Abscisic acid and stomatal closure
Abscisic acid (ABA) is a stress hormone that is secreted in response to difficult
environmental conditions such as very high temperatures or much reduced water
supplies. ABA triggers the closure of stomata to reduce transpiration and prevent
water loss.
ABA binds to cell surface receptors
inhibits proton pumps: stop H
+
pumped out
stimulates movement of Ca2+ through the cell surface membrane and tonoplast
Ca2+ acts as a 2nd messenger to activate channel proteins to open that allow
negatively charged ions to leave the guard cell. This in turn
opens channel proteins that allow K
+
to leave the cell
closes channel proteins that allow K
+
to enter the cell
–> net movement: K
+
leaves cell
Loss of ions = higher water potential inside cell = water passes out by osmosis = guard
cells become flaccid –> stomata close