measurements. A comparison between fluxes and ΔΔC m , however, is of great utility in gauging the relative apportionment of NH 3 and NH 4 + fluxes, as we show here.…”

Ammonium Inhibition of Arabidopsis Root Growth Can Be Reversed by Potassium and by Auxin Resistance Mutations aux1, axr1, and axr2

“…Because the size and charge of potassium ions are similar to those of ammonium ions (Buurman et al, 1991), ammonium can inhibit potassium uptake (Smith and Epstein, 1964;Ajayi et al, 1970;Rufty et al, 1982;Deane-Drummond and Glass, 1983;Scherer et al, 1984;Vale et al, 1987Vale et al, , 1988 and is transported by a potassium channel (Schachtman et al, 1992).…”