The opening and closing of the stomatal pore depends upon:

The accumulation and loss of K+ ions cause the opening and closing of stomata. During day time, K+ ions are accumulated by the guard cells which increases the solute concentration and decreases the water potential. Thus, water enters the cell by endosmosis, cells become turgid and the stomatal pore open. During night time, guard cells lose the K+ ions which decreases the solute concentration and increases the water potential. Thus, water leaves the cell by exosmosis, cells lose turgidity and the stomatal pore close.