CS- MECHANISM AND EFFECTS ON H+ RELEASE, TRANSMEMBRANE ELECTRIC-POTENTIAL AND CELL PH( UPTAKE IN SUBAPICAL MAIZE ROOT SEGMENTS )

Subapical segments from maize (Zea mays) root took up Cs+; at low exte rnal concentrations (less than or equal to 0.25 mM) the kinetic consta nts 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 m u mol h(-1) (g FW)(-1), respectively). Competition experiments suggest ed that the three cations use the same transport system. At higher con centrations (>0.25 mM) the discrimination between Cs+ and Rb+ increase d. At low external concentrations, the release of H+ into the medium w as 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 depolar ised when Cs+, Rb+ or K+ were present in the incubation media; this de polarisation was greater in the presence of K+ and was particularly ev ident 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 infl ux of Cs+ was accompanied by less dark fixation of CO2 than was that o f K+ or Rb+. These results are consistent with the activity of a syste m showing low selectivity between Cs+, Rb+ and K+, operating primarily at low external concentrations, which possibly uses a cation-H+ co-up take mechanism, and of a channel-mediated system more selective for K, operating at higher concentrations; these systems differ in their in volvement of H+ transport activity and the metabolic pH-stat mechanism .