When stomata opens, the pH of guard cells

Malate or potassium ion pump hypothesis was first proposed by Levitt, 1974. According to this theory, rise in pH of guard cells (due to photosynthesis and consequent decrease of carbon dioxide) causes hydrolysis of starch to form phosphoenol pyruvate (PEP). PEP combines with carbon dioxide to form organic acids like oxalic acid. The organic acids get ionised and the proton passes out of guard cells and potassium ion is actively absorbed and accumulated. Potassium ions combine with malate and pass into small vacuoles. Chloride ions are also absorbed from outside to maintain electroneutrality. As a result of accumulation of ions there is a reduction in water potential of guard cells. The reduction in water potential causes absorption of water from adjacent cells, guard cells become turgid and stomata open.