abscisic acid (abscisin II, dormin) A terpenoid (see TERPENE) compound that is one of the five major plant hormones. Although it is synthesized principally in the chloroplasts, it occurs throughout the plant body and is particularly concentrated in the leaves, fruits, and seeds. It has powerful growth-inhibiting properties generally and also promotes leaf abscission and the senescence of plants and/or their organs, and induces the closing of stomata and dormancy in seeds and buds. Its effect is antagonistic to the plant growth hormones, and is thought to act by inhibiting the synthesis of protein and nucleic acids.

abscisic acid (ABA) A naturally occurring plant growth substance that appears to be involved primarily in seed maturation, stress responses, and in regulating closure of leaf pores (stomata). In seeds, it promotes the synthesis of storage protein and prevents premature germination. In leaves, abscisic acid is produced in large amounts when the plant lacks sufficient water, promoting closure of stomata and hence reducing further water losses. Levels of abscisic acid increase suddenly in response to various forms of stress, including heating, waterlogging, and chilling. It was formerly believed to play a role in abscission, hence the name.