Abscisic Acid

e. Abscissic Acid : Abscisic acid (ABA) is a naturally occurring growth-inhibiting hormone. Carns and Addicott (1961-65) found that the shedding of cotton bolls was caused by chemical substances they called abscisin I and II, while Wareing (1963) isolated a related substance from the buds of Acer that could induce bud dormancy and named it dormin; these two lines of research turned out to describe the same compound, which was given the common name abscisic acid. It is synthesised in leaves, fruits, roots and seeds, and is chemically a 15-carbon sesquiterpenoid, built like gibberellin from mevalonic acid.

Physiological effects and application of ABA : ABA promotes the abscission of leaves and induces dormancy in buds and seeds, acting by inhibiting the growth processes that would otherwise continue. It accelerates the senescence of leaves, flowers and fruits, inhibits and delays cell division and cell elongation, and suppresses cambial activity by inhibiting mitosis in the vascular cambium. A distinctive action of ABA is on the stomata: it causes an efflux of K+ ions from the guard cells, leading the stomata to close -- for this reason ABA is often called an antitranspirant. More broadly, ABA acts as the plant's principal stress hormone, helping the plant cope with adverse environmental conditions. Its effect on flowering is photoperiod-dependent: it inhibits flowering in long-day plants but stimulates flowering in short-day plants.