Sodium transport measured in plasma membrane vesicles isolated from wheat genotypes with differing K+/Na+ discrimination traits

Right-side-out plasma membrane vesicles were isolated from wheat roots using an aqueous polymer two-phase system, The purity and orientation of the vesicles were confirmed by marker enzyme analysis, Membrane potential (Delta Psi)-dependent Na-22(+) influx and sodium/proton (Na+/H+) antiport-mediated efflux across the plasma membrane were studied using these vesicles, Membrane potentials were imposed on the vesicles using either K+ gradients in the presence of valinomycin or H+ gradients, The Delta Psi was quantified by the uptake of the lipophilic cation tetraphenylphosphonium. Uptake of Na+ into the vesicles was stimulated by a negative Delta Psi and had a K-m for extravesicular Na+ of 34.8+/-5.9 mol m(-3). The Delta Psi-dependent uptake of Na+ was similar in vesicles from roots of hexaploid (cv, Troy) and tetraploid (cv, Langdon) wheat differing in a K+/Na+ discrimination trait, and was also unaffected by growth in 50 mol m(-3) NaCl. Inhibition of Delta Psi-dependent Na+ uptake by Ca2+ was greater in the hexaploid than in the tetraploid. Sodium/proton antiport was measured as Na+-dependent, amiloride-inhibited pH gradient formation in the vesicles, Acidification of the vesicle interior was measured by the uptake of C-14-methylamine. The Na+/H+ antiport had a K-m for intravesicular Na+ of between 13 and 19 mol m(-3). In the hexaploid, Na+/H+ antiport activity was greater when roots were grown in the presence of 50 mol m(-3) NaCl, and was also greater than the activity in salt-grown tetraploid wheat roots, Antiport activity was not increased in a Langdon 4D chromosome substitution line which carries a trait for K+/Na+ discrimination, It is concluded that neither of the transport processes measured is responsible for the Na+/K+ discrimination trait located on the 4D chromosome of wheat.