Question

Describe the active $K^+$ active ion theory for stomatal opening.

Answer 1

Hint: According to this theory, the stomatal opening and closing depending on the generation of a potassium ion gradient. ATP produced in the guard cells during photosynthesis is utilized to pump the potassium ions of the adjacent cells into the guard cells.

Complete answer: During daylight the carbon dioxide concentration decreases, followed by the formation of malic acid in the guard cells.
1. Malic acid dissociates into malate ions and hydrogen ion.
2. The potassium ions are taken inside the guard cells and hydrogen ions are transported out of the guard cells which causes the osmotic potential of the guard cells to increase.
3. Endosmosis into guard cells makes them become turgid and this in turn leads to the opening of the stomatal pores.
4. During the dark carbon dioxide concentration subsequently increases.
5. Due to the decrease in the pH of the guard cells, ABA gets released to prevent the potassium ions ion exchange. Malate ions and hydrogen ions combine to form the malic acid in guard cells. Potassium ions are further transported back into the surrounding subsidiary cells resulting in decreased osmotic concentration of guard cells.
6. This ultimately leads to the exosmosis from guard cells and they become flaccid.
7. As a result of this, the stomatal pores are closed.

Note: $K^+$ ion theory was proposed by Levitt in 1974.
Experimental pieces of evidence show that stomata also open even in carbon dioxide-free air and in absence of light by increasing the $K^+$ concentration in the guard cells.