PERMEATION OF CA2+ AND MONOVALENT CATIONS THROUGH AN OUTWARDLY RECTIFYING CHANNEL IN MAIZE ROOT STELAR CELLS

A depolarization-activated outwardly-rectifying channel (OR), most lik ely involved in the passive release of K+ from the root symplasm into the stelar apoplast (for subsequent transport to the shoot via the xyl em vessels), has been characterized in the plasma membrane of maize ro ot stelar cells (Roberts and Tester, 1995), In the present study, the selectivity of this channel was further characterized using single cha nnel current-voltage curves generated using a voltage ramp protocol. T his protocol permitted the accurate and unambiguous measurement of the reversal potentials of currents resulting from single channel opening s, Using the voltage ramp protocol, it was shown that the OR allows bo th K+ efflux and Ca2+ influx at potentials positive of E-K and negativ e of E-Ca, The OR had a P-Ca/P-K of 1.72-0.21 decreasing as extracellu lar Ca2+ was increased, The permeability of the OR for monovalent cati ons other than K+ was also investigated. In bi-ionic conditions, a rel ative permeability sequence of K+ greater than or equal to Rb+ > Cs+ > Na+ = NH4+ was determined Eisenman sequence IV), The physiological im plications of the selectivity of the OR are discussed.