ABA-induced stomatal closure is extremely rapid and involves changes in ion channel (mech.)

ABA triggers an increase in cytosolic calcium (Ca2+), which activates anion channels (A- ) allowing Cl- to leave the cell. ABA also activates channels that move potassium out of the cell (K+out) and inhibits channels that move potassium into the cell (K+ in). The net result is a large movement of ions out of the cell. As ions leave the cell, so does water (by osmosis), causing the cells to lose volume and close over the pore.