Membrane depolarization induces K+ efflux from subapical maize root segments

•The role of potassium efflux from maize (Zea mays) root segments in maintaining transmembrane electric potential difference (Em) was studied in vivo, together with the involvement of outward rectifying K+ channels (ORCs). •Measurements were made of the efflux of potassium (K+) from roots when its uptake was competitively inhibited by rubidium (Rb+), of the Em of the root cells by microelectrodes and of the unidirectional fluxes of monovalent cations. •The influx of Rb+, caesium (Cs+) or ammonium (NH4+) into the segments induced an efflux of K+. Lithium (Li+) and sodium (Na+) were not taken up and did not induce K+efflux. The permeating cations induced membrane depolarizations, which were closely related to the values of K+ efflux. Two K+-channel blockers, tetraethylammoniumchloride and quinidine, inhibited K+ efflux. The inhibition was accompanied by a higher membrane depolarization induced by Rb+, whose influx was not affected. •The results suggest that a depolarizing event caused by cation uptake increased K+ efflux from the cells, probably through the activation of ORCs involved in restoration and stabilization of Em.