Potassium uptake in the halophyte Halimione portulacoides L. Aellen

•K+ transport in K+ and Na+-starved Halimione portulacoides roots has a Km of 300 μM.•K+ transport decreases at high salinity but membrane potential is hardly affected.•K+ transport is inhibited by barium, cesium and unaffected by TEA+, ammonium and pH.•Cytosolic K+ levels decrease from 75 mM without Na+ to 55 mM at 300 mM Na+.•K+ transport is mediated by AKT-type channels in plants grown at high or low salinity.

The characteristics of root K+ uptake in the halophyte Halimione portulacoides have been studied using electrophysiological techniques. Plantlets grown in the absence of both K+ and Na+ are able to take up K+ at micromolar external concentrations and root cells of these plants depolarize in response to these K+ concentrations. Depolarizations show saturation kinetics with a Km around 300 μM K+. Rubidium also induces membrane depolarizations and inhibits K+ transport at low micromolar concentrations whereas Cs+ hardly depolarizes the membrane. The addition of millimolar Na+ reduces both the affinity for K+ and the maximum transport rates, and membrane potentials become more positive only when Na+ concentration reaches 300 mM. Plantlets grown at high Na+ show the same effects on K+ transport but membrane potentials are maintained at highly negative values. Plantlets grown in the absence or the presence of Na+ show similar responses to different inhibitors of K+ uptake. TEA+ does not affect K+ transport, Cs+ produces a partial inhibition whereas Ba2+ completely blocks the process. Besides, the external pH does not affect K+ transport. However, NH4+ inhibits partially this process in plants grown without Na+ but has no effect on plants grown at 300 mM Na+. Altogether these results point out that the transport of K+ in this species is mediated mainly by channels, most probably of the AKT-1 type, both in plants grown with or without Na+; however, the operation of potassium transporters cannot be excluded in plants grown in the absence of Na+. Cytosolic K+ activities were also measured in plantlets grown without or with 300 mM Na+, showing lower values at high Na+. Calculation of the electrochemical potential gradient for K+ in these plants indicates that the transport would be passive, i.e. mediated by channels, at concentrations higher than 20 μM or 120 μM, in the absence or the presence of 300 mM Na+, respectively.