During the opening of stomata, malic acid dissociates into malate ion and ${\mathrm{H}}^{+}$ ions. An energy-dependent proton pump starts and transports ${\mathrm{H}}^{+}$ ions out of guard cells and ${\mathrm{K}}^{+}$ ions enter inside. The K⁺ ions combine with malate ion to form potassium malate and thus increase the OP of guard cells. As a result, endosmosis occurs and stomata open.