Starch-sugar inter-conversion theory
During the day, in the presence of light, photosynthesis occurs and uses up the carbon (IV) oxide in the cytoplasm thus raising the pH (raises alkalinity). Alkalinity activates enzymes that catalyze the hydrolysis of starch, which is osmotically inactive, to glucose, which is osmotically active. The guard cells become hypertonic, hence draw water from the adjacent epidermal cells by osmosis and become turgid causing the stoma to open.
At night, photosynthesis does not take place. Respiration occurs producing Carbon (IV) oxide that lowers the pH (increases acidity). Increased acidity inactivates the enzymes that catalyse the conversion of starch to glucose. The guard cells become hypotonic to the adjacent epidermal cells. Guard cells then lose water through osmosis to the adjacent epidermal cells and become flaccid, causing the stoma closes.
Limitation: It does not explain why those guard cells lacking starch still affect the size of the stomata in some plants.
