Cs+ uptake in subapical maize [Zea mays] root segments: Mechanism and effects on H+ release, transmembrane electric potential and cell pH
1997
Sacchi, G.A. (Universita degli Studi di Milano (Italy)) | Espen, L. | Nocito, F. | Cocucci, M.
Subapical segments from maize (Zea mays) root took up CS+; at low external concentrations the kinetic constants for the influx of this cation were similar to hose for the uptake of Rb+ and K+. Competition experiments suggested that the three cations use the same transport system. At higher concentrations 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 (5mM) the H+ release was higher in the presence of K+ than with Rb+ or Cs+. The transmembrane electrical potential difference (Em) was depolarized when Cs+, Rb+ or K+ were present in the incubation media; this depolarization was greater in the presence of K+ and was particularly evident at higher concentrations: the Em was - 85 mV, -105 mV and -119 mV in 5 mM K+, Rb+ or Cs+, respectively. Tetraethylammonium-chloride, a K+ channel blocker, strongly hyperpolarized Em 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
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