Stomatal movement is controlled by external and internal signals such as light, phytohormones or cytoplasmic Ca(2+). Using Vicia faba L., we have studied the dose-dependent effect of auxins on the modulation of stomatal opening, mediated through the activity of the plasma-membrane H(+)-ATPase. The patch-clamp technique was used to elucidate the electrical properties of the H(+)-ATPase as effected by growth regulators and seasonal changes. The solute composition of cytoplasmic and extracellular media was selected to record pump currents directly with high resolution. Proton currents through the ATPase were characterized by a voltage-dependent increase in amplitude, positive to the resting potential, reaching a plateau at more depolarized values. Upon changes in extracellular pH, the resting potential of the cell shifted with a non-Nernst potential response (Β±21 mV), indicating the contribution of a depolarizing ionic conductance other than protons to the permeability of the plasma membrane. The use of selective inhibitors enabled us to identify the currents superimposing the H(+)-pump as carried by Ca(2+). Auxinstimulation of this electroenzyme resulted in a rise in the outwardly directed H(+) current and membrane hyperpolarization, indicating that modulation of the ATPase by the hormone may precede salt accumulation as well as volume and turgor increase. Annual cycles in pump activity (1.5-3.8 ΞΌA Β· cm(-2)) were expressed by a minimum in pump current during January and February. Resting potentials of up to -260 mV and plasmamembrane surface area, on the other hand, did not exhibit seasonal changes. The pump activity per unit surface area was approximately 2- to 3-fold higher in guard cells than in mesophyll cells and thus correlates with their physiological demands.