The opening and closing of stomata regulates the transpiration. What is its mechanism?

Most plants keep their stomata open during the day and close them at night. The regulation of transpiration through stomata depends upon guard cells. Each stoma is surrounded by two guard cells, which are attached to each other at their ends. The inner concave sides of guard cells are thicker than the outer convex sides.

Mechanism

Initially, it was thought that concentration of glucose in guard cells is responsible for opening and closing stomata. When guard cells become turgid, their shapes are like two beans and stoma between them opens. When the guard cells loose water and become flaccid, their inner sides touch each other and the stoma closes.

Recently, it is has been revealed that opening and closing of stomata depends upon the movement of Potassium ions in and out of guard cells. The blue wavelengths of daylight cause the K⁺ to flow into the guard cells, from the surrounding epidermal cells. Water passively follows these ions into the guard cells. The guard cells become turgid and open. During the night time, the K⁺ flows back to the surrounding epidermal cells, which also lead to loss of water. Guard cells become flaccid and stomata close.