e. Abscissic Acid :
It is a natural growth inhibiting hormone. Carns and Addicott (1961-65) observed that the shedding of cotton balls was due to a chemical substance abscisin I and II. Wareing (1963) isolated a substance from buds of Acer that can induce bud dormancy and named it dormin. These two identical chemical substances were given the common name abscissic acid. It is synthesized in leaves, fruits, roots, seeds etc. Chemically, it is a 15-carbon sesquiterpenoid and is synthsized from mevalonic acid.
Physiological effects and application of ABA:
It promotes abscission of leaves and induces dormancy in many plants. It controls the dormancy in buds and seeds by inhibiting growth processes. It accelerates the senescence of leaves, flowers and fruits. It inhibits and delays cell division and cell elongation and suppresses cambium activity by inhibiting mitosis in vascular cambium. ABA could cause efflux of k+ ions from the guard cells and result in closure of stomata. So, it is known as an antitranspirant. It acts as a stress hormone by inducing the plant to bear the adverse environmental conditions. It inhibits flowering in long day plants but stimulates flowering in short day plants.
