Effects of rubidium and potassium on potassium efflux from subapical segments of maize roots

Subapical maize (Zea mays) root segments incubated in the presence of Rb+ released K+ and absorbed Rb+. In the presence of other cations (Li+, Na+ and Ca2+) there was no K+ release. K+ release per unit of area through the cut ends was similar to the one through the intact surfaces of the root segments, indicating that it depended on efflux from root cells. K+ efflux greatly increased with increasing Rb+ concentrations up to 0.1 mM; at higher concentrations, the increase in K+ efflux was slight. K+ efflux decreased slightly in the presence of fusicoccin (FC) and increased in the presence of NaN3 or carbonylcyanide m-chlorophenyl hydrazone (CCCP). Rb+ influx, on the contrary, increased greatly in the presence of FC and decreased in the presence of NaN3 or CCCP. The different kinetics of K+ efflux and Rb+ influx suggest that they are mediated by different transport systems. A physiological role for K+ efflux promoted by Rb+ is suggested by results, obtained using Rb-86+-preloaded root segments, showing that incubation in K+ or Rb+ induced a similar efflux of the radiotracer. These results suggest that incubation in Rb+ was an useful method to measure K+ efflux, and that the presence of Rb+ or K+ in the medium increased K+ permeability, probably through the opening of K+ channels, which might play a role in restoration and stabilization of transmembrane electric potential.