Abscisic Acid (ABA) (Stress Phyto Hormone)
In 1963, the hormone was first isolated by Addicott et al., from young cotton bolls and named as Abscission II. Eagles and Wareing during 1963–64 isolated a dormancy inducing substance from leaves of Betula and called it as dormin. In 1965, it was found by Cornsforth et al., that both dormin and abscission are chemically same compounds and called Abscisic Acid (ABA).
This hormone is found abundantly inside the chloroplast of green cells.
The hormone is formed from mevalonic acid pathway or xanthophylls.
Abscisic acid is transported to all parts of the plant through diffusion as well as through phloem and xylem.
It has carotenoid structure.
The inhibition of IAA induces straight growth of rice seedling coleoptiles.
• It helps in reducing transpiration rate by closing stomata. It inhibits K+ uptake by guard cells and promotes the leakage of malic acid. It results in closure of stomata.
• It spoils chlorophylls, proteins and nucleic acids of leaves making them yellow.
• Inhibition of cell division and cell elongation.
• ABA is a powerful growth inhibitor. It causes 50% inhibition of growth in Oat coleoptile.
• It induces bud and seed dormancy.
• It promotes the abscission of leaves, flowers and fruits by forming abscission layers.
• ABA plays an important role in plants during water stress and during drought conditions. It results in loss of turgor and closure of stomata.
• It has anti-auxin and anti-gibberellin property.
• Abscisic acid promotes senescence in leaves by causing loss of chlorophyll pigment decreasing the rate of photosynthesis and changing the rate of proteins and nucleic acid synthesis.
· In Cannabis sativa, induces male flower formation on female plants.
· Induction of flowers in short day plants.
· It promotes sprouting in storage organs like Potato.
· ABA plays an important role in plants during water stress drought conditions.
· It inhibits the shoot growth and promotes growth of root system. This character protect the plants from water stress. Hence, ABA is called as stress hormone.