The relative contribution of the high-affinity K+ transporter AtHAK5 and the inward rectifier K+ channel AtAKT1 to K+ uptake in the high-affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col-0). The results obtained with wild-type lines, with T-DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the -sensitive and the Ba2+-sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high-affinity range of Rb+ concentrations. Using Rb+ as a K+ analogue, it was shown that AtHAK5 mediates absorption at lower Rb+ concentrations than AtAKT1 and depletes external Rb+ to values around 1 μM. Factors such as the presence of K+ or during plant growth determine the relative contribution of each system. The presence of in the growth solution inhibits the induction of AtHAK5 by K+ starvation. In K+-starved plants grown without , both systems are operative, but when is present in the growth solution, AtAKT1 is probably the only system mediating Rb+ absorption, and the capacity of the roots to deplete Rb+ is reduced.