JAMB - Biology (2004 - No. 19)

The opening of stomata is indeed driven by an increase in solute concentration within the guard cells, leading to water entry and cell swelling, which then causes the stoma to open. In the process of stomatal opening, guard cells actively accumulate solutes, particularly potassium ions (K+) and sometimes chloride (Cl-) and malate (Mal). This increase in solute concentration lowers the water potential inside the guard cells. 

The lower water potential inside the guard cells causes water to move into them from the surrounding cells via osmosis. As water enters the guard cells, they swell, leading to an increase in turgor pressure (the pressure of the cell contents against the cell wall).