Operates as a K(+)/H(+) antiporter that maintains K(+) homeostasis in guard cells and could regulate pH. Plays a critical role in osmoregulation through the control of stomates opening. Expressed in leaves and stems. Preferentially expressed in guards cells. Impaired light-induced stomatal opening. Belongs to the monovalent cation:proton antiporter 2 (CPA2) transporter (TC 2.A.37) family. CHX (TC 2.A.37.4) subfamily.