• 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

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