Acid-Induced Stomatal Opening in Vicia faba L. and the Role of Guard Cell Wall Elasticity

When epidermal peels of Vicia faba L. were treated with solutions of varying pH, stomatal aperture was significantly increased at pH 4.0, 3.0, and 2.7 in darkness, but not in light. This effect was independent of the presence of KCl in the medium. Using a short-term plasmolytic method, estimates were made of the osmotic pressure (II_i) and the volumetric elastic modulus of guard cells, the aperture of which varied due to pretreatments at different pH, in darkness or light. In darkness, the lower pH pretreatments induced an increase in II_i and a decrease in volumetric elastic modulus. In comparison to the response in unbuffered solutions, 10 and 25 millimolar Mes buffer at pH 6.5 significantly reduced the degree of stomatal opening induced by light or by fusicoccin. These results indicate that acid-induced stomatal opening is, at least partially, due to an increase in guard cell wall elasticity which occurs in association with changes in II_i. It is suggested that the observed inhibitory effect of Mes buffer on stomatal opening may be due to a reduction in the degree of acidification of the guard cell wall and a consequent decrease of cell wall elasticity.