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Summary The membrane potentials of corn coleoptile cells are measured during the alteration of the potassium ion concentration within the medium. The relative change Δ E E A of the membrane potential is logarithmic-linear in respect to the ionic strength of the medium as well as to the calcium ion content. From these findings equations are derived which express the membrane potentials as functions of both calcium and potassium ion concentrations of the medium. It is suggested from these equations that low concentrations of calcium (up to 0.2 mM) cause a hyperpolarization followed by a depolarization at higher calcium levels (0.2—6 mM). Furthermore, rising concentrations of calcium ions within the medium depress the potassium-induced drop of the membrane potential, especially at low potassium ion concentrations (up to 10 mM).

Summary The cell potential of corn (Zea mays L.) coleoptile cells is decreased within a few seconds with increasing concentrations of potassium and sodium ions in the external solution. The effects of these ions upon the membrane potential is reduced by both IAA and calcium ions. Both calcium ions and IAA together are less active than calcium ions alone. We conclude that calcium ions as well as IAA act at the plasma membrane.