Growth and Development of Horticultural Crops (1+1)

Introduction-Abscisic Acid (ABA)

• In 1963, a substance strongly antagonistic to growth was isolated by Addicott from young cotton fruits and named Abscisin II. Later on, this name was changed to Abscisic acid (ABA). The chemical name of abscisic acid is 3-methyl 5-1’ (1’-hydroxy, 4-oxy-2’, 6’, 6’-trimethyl2-cyclohexane-1-yl) –cis, trans-2,4-penta-dienoic acid.
  
• Warning et al. (1963, 64) pointed out the presence of a substance in birch leaves (Betula pubescens, a deciduous plant) which inhibited growth and induced dormancy of buds and, therefore, named it ‘dormin’. But, very soon as a result of the work of Cornforth et al. (1965), it was found to be identical with abscisic acid.
  
• Abscisic acid is a 15-C sesquiterpene compound (molecular formula C15H12O4) composed of three isoprene residues and having a cyclohexane ring with keto and one hydroxyl group and a side chain with a terminal carboxylic group in its structure. ABA resembles terminal portion of some carotenoids such as violaxanthin and neoxanthin and appears to be a breakdown product of such carotenoids.Any change in its molecular structure results in loss of activity. ABA occurs in cis and trans isomeric forms that are decided by orientation of –COOH group around 2nd carbon atom in the molecule. Almost all naturally occurring ABA in plants exist in cis form that is biologically active and the name abscisic usually refers to this form. Trans-ABA is inactive form but can be interconvertible with cis ABA.