(a) : Absisic acid is a hormone produced during adverse environmental condition. It also causes the closure of stomata under conditions of water stress as also under high concentration of CO2​ in the guard cells. Abscisic acid inhibits the K+ uptake by guard cells and promotes the leakage of malic acid. It results in reduction of osmotically active solutes so that the guard cells become flaccid and stomata get closed.
