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Plant Growth Regulators
A typical example of plant growth regulator is a plant hormone. Plant hormones are the collective term for compounds produced in minute amounts by plants to regulate their own physiological functions. To date, seven natural plant hormones have been discovered: auxins, cytokinins, ethylene, jasmonates, abscisic acid, gibberillins and brassinosteroids (Fig. 1). Unnatural plant growth regulators or plant growth inhibitors are also included in this section.
Historically, auxins were first discovered as substances which showed phototropism. To date, it has been revealed that they play numerous roles such as initial development, budding, root growth development, growth of flower parts and cell division. Natural auxins are 3-indoleacetic acid, 3-indolebutyric acid and phenylacetic acid. Some unnatural synthetic compounds also exhibit the same activities.
Cytokinins are regarded as substances which stimulate cell division, shoot initiation and bud formation, when addition auxins are added. Typical structure features are adenine with an isopentenyl unit at N6 position, or with an isopentenyl unit with the methyl terminus being hydroxylated.
Jasmomates have a distinct fragrance and are biosynthesized from linolenic acid, an unsaturated fatty acid. They inhibit growth in adverse conditions, and stimulate tuber formation. They promote senescence of leaves, suppression of fruit growth, and the induction of tuber formation in potatoes.
Abscidic acid is occasionally classified as a sesquiterpene, however, it is biosynthesized from a carotenoid (C40) precursor. It stimulates the closure of stomata in the absence of water and induces seeds to synthesize storage proteins. It is also released when a plant experiences stress, as in lack of nutrition, pests, root distress, or disease.
- b)R. Arteca, in Plant Growth Substances:
- Principles and Applications, Chapman & Hall, New York, 1996.