What causes the opening and closing of guard cells of stomata during transpiration?

The tiny pores present on the surfaces of leaves, called stomata, help in the exchange of gases. Each stoma consists of bean-shaped or dumbbell-shaped guard cells. The epidermal cells surrounding the guard cells are modified to form subsidiary cells. The opening and closing of the guard cells is caused by a change in their turgidity. The inner walls of the guard cells are thick and elastic, while the outer walls are thin. The numerous microfibrils present in the guard cells facilitate the opening and closing of the guard cells.
At the time of the opening of the stomata, the turgidity of the guard cells increases. As a result, the outer walls bulge and the inner walls become crescent-shaped. The stomatal opening is facilitated by the radial arrangement of the microfibrils.
At the time of the closing of the stomata, the guard cells lose their turgidity, the outer and inner walls retain their original shapes, and the microfibrils get arranged longitudinally.

 A change in the turgidity of guard cells results in closing or opening of stomata. The inner wall of the guard cell; towards the stomatal aperture; is thick and elastic. An increase in turgidity results in the thin outer walls to bulge out. This forces the inner wall into a crescent shape and results in opening of stoma. The orientation of the microfibrils in the cell walls of the guard cells also helps in opening of stomata. These meicrobifibrils are radially oriented and thus make it easy for the stoma to open up. A loss in turgidity of the guard cells, leads to resumption of shape of the elastic inner wall of the guard cell and the stoma closes.

Normally stomata are open in the day time and close during the night.

The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells. The inner wall of each guard cell, towards the pore is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture thin outer walls bulge out and force the inner walls into a crescent shape. The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells.

The general mechanism of stomatal movement is based on entry and exit of water molecules in guard cells. When the turgidity increases within the guard cells the stomata open. When the guard cells lose water it becomes flaccid and the stomata closes.