Starch-Sugar Interconversion:
The stomata normally open during day and close during night. Actually, during daytime at pH7, the starch is converted to glucose 1 phosphate, which is soluble in water, so the osmotic potential of the guard cell increases, endosmosis occurs and the guard cell opens. during night time, the pH is reduced to 5, which is due to the accumulation of CO2 and malic acid, the soluble sugar is reconverted to starch, which is insoluble in water and the guard cell becomes flaccid and closes.
Potassium infiltration theory:
Starch sugar inter conversation theory only explains one time opening and time closing during the evening. But the stomata cannot be kept open all through the day, so it needs to exhibit intermittent opening and closing of stomata during the daytime. This is brought about by infiltration of potassium ion which produces potassium malate inside the guard cell, it is osmotically active and guard cell, malic acid is regenerated, guard cell becomes flaccid and stomata close.