Fluxes of H+ and K+ in Corn Roots

Abstract

We report here on an experimental system that utilizes ion-selective microelectrodes to measure the electrochemical potential gradients for H+ and K+ ions within the unstirred layer near the root surface of both intact 4-day-old corn seedlings and corn root segments. Analysis of the steady state H+ and K+ electrochemical potential gradients provided a simultaneous measure of the fluxes crossing a localized region of the root surface. Net K+ influx values obtained by this method were compared with unidirectional K+ (86Rb+) influx kinetic data; at any particular K+ concentration, similar values were obtained by either technique. The ion-specific microelectrode system was then used to investigate the association between net H+ efflux and net K+ influx. Although the computed H+:K+ stoichiometry is dependent upon the choice of diffusion coefficients, the values obtained were extremely variable, and net K+ influx rarely appeared to be charge-balanced by H+ efflux. In contrast to earlier studies, we found the cortical membrane potential to be highly K+ sensitive within the micromolar K+ concentration range. Simultaneous meaurements of membrane potential and K+ influx, as a function of K+ concentration, revealed similar Km values for the depolarization of the potential (Km 6-9 micromolar K+) and net K+ influx (Km 4-7 micromolar K+). These data suggest that K+ may enter corn roots via a K+-H+ cotransport system rather than a K+/H+ antiporter.