Question

Describe the mechanism of opening and closing of stomata.

Answer 1

Mechanism of opening and closing of stomata:

1. The opening and closing of stoma is controlled by turgor of guard cells.

2. During day time, guard cells become turgid due to endo-osmosis.

3. Thus turgor pressure is exerted on the thin walls of guard cells.

4. Being elastic and thin, lateral walls are stretched out.

5. Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma.

6. During night time, guard cells become flaccid due to exo-osmosis.

7. Flaccidity closes the stoma almost completely.

8. Endo-osmosis and exo-osmosis occur due to diurnal changes in the osmotic potential of guard cells.

9. According to starch-sugar inter-conversion theory (Steward 1964), during day time, enzyme phosphorylase converts starch to sugar, thus increasing the osmotic potential of guard cells causing entry of water, thereby guard cells are stretched and stoma widens. The reverse reaction occurs at night bringing about the closure of the stoma.

\[\ce{\underset{\text{(Stomata open)}}{Starch}⇌[Phosphorylase (Day)][(Night)]\underset{\text{(Stomata close)}}{Sugar}}\]

10. According to the theory of proton transport (Levitt-1974), stomatal movement occurs due to the transport of protons H⁺ and K⁺ ions. During the daytime, starch is converted into malic acid. Malic acid dissociates to form malate ions and protons. Protons are transported to subsidiary cells and K⁺ ions are imported from them.
Potassium Malate is formed that increases osmolarity and causes endoosmosis. Uptake of K⁺ ions is always accompanied by Cl^– ions. At night, uptake of K⁺ and Cl^– ions is prevented by abscisic acid, changing the permeability of guard cells. Due to this guard cells become hypotonic and thereby become flaccid.