Guard Cell Protoplast Isolation and Electrophysiological Recordings. Plants were grown on compost for 5 weeks in a greenhouse. Guard cell protoplasts were enzymatically isolated, and patch-clamp experiments were carried out as described (10). The pipette solution contained 100 mM K-glutamate, 2 mM MgATP, 5 mM EGTA, 1 mM CaCl₂ (50 nM free Ca²⁺), 0.5 mM MgCl₂, 300 mM d-mannitol, and 20 mM Hepes-KOH (pH 7.25). The bath solution contained 20 mM CaCl₂, 2 mM MgCl₂, 100 mM K-glutamate, 225 mM d-mannitol, and 10 mM Mes-HCl (pH 5.5). Liquid junction potentials at the pipette–bath interface were measured and corrected.