Ca2+‐dependent activation of guard cell anion channels, triggered by hyperpolarization, is promoted by prolonged depolarization

*SUMMARY Rapid stomatal closure is driven by the activation of S-type anion channels in the plasma membrane of guard cells. This response has been linked to Ca 2+ signalling, but the impact of transient Ca 2+ signals on S-type anion channel activity remains unknown. In this study, transient elevation of the cytosolic Ca 2+ level was provoked by voltage steps in guard cells of intact Nicotiana tabacum plants. Changes in the activity of S-type anion channels were monitored using intracellular triple-barrelled micro-electrodes. In cells kept at a holding potential of )100 mV, voltage steps to )180 mV triggered elevation of the cytosolic free Ca 2+ concentration. The increase in the cytosolic Ca 2+ level was accompanied by activation of S-type anion channels. Guard cell anion channels were activated by Ca 2+ with a half maximum concentration of 515 nM (SE = 235) and a mean saturation value of )349 pA (SE = 107) at )100 mV. Ca 2+ signals could also be evoked by prolonged (100 sec) depolarization of the plasma membrane to 0 mV. Upon returning to )100 mV, a transient increase in the cytosolic Ca 2+ level was observed, activating S-type channels without measurable delay. These data show that cytosolic Ca 2+ elevation can activate S-type anion channels in intact guard cells through a fast signalling pathway. Furthermore, prolonged depolarization to 0 mV alters the activity of Ca 2+ transport proteins, resulting in an overshoot of the cytosolic Ca 2+ level after returning the membrane potential to )100 mV.