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

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 Ca²⁺ signalling, but the impact of transient Ca²⁺ signals on S‐type anion channel activity remains unknown. In this study, transient elevation of the cytosolic Ca²⁺ 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 Ca²⁺ concentration. The increase in the cytosolic Ca²⁺ level was accompanied by activation of S‐type anion channels. Guard cell anion channels were activated by Ca²⁺ with a half maximum concentration of 515 nm (SE = 235) and a mean saturation value of ?349 pA (SE = 107) at ?100 mV. Ca²⁺ 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 Ca²⁺ level was observed, activating S‐type channels without measurable delay. These data show that cytosolic Ca²⁺ 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 Ca²⁺ transport proteins, resulting in an overshoot of the cytosolic Ca²⁺ level after returning the membrane potential to ?100 mV.