BIOLOGY Volume 2 - A Guide to General Biology - 2004

16. PLANT COORDINATION AND REGULATION

16.2. Plant Growth Substances

16.2.8. Abscisic Acid

Discovery of Abscisic acid

Sometime later, plant physiologists obtained evidence that normal plant development requires not only growth stimulators such as Auxins, Gibberellins, and Cytokinins, but also growth inhibitors. The existence of such agents—responsible, in particular, for seed dormancy—had been hypothesized for quite some time, but attempts to isolate them were undertaken only in the late 1950s by a group of scientists from the University of Wales, Aberystwyth, led by Wareing. In 1963, it was demonstrated that an extract from birch leaves induces bud dormancy. These leaves were placed under short-day conditions, simulating the onset of winter. Purified crystals of an active substance of the same type were isolated in 1964 from sycamore leaves. This substance was named dormin. It turned out to be identical to a compound isolated by another group in 1963 from young cotton bolls, which accelerated their abscission and was named abscisin II (from the Latin abscidere, to cut off; abscisin I acts similarly, but differs in chemical Structure and is less active). In 1967, it was decided to rename this substance abscisic acid (ABA). Abscisic acid has been found in all plant groups, from mosses to angiosperms. A functionally analogous lunularic acid has been isolated from liverworts and Algae.

Structure of abscisic acid

Like gibberellins, ABA belongs to the terpenoids and has a complex structure (Fig. 16.24). It is the only growth substance of this chemical Class.

Fig. 16.24. Structural formula of abscisic acid.

Synthesis and distribution of ABA

ABA is produced in leaves, stems, fruits, and seeds. The fact that isolated METABOLISM/14.html">Chloroplasts retain The ability to synthesize it suggests a metabolic link between this substance and carotenoid pigments, which are also formed in chloroplasts. Like other phytohormones, ABA is transported through the plant's Vascular System, primarily via the phloem. In addition, it spreads by diffusion from the ROOT cap (see the section on geotropism).

Action of ABA

Table 16.4 summarizes data on the effects of ABA on Plant GROWTH AND DEVELOPMENT. It is the primary growth inhibitor and an antagonist to all three classes of growth-stimulating substances. Classic Examples of ABA action include the induction of bud dormancy (including apical dominance), seed dormancy, and organ abscission (Section 16.3.4). Furthermore, ABA plays a role in wilting, flowering, and leaf senescence processes; it is also possible that it regulates plant geotropic responses. It is associated with stress responses, particularly Water loss during drought. For instance, in wilting tomato leaves, the concentration of ABA is 50 times higher than in leaves maintaining normal turgor. It is believed that this phytohormone triggers stomatal closure. High concentrations of ABA completely arrest plant growth.

MECHANISM OF ACTION of ABA

There are currently no data available on this topic.

Practical Applications OF ABA

ABA can be used to spray fruit trees at the end of the growing season to induce simultaneous fruit drop, thereby shortening the harvest period.