No uptake of anions required by opening stomata of Vicia faba: Guard cells release hydrogen ions

Abstract

Epidermal strips from leaves of Vicia faba L. with ruptured epidermal cells and intact guard cells were exposed to solutions of K⁺ in association with non-absorbable anions. KCl served as control. Stomata exposed to a range of concentrations of K iminodiacetate, K 4,4-dimethyl-4,7-diazadecane-1,10-disulfonate and K benzene sulfonate opened as widely as on KCl, indicating that K⁺ can be taken up by guard cells without the necessity of an anion traveling along. Electroneutrality was maintained by an exchange of K⁺ for H⁺. Release of H⁺ from guard cells was recorded as a drop in the pH of the solution on which the epidermal samples floated. Formation of acid equivalents by the guard cells was also recorded by automatic titration of the bathing solution at constant pH while CO₂ was continuously being removed. A considerable amount of H⁺ was released from the epidermis by ion exchange (about 8x10^-10 eq/mm²). Subtracting this quantity from the total amount of H⁺ titrated resulted in an estimate of acid production during stomatal opening of 1.2 to 7x10^-10 eq/mm² or 1.5 to 8.5x10^-12 eq/stoma. These amounts are equivalent to the known capacity of the guard cells of Vicia faba to absorb K⁺.