Transmembrane K+ transport system : selectivity, reciprocal integration and metabolic implications in maize roots

Summary
Subapical segments from maize roots took up Cs+, Rb+ and K+. At low external
concentrations (up to 0.5 mM) the kinetics of Cs+, Rb+ or K+ influxes were very similar
and competition experiments suggested that the three cations use the same transport
system. At higher concentrations Cs+ was discriminated. Up to 0.5 mM, the net H+
efflux was similarly stimulated by the presence of Cs+, Rb+ or K+, at higher
concentration the net H+ efflux was higher in K+. Potassium induced at 5 mM a
depolarisation of the transmembrane electric potential difference induced by Rb+ and particularly by Cs+. JEA+, a K+- channel blocker, strongly
hyperpolarized Em in the presence of K+, but had slight or no effect in Rb+ or Cs+,
respectively. Cs+ influx was accompanied by a C02 dark fixation lower than that
induced by K+ and Rb+. These results are consistent with the activity of a cation-H+
co- uptake mechanism, showing low selectivity between Cs+, Rb+ and K+, primarily
operating at low external concentrations, and of a channel- mediated system more
selective for K+, operating at higher concentrations; these two systems differently
involve H+ transport activity and metabolic pH- stat mechanism. An effect of Cs+ on
outward directed K+ channels, mediating the efflux of the cation from the cells, is also
reported. These results are discussed in term of inhibition of K+ xylem loading