Mechanism of Opening and Closing of Stomata

Environmental factors such as light intensity and quality, temperature, relative humidity, and intracellular CO₂ concentrations are sensed by guard cells, and these signals are integrated into well-defined stomatal responses

If leaves kept in the dark are illuminated, the light stimulus is perceived by the guard cells as an opening signal, triggering a series of responses that result in opening of the stomatal pore.

Stomata open and close due to changes in turgor pressure of the guard cells.

They open when water is absorbed by the guard cells from the surrounding epidermal cells.

The guard cells then become fully turgid and they push into the epidermal cell besides them.

When water is lost and the guard cells become flaccid, the pore closes again.

Stomata tend to open in daylight and close in the dark (but there are exceptions to this diurnal pattern).

Proposed the proton transport concept to explain the mechanism of the opening and closing of the stomata.

ABA is also involved in the closure of stomata under conditions of  drought .

Green plants that are experiencing drought produce ABA in their chloroplasts, including those in the guard cells, and this ‘stress hormone’ triggers stomatal closure.

This response involves calcium ions as the ‘second messenger’.

ABA triggers release of calcium ions from the cell sap in the guard cell vacuoles into the guard cell cytosol, via activated calcium ion channels in the tonoplast.

Consequently, the concentration of calcium ions in the cytosol is abruptly increased, and directly triggers:

1.  a sudden efflux of anions such as chloride ions from the cytosol, across the cell surface membrane, by activated anion channels

2.  a sudden efflux of potassium ions from the cytosol, across the cell surface membrane, by activated potassium ion pumps.

3.  inhibition of efflux of potassium ions into the guard cells (a feature of the stomatal opening mechanism).

As a result, the water potential of the guard cells is abruptly raised. It is now higher than the water potential of the surrounding cells, and a net efflux of water occurs from the guard cells. The guard cells become flaccid and the pores close.