ABA causes stomata to close by reversing the accumulation of potassium salts in guard cells and inducing the conversion of sugars into starch.
The resulting fall in solute concentration leads to loss of water from the guard cell into the apoplast by osmosis, a fall in guard cell turgor, and closing of the stomatal pore.
#Unit 6. System Physiology – Plant#The Stomatal Biology#Part B Pointers
