Question

Which of the following options best describes the mechanism that causes a stoma to open?

• A. $K^{+}$ enters the guard cells and water follows passively, making the cells turgid.
• B. $K^{+}$ activates water pumps in the guard cell membrane that make them turgid.
• C. $K^{+}$ leaves the guard cells and water follows passively, making the cells flaccid.
• D. Loss of $K^{+}$ from guard cells creates positive pressure and expands the guard cells.

Answer 1

The correct option is A $K^{+}$ enters the guard cells and water follows passively, making the cells turgid.

Malate or $k^{+}$ ion pump hypothesis theory was put forward by Levitt $\left(1974\right)$. It is considered as the modern one. It states that the change in the turgor pressure of the guard cells regulates the opening and closing of stomata. In the presence of sunlight, malic acid dissociates into the hydrogen ion and malate ion in the guard cells. When the hydrogen ion is transported to epidermal cell from guard cell the K${}^{+}$ transports to guard cells from epidermal cells. In the guard cell K${}^{+}$ is maintained by the malate ion and in its increased concentration, osmotic concentration is also increased which causes water to enter guard cell passively and make the cell turgid. Thus, the correct answer is option A.