stomatal closure

• Plants need to be able to close their stomata in order to reduce water loss through transpiration. They do this using guard cells.
• Guard cells are found either side of a stomatal pore. When the guard cells are full of water, they are plump and turgid and the pore is open. When the guard cells lose water, they become flaccid, making the pore close
• The plant hormone abscisic acid (ABA) is able to trigger stomatal closure:

  1. ABA binds to receptors on the guard cell membranes. This causes specific ion channels to open, which allows calcium ions to enter the cytosol from the vacuole.
  2. The increased concentration of calcium ions in the cytosol causes other ion channels to open. These ion channels allow ions (such as potassium ions) to leave the guard cells, raising the water potential of the cells.
  3. Water then leaves the guard cells by osmosis.
  4. The guard cells become flaccid and the stomata close.
• opening and closing of the stomata

  A) pore open

  B) turgid

  C) flaccid

  D) pore closed