Abscisic acid (ABA) is a plant growth regulator that derived its name because it was initially thought to be responsible for abscission (leaves or fruits falling from the plants). Abscisic acid inhibits growth and so works antagonistically to auxins and gibberellins. Ethylene in excess amount can also play a major for abscission. In leaves, ABA weakens the petiole and results in shedding of the leaves.