Extracellular calcium is involved in stomatal movement through the regulation of water channels in broad bean

Involvement of extracellular Ca²⁺ in stomatal movement through the regulation of water channels was investigated in broad bean (Vicia faba L.). Leaf peels were first incubated to open stomata, and then transferred to buffers in the presence of different CaCl₂ concentrations. Stomatal status was observed under magnification and stomatal aperture (pore width/length) was measured. Stomatal closure was significantly induced and aperture oscillation occurred at lower extracellular concentrations of calcium (Ca²⁺]_ext), while at higher concentrations, no significant change in stomatal aperture was observed, which was similar to the response recorded with HgCl₂. Lower Ca²⁺]_ext-induced stomatal closure could be reversed using depolarizing buffer. It is suggested that lower Ca²⁺]_ext regulates water channels through an indirect way and at higher concentrations, extracellular Ca²⁺ is involved in regulating stomatal aperture by directly influencing water channels to retard aperture change.