Sodium and Potassium Fluxes and Compartmentation in Roots of Atriplex and Oat

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Abstract
K+ and Na+ fluxes and ion content were studied in roots of Atriplex nummularia Lindl. and Avena sativa L. cv. Goodfield grown in 3 mM K+ with or without 3 or 50 mM NaCl. Compartmental analysis was carried out with entire root systems under steady-state conditions. Increasing ambient Na+ concentrations from 0 to 50 mM altered K+, in Atriplex, as follows: slightly decreased the cytoplasmic content (Qc) the vacuolar content (Qr), and the plasma membrane influx and efflux. Xylem transport for K+ decreased by 63% in Atriplex. For oat roots, similar increases in Na+ altered K+ parameters as follows: plasma membrane influx and efflux decreased by about 80%. Qc decreased by 65%, and xylem transport decreased by 91%. No change was observed in Qv for K+. Increasing ambient Na+ resulted in higher (3-5-fold) Na+ fluxes across the plasma membrane and Qc of both species. In Atriplex, Na+ fluxes across the tonoplast and Qv increased as observed in Na+ was increased. In oat no significant change was observed in Na+ flux across the tonoplast or in Qv as external Na+ was increased. In oat roots, Na+ reduced K+ uptake markedly; in Atriplex, this was not as pronounced. Even at high Na+ levels, the influx transport system at the plasma membrane of both species perferred K+ over Na+. Based upon the Ussing-Teorell equation, active inward transport of K+ occurred across the plasma membrane, and passive movement of K+ occurred across the tonoplast in both species. Na+, in oat roots, was actively pumped out of the cytoplasm to the exterior, whereas, in Atriplex, Na+ was passively distributed between the free space, cytoplasm, and vacuole.
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