Which one gives the most valid and recent explanation for stomatal movement?

The most valid and recent explanation for stomatal movement is the potassium influx and efflux theory. This theory was proposed by Levitt (1974) and elaborated by Raschke (1975) and Bowling (1976).

The organic acids, viz, malic acid, dissociate into malate ion and ${\text{H}}^{+}$ in the guard cells. ${\text{K}}^{+}$ are taken into the guard cells in exchange of ${\text{H}}^{+}$. ${\text{K}}^{+}$ ions accumulate in large numbers in guard cells. Increased concentration of ${\text{K}}^{+}$ and malate ions in the vacuole of guard cells results in the production of osmotically active salts namely potassium chloride and potassium malate. This decrease water potential resulting in endosmosis. Increased turgor of guard cells makes the outer wall of the guard cells to become more convex. The inner thick wall guard cell moves apart. As a result, stoma opens.

Carbon dioxide concentration is increased in dark in the substomatal cavity because photosynthesis is stopped but respiration is continued. Higher concentration of ${\text{CO}}_2$ in sub stomatal cavity prevents proton gradient across protoplasmic membrane in guard cells. As a result, active transport of ${\text{K}}^{+}$ into guard cells ceases. Malate ions present in the guard cell cytoplasm combine with ${\text{H}}^{+}$ to produce malic acid. Excess of malic acid inhibits its own synthesis by decreasing the activity of PEP carboxylase. These changes cause reversal of ion movement so that ${\text{K}}^{+}$ and ${\text{Cl}}^{-}$ is transported out of guard cells into surrounding epidermal cells. The osmotic pressure of guard cells is thus decreased and water potential increases. This results in movement of water from guard cells to surrounding cells. The guard cells become flaccid and the stomata gets closed.