Transpiration:

Transpiration is the loss of water from the plant surface through evaporation.

Opening and closing of stomata:

Most plants keep their stomata open during the day and close them at night.

Functions of guard cells:

It is the responsibility of the stomata to regulate transpiration via guard cells. The two guard cells of a stoma are attached at their ends. The inner concave sides of guard cells that enclose a stoma are thicker than the outer convex sides. When these guard cells get water and become turgid, their shapes are like two beans and the stoma between them opens. When the guard cells lose water and become flaccid, their inner sides touch each other, and the stoma closes.

The function of Potassium ions in opening and closing of stomata:

Recent studies have revealed that stomata open and close due to the movement of potassium ions in and out of guard cells. According to blue wavelengths of daylight open stomata by allowing K to flow into the guard cells, from the surrounding epidermal cells. Water passively follows these ions into the guard cells, and as their turgidity increases the stoma opens. As the day progresses, guard cells make glucose. Due to a higher concentration of glucose, their water potential decreases and water stays in them. At the end of the day, the K' flows back from guard cells to the epidermal cells, and the concentration of glucose also falls. This initiates the loss of water and reduced turgor pressure in guard cells, which causes the closure of the stoma.