<p>The stomatal opening is currently known to open with sucrose and K⁺ acting as the main osmotic substances. However, all theories related to stomatal closing focus solely on closure through the signal transduction pathway mediated by abscisic acid (ABA). There is now an urgent need for the researches that explore the new perspective that stomatal closing may be induced not by ABA but by the release of sucrose and K⁺ from the vacuole. Since stomatal researches used physical force to peel off the lower epidermis of leaves, it has been newly recognized that the main osmotic agent for stomatal opening is K⁺. However, recently, the theory that sucrose is very likely to be the main osmotic material for stomata opening has been receiving renewed attentions and supports. While the dominant view is that K⁺ is the most important osmotic material for stomatal opening, there is also a coexisting view that sucrose is also the most important osmotic material for stomatal opening. A study on stomatal closure was also performed using separated epidermis, and it was reported that ABA induces an increase in the cytoplasmic Ca²⁺ concentration, which closes stomata. ABA-mediated stomatal closing mechanisms are triggered by the release of K⁺, malate²⁻, and Cl⁻ from the vacuole into the cytoplasm. The mechanism of stomatal closing may have opposite properties to the mechanism of stomatal opening. However, if the main osmotic agent for stomatal opening is sucrose, the mechanism of stomatal closing would be different.</p>