Plant Hormones - Abscisic Acid (ABA)

Abscisic acid is a plant hormone found in chloroplasts of leaves and in high amounts in fruits and seeds. It acts as a growth inhibitor and is present in angiosperms, gymnosperms, pteridophytes, and some mosses.

• Discovery — Carns & Addicott (1961–65): abscisin from cotton; Wareing (1963): dormin from Acer buds. Both are named ABA. Chemically: 15-carbon sesquiterpenoid from mevalonic acid.
• Nature & Transport — Natural growth inhibitor and stress hormone. Also called anti-gibberellin. Transport is non-polar. Found in leaves, fruits, roots, and seeds.
• Stomatal Closure — Causes K⁺ efflux from guard cells → stomata close during stress. Known as an antitranspirant.
• Dormancy & Abscission — Induces bud and seed dormancy. Causes abscission of leaves, flowers, and fruits. Regulates fruit drop.
• Senescence & Stress — Accelerates senescence. Helps plants tolerate drought, salinity, cold and frost (stress hormone).
• Inhibitory Effects — Inhibits cell division, elongation, and cambium activity. Inhibits flowering in long-day plants; promotes in short-day plants.
• Other Effects — Induces carotenoid synthesis; turns green oranges yellow; causes geotropism when applied to roots.