Malate or K+ ion pump hypothesis : Proposed by Levitt. According to this theory, the change in turgor pressure of the guard cells that open and close the stomata results in absorption and loss of K+ ions by guard cells.
Opening of stomata in light:
(a) Starch in guard cells is metabolised into phosphoenol pyruvate (PEP). It is later converted into organic acids, particularly malic acid. It is catalyzed by an enzyme PEP carboxylase.
(b) Malic acid dissociates into H+ and malate ions in the guard cells.
© H+ ions from guard cells, are transported to epidermal cells and K+ ions from epidermal cells to guard cells through hydrogen - potassium ion exchange pump in the plasma membrane.
(d) The process of potassium exchange requires ATP and thus it is an active process.
(e) In the guard cells K+ ions are balanced by malate ions. Besides, small amount of Cl“ ions are also absorbed which neutralize a small percentage of K+ ions.
(f)Increased K+ ions and anion concentration in the guard cells increases their osmotic concentration. As a result, water enters the guard cells by endosmosis.
(g) This increases the turgor pressure of the guard cells and the stoma opens.
Closing of Stomata in Dark:
(a) During night (dark) C02 is not utilized in photosynthesis, hence its concentration increases in the sub-stomatal cavity.
(b) Organic acids are converted back into starch. An inhibitor hormone, abscisic acid (ABA) functions
in the presence of C02. It inhibits K+ ions uptake by changing the diffusion and permeability of the
guard cells.
© The K+ ions are transported back to the epidermal or subsidiary cells from the guard cells.
(d) The osmotic concentration of the guard cells decreases. This results in the movement of water
out of the guard cells (exosmosis). The guard cells now become flaccid and the stoma closes.