Electrical Potentials in Stomatal Complexes

Abstract

Guard cells of several species, but predominantly Commelina communis, were impaled by micropipette electrodes and potential differences measured that occurred between cell compartments and the flowing bathing medium. The wall developed a Donnan potential that was between -60 and -70 mV in 30 mm KCl at pH 7. The density of the fixed charges ranged from 0.3-0.5 M; its dependence on pH was almost identical with the titration curve of authentic polygalacturonic acid. The vacuolar potential of guard cells of C. communis L., Zea mays L., Nicotiana glauca Graham, Allium cepa L. and Vicia faba L. was between -40 and -50 mV in 30 mM KCl when stomata were open and about -30 mV when stomata were closed. The vacuolar potential of guard cells of C. communis was almost linearly related to stomatal aperture and responded to changes in the ionic strength in the bathing medium in a Nernstian manner. No specificity for any alkali ion (except Li+), ammonium or choline appeared. Li caused hyperpolarization. Ca in concentrations between 1-100 mM in the medium led to stomatal closure, also caused hyperpolarization and triggered transient oscillations in the intracellular potential. Gradients in the electrical potential existed across stomatal complexes with open pores. When stomata closed, these gradients almost disappeared or slightly reverted; all epidermal cells were than at potentials near -30 mV in 30 mM KCl.