Explanation

ABA theory to explain the mechanism of stomatal closure was proposed by Cowan et al, in 1982. According to it formation of abscisic acid (during drought or mid-day) promotes reversal of $H^{+} \rightleftharpoons K^{+}$ pump and increases availability of $H^{+}$ inside the guard cell cytoplasm and stomata close. Active $K^{+}$ transport theory for opening and closing of a stomata, was proposed by Imamura (1943) and Fujino (1967). Proton ${(H}^{+} - K^{+})$ transport theory was proposed by Levitt (1974).