Closing of stomata is induced by abscisic acid

The scarcity of water in the soil causes the root to release abscisic acid (ABA). The ABA binds to the receptor protein of the guard cell. It raises the pH of the cytosol of the cell. The calcium ions enter into the cell from the outside as well as from the endoplasmic reticulum and vacuoles and making the intracellular calcium concentration to increase. The increase in intracellular calcium ions makes the chloride ions and organic ions to move out of the cell and prevent the further uptake of potassium ions. This makes the water potential rise in the guard cell resulting in the movement of water out of the cell by diffusion. This shrinks the guard cell and closes the stomatal pores.

The structure of stomata varies in different plants:

Anomocytic stomata are enclosed by guard cells. They have the same size, shape, and arrangement as guard cells.

Anisocytic stomata are specific in that they have two large subsidiary cells and one smaller cell around the guard cells.

Diacytic stomata are enclosed by two subsidiary cells and they are positioned at a right angle to guard cells.

Paracytic stomata may have one or more subsidiary cells. They are positioned parallel to the stomatal pore and guard cells.

Gramineous stomata are bordered by two lens-shaped subsidiary cells. The guard cells are thick at both ends and narrower at the middle.

Actinocytic stomata are bordered by at least five radiating subsidiary cells. It is a rare type of stomata.

Tetracytic stomata are bordered by four subsidiary cells. Two stomata are present at the opening ends and two stomata are next to guard cells.