Question

Select the correct events leading to opening of the stomata.
I. Decline is guard cells solutes.
II. Lowering of osmotic potential of guard cells.
III. Rise in potassium levels in guard cells.
IV. Movement of water from neighbouring cells into guard cells.
V. Guard cells becoming flaccid.
Select the correct answer using the codes given below.

• A. I and V
• B. II, III and IV
• C. I, III and IV
• D. II, IV and V

Answer 1

The correct option is B

II, III and IV

Levitt (1974) proposed malate or $K^{+}$ ion pump mechanism in guard cells for the stomatal opening and closing. According to it, in the presence of light, starch in the guard cells is metabolized to phospho-enol pyruvate (PEP). It is later converted to malic acid. The malic acid dissociates into $H^{+}$ and malate ions. The $H^{+}-K^{+}$ exchange pump in the cell membrane of a guard cell pumps $H^{+}$ ions out and $K^{+}$ ions in. Thus, there is an accumulation of $K^{+}$ ions in the guard cells. This lowers the osmotic potential in the guard cells. Hence, endosmosis takes place and the guard cells become turgid and stomatal opening takes place.