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
abscisic acid or (formerly) abscisin II or dormin or ABA; 5-(1- hydroxy-2,6,6-trimethyl-4-oxocyclohex-2-en-1-yl)-3-methylpenta- 2,4-dienoic acid; a chiral sesquiterpene. The naturally occurring form, the 2Z,4E,S isomer, also designated (S)-abscisic acid, is a phytohormone formed by the degradation of carotenoids. It controls abscission in flowers and fruit but probably not in leaves, and is also implicated in geotropism,