closing of stomata

• stomatal closure can be triggered by water stress, high temperature, increasing co2 levels in the leaf mesophyll and low light intensity
• the hormone abscisic acid (ABA) is secreted by plant cells when transpiration rate is high and soil water is low
• Abscisic acid molecules bind to receptor proteins in the plasma membrane of the guard cells. This increases the permeability of calcium channels in the plasma membrane. Calcium channels open and calcium ions from surrounding subsidiary cells enter into the guard cell.
• the accumulation of calcium ions in the cytosol inhibits the proton pump (H+-ATPase). There is an increase of hydrogen ions (H+)in the cytosol.
• The outward potassium channels open and large amount of potassium ions diffuse out of the guard cell into the subsidiary cells down along an electrochemical gradient. Chloride ions also diffuse from guard cell into the subsidiary cells.
• The water potential in the guard cells increases and water diffuse out to neighbouring subsidiary cells by osmosis. The turgor pressure in the guard cells decreases, the cells becomes flaccid and the stomatal pore closes.