Abscisic Acid

A plant hormone (C₁₅H₂₀O₄) and weak acid that generally acts to inhibit growth, induces dormancy, and helps the plant tolerate stressful conditions by closing stomata. Abscisic acid was named based on a belief that the hormone caused the abscission (shedding) of leaves from deciduous trees during the fall. At times when a plant needs to slow down growth and assume a resting stage (dormant), abscisic acid is produced in the terminal bud, which slows down growth and directs the leaf primordia to develop scales that protect the dormant bud during winter. Since the hormone also inhibits cell division in the vascular cambium, both primary and secondary growth is put on hold during winter. This hormone also acts as a stress agent, helping a plant deal with adverse conditions. For example, ABA accumulates on leaves and causes stomata to close, reducing the loss of water when a plant begins to wilt.
In 1963 abscisic acid was first identified and characterized by Frederick Addicott and colleagues. In 1965 the chemical structure of ABA was defined, and in 1967 it was formally called abscisic acid.