Cs^+ Uptake in Subapical Maize Root Segments: Mechanism and Effects on H^+ Release, Transmembrane Electric Potential and Cell pH

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Subapical segments from maize (Zea mays) root took up Cs^+; at low external concentrations (≦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 μM and V_<max>=3.62, 3.94 and 4.09 μmol 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 CO_2 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.

Cs^+ Uptake in Subapical Maize Root Segments: Mechanism and Effects on H^+ Release, Transmembrane Electric Potential and Cell pH

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