Cs+ uptake in subapical root segments : mechanism and effects on H+ release, transmembrane electric potential and cell pH

Subapical segments from maize (Zea mays) root took up Cs+; at low external concentrations (less than or equal to 0.25 mM) the kinetic constants for the influx of this cation were similar to those for the uptake of Rb+ and K+ (K-m=26, 26 and 22 mu M and V-max=3.62, 3.94 and 4.09 mu mol h(-1) (g FW)(-1), respectively). Competition experiments suggested that the three cations use the same transport system. At higher concentrations (>0.25 mM) the discrimination between Cs+ and Rb+ increased. At low external concentrations, the release of H+ into the medium was promoted similarly by Cs+, Rb+ or K+; at higher concentration (5 mM) the H+ release was higher in the presence of K+ than with Rb+ or Cs+. The transmembrane electrical potential difference (E(m)) was depolarised when Cs+, Rb+ or K+ were present in the incubation media; this depolarisation was greater in the presence of K+ and was particularly evident at higher concentrations: the E(m) was -85 mV, -105 mV and -119 mV in 5 mM K+, Rb+ or Cs+, respectively. Tetraethylammonium-chloride, a K+-channel blocker, strongly hyperpolarized E(m) in the presence of K+ but had little or no effect with Rb+ or Cs+, respectively. The influx of Cs+ was accompanied by less dark fixation of CO2 than was that of K+ or Rb+. These results are consistent with the activity of a system showing low selectivity between Cs+, Rb+ and K+, operating primarily at low external concentrations, which possibly uses a cation-H+ co-uptake mechanism, and of a channel-mediated system more selective for K+, operating at higher concentrations; these systems differ in their involvement of H+ transport activity and the metabolic pH-stat mechanism.