Opening and closing of stomata is due to the : -

Each stoma is surrounded by two guard cells.
When water enters the guard cells (after $K^+$ ions move in), they become turgid. Because their inner walls are thick and inelastic and the cellulose microfibrils are arranged radially, they bend outward and the stoma opens.
When the guard cells lose water and become flaccid, the stoma closes.
So the opening and closing of stomata is due to the change in turgor pressure of the guard cells.