In the mechanism of opening and closing of stomata, the important factor is

In the mechanism of opening and closing of stomata, the important factor is the shape of the guard cells.
When the concentration of potassium ions increases in the guard cell, it allows the guard cell to absorb water from the surrounding cells. This turgidity of the guard cell cause opening of the stomata. However, decrease in the concentration of potassium ions, makes guard cell flaccid and ultimately makes it close.