F. B. Salisbury and W. A. Jensen (1984) introduced abscisic theory.
(i) Stomatal Opening Mechanism
According to this theory, the concentration of CO2 in the guard cell decreases due to photosynthesis during the day. So K+ enters the guard cell. The entry of K+ increases the concentration of the guard cell and water enters the guard cell by the process of endorespiration. The guard cell takes in water and bends like a bow and the stomata open.
(ii) Mechanism of stomatal closure
Abscisic acid enters the guard cell from the mesophyll tissue during the night and K+ is released. This reduces the density of the guard cells and water escapes from the guard cells in the process of exudation. The guard cells lose water and become silent and the stomata close.