Temperature dependencies and apparent activation energies of stomatal opening and closing

Abstract

Stomatal opening movements in response to illumination, and stomatal closure following darkening were studied in leaf sections of Zea mays, using air-flow porometers. Stomatal opening is characterized by a phase of linear increase of air flow through the leaf (slope = “opening velocity”); stomatal closure follows a relaxation curve from which a time constant (“closing coefficient”) can be derived. Apparent energies of activation, μ, were computed for the opening velocity and for the closing coefficient from stomatal movements recorded at tissue temperatures between 5° and 50°. It was assumed that the closing coefficient can be used as a measure of the closing force, and that the opening force has to exceed the closing force in order to bring about stomatal opening. μ is about 7 kcal mole^-1 for the closing coefficient and between 12 and 18 kcal mole^-1 for the opening force. Thus, during stomatal opening, metabolism must provide energy to build up a pressure difference between guard cells and the surrounding tissue. The process controlling the velocity of closure is essentially a passive loss of water (and solutes?) from the guard cells. The μ of 7 kcal mole^-1 found for the closing coefficient is, however, higher than that for the viscosity of water or for the coefficient of self diffusion of water. It is, therefore, concluded either that water interacts with the cell structures which it has to permeate during stomatal closure, or that the rate of salt loss from guard cells controls the velocity of stomatal closure. The closing force decreases when leaf temperature rises above 35° or falls below 15°. Therefore, stomata of maize open relatively faster and wider above 35° and below 15°, and the μ's of the opening velocity appear to be very large above 35° (up to 50 kcal mole^-1) while they have a negative sign below 15°.