Explanation

According to active $K^{+}$ theory of Levitt, the stomatal opening and closing is regulated by ATP driven $K^{+}$ exchange pump. According to this theory, there is a accumulation of $K^{+}$ in the guard cells during day time 200. When guard cells have more $K^{+}$, endosmosis takes place, resulting in the lowering of osmotic potential of guard cells. They starts to absorb water from neighbouring cells and becomes turgid to make a pore or opening in the stoma. Thus, stomatal opening takes place. Due to the loss of $K^{+}$ the osmotic concentration of guard cells in comparison to adjoining epidermal cells decreases. Therefore, exosmosis takes place and guard cells becomes flaccid due to the loss of turgidity. Thus, stomatal closure takes place