Imamura and M. Fujino (1959) found a direct correlation between stomatal movement and K⁺ ion concentration of guard cells. Fujino proposed that stomatal opening and closing are a result of an active transport of K+ ion into the guard cells. They showed the accumulation of K+ in the guard cells during stomatal opening. By using the electron probe micro analyzer, it has been found that opening of stomata in light is accompanied by the transport of K⁺ into the guard cells from the adjacent cells.  It has been found that anions which balance the positive charge of K⁺ are those of organic acid especially malic acid. The influx of K⁺ into the guard cells is accompanied by the synthesis of malic acid.

Opening of stomata in light:  Stomata open in light due to following reactions.

1. In light starch in the guard cells is metabolised into Phospho Enol Pyruvate (PEP). It is later converted into organic acids, particularly malic acid. The reaction takes place in the presence of an enzyme phospho enol pyruvic carboxylase.
2. Malic acid dissociates into malate and hydrogen ions in the guard cells.
3. Hydrogen ions from guard cells are transported to epidermal cells and K⁺ ions from epidermal cells are absorbed into the guard cells.
4. In the guard cells, K⁺ ions are balanced by malate anions. Besides, small amount of Cl^-ions are also absorbed which neutralize a small percentage of K+ ions.
5. The process of K+ ions exchange requires ATP and thus, it is an active process.
6. Increased K⁺ ions and anion concentration in the guard cells increases their osmotic concentration. Hence, water enters the guard cells by endosmosis.
7. Turgor pressure of the guard cells increases due to endosmosis and the stoma opens.