Explanation

ABA (abscisic acid) causes stomata to close by inhibibition of an ATP dependent pump in the plasma membrane of guard cells. The application of exogenous ABA on leaves of normal plants causes closing of stomata within a few minutes. ABA acts in the presence of $CO_{2}$, which decreases the pH of guard cells. ABA includes the loss of $K^{+}$ions, which decreases the osmotic concentration of guard cells as compared to adjacent epidermal cells. This causes exosmosis and turgidity of guard cell decreases.