P.F. Wareing in U.K. and F.T. Addicott in USA in 1961 discovered growth inhibiting hormone. It was named abscisic acid (ABA). Abscisic acid was discovered from the leaves of Acer pseudoplatanus and fruits of cotton plant. It exists universally in higher plants. It occurs in large amount in the fruits of Avocado and rose, dormant tubers of potato and seeds of apple.
ROLE OF ABSCISIC ACID IN PLANT GROWTH (PHYSIOLOGICAL ROLES)
- Growth inhibitor
ABA is growth inhibitor hormone. It inhibits the growth of root, stem, leaf and coleoptile. Even its very low concentration is effective. Gibberellin reverses the effect of ABA. Thus plant resumes normal growth. Therefore, abscisic acid and gibberellic acid have opposite effects on plants.
- Role in apical dominance
ABA makes the apical bud dormant in temperate species like Betula. Morus and fig. The growth a shoot apical bud ceases. It is eovered with scale leaves. These leaves are small, non green and membranous.
- Role in seed dormancy
ABA also makes the seeds of several species dormant. Such seeds fail to germinate even under favourable conditions. Its affect can be reversed by the application of gibberellic acid.
- Dormancy of bud
The shoot buds of temperate tree species become dormant in autumn season. Similarly, freshly harvested seeds of apple and peach do not germinate. The potato tubers fail to sprout soon after harvesting. All these are caused by accumulation of large amount of ABA in these organs. The concentration of ABA decreases and concentration of gibberellins increase in spring. Therefore, shoot buds sprout and the seeds start germinating in spring.
- Effect of Drought
A large quantity of ABA accumulates in plant during drought or water deficiency. It has a biological advantage for the plant. ABA closes the stmnata. It prevents further loss of water,. Thus plant is able to conserve
available moisture and withstand the drought.
- Promotion of abscission
Application of ABA promotes the abscission of leaves, flowers and fruits