Mechanism of Stomatal Action

The mechanism namely the opening and closing of stomata depends upon the turgor pressure in the guard cells. When the guard cells are turgid, the stoma opens and when the guard cells lose water, stoma closes.

Stomatal Movement in Dicot Plants

Stomatal Movement in Monocot Plants

Active Potassium Theory (Levitt 1974)

It was observed by Fujino (1967) that opening of stomata occurs due to the influx of K⁺ ions into the guard cells. The source of K⁺ ions are the neighbouring subsidiary and epidermal cells, there by increasing the concentration from 50mM to 300mM in guard cells.

The increase in K⁺ ion concentration increases the osmotic concentration of guard cells thus leading to stomatal opening. ATP helps in entry of K⁺ ions into the guard cells.

Role of potassium, chloride and malate ions in stomatal opening (PEPcase = Phosphoenol pyruvate carboxylase)

Levitt (1974) observed that proton (H⁺) uptake by guard cells, chloroplasts takes place with the help of ATP. This leads to increase in value of pH in guard cells. Rise in pH converts starch into organic acid like malic acid.

The uptake of K⁺ ions is balanced by:

• Uptake of chloride (Cl^-) ions

• Transport of H⁺ ions released from organic acid (malic acid)

• By negative charges of organic acids when they lose H⁺ ions

Thus all these factors lead to the opening of stomata. The stomata closure is due to excretion of K⁺ ions from guard cells surrounding epidermal and subsidiary cells.

The stomatal closure is considered to be brought about by a passive or highly catalysed excretion of K⁺ ions and Cl^- ions from the guard cells to the epidermal tissue in general and the subsidiary cells in particular. It is believed that subsidiary cells have an active reabsorption mechanism of K⁺ ions.

Role of ABA (Abscissic acid) in stomatal closure

ABA blocks the active excretion of H⁺ from guard cells. ABA brings about closure of stomata.

Number of stomata on the upper and lower surfaces of leaves