Abscisic Acid & Stomatal Closure (CIE A Level Biology)

• During times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata
  • Certain environmental conditions can cause water stress, such as very high temperatures or reduced water supplies
• Guard cells have ABA receptors on their cell surface membranes
• ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H⁺) ions out of the guard cells
• ABA also causes calcium (Ca²⁺) ions to move into the cytoplasm of the guard cells through the cell surface membranes
• The calcium ions act as second messengers:
  • They cause channel proteins to open that allow negatively charged ions to leave the guard cells
  • This stimulates the opening of further channel proteins that allow potassium (K⁺) ions to leave the guard cells
  • The calcium ions also stimulate the closing of channel proteins that allow potassium (K⁺) ions to enter the guard cells
• This loss of ions increases the water potential of the guard cells
• Water leaves the guard cells by osmosis
• The guard cells become flaccid, causing the stomata to close

ABA causing the closure of the stoma diagram

Closure of a stoma in response to abscisic acid (ABA)