The opening and closing of stomata is a function of turgor changes in guard cells. In light malic acid is produced in guard cells which dissociates into hydrogen and malate ions, resulting in intake of $\mathrm{K}^{+}$ions through $\mathrm{H}^{+}-\mathrm{K}^{+}$ion exchange. There is increase in concentration of $\mathrm{K}^{+}$and malate ions in guard cells, endosmosis of water into guard cells occurs resulting in increased turgor pressure which results in stomatal opening. Stomata are chiefly concerned with gaseous exchange during photosynthesis and respiration and water loss through transpiration.