K/Na selectivity at the plasmalemma of cortical root cells and preferential release of sodium to the xylem vessels in roots of <i>Atriplex hortensis</i>

Abstract

Using excised roots of Atriplex hortensis L., cv. Gelbe Gartenmelde, the uptake, accumulation and xylem transport of K + and Na + have been measured. Influx as well as xylem transport proved to discriminate little between K + and Na + , when considered in relation to the external solution. Both K + and Na + inhibited the uptake and xylem transport of each other to about the same degree. Measurements of intracel‐lular Na + fluxes by means of compartment analysis indicated that the low degree of K/Na discrimination during uptake was due to low influx selectivity. Moreover, K + /Na + exchange at the plasmalemma was not very efficient in Atriplex roots. In order to establish the basis of the low K/Na discrimination in xylem transport, the rates of K + and Na + transport were related to the cytoplasmic K + and Na + concentrations to yield the selectivity ratio of transport, S(transport) = (ϕ cx (K) × [Na + ] c )/(ϕ cx (Na) × [K + ] c ). Under all conditions this ratio was far below one indicating that Na + was favoured during xylem release in excised roots of Atriplex at low external concentrations. The implications of this discrimination in favour of Na+ are discussed with respect to salt tolerance of A. hortensis .