Auxin



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What is Auxin?

Auxin is a generic name given to the growth hormone produced by the tip of coleoptiles and specifically required for elongation process.

Occurrence of Auxin

Auxins are universally present in plants and their highest concentration is in the growing tips of coleoptiles, leaves and roots. An auxin synthesized in one tissue is frequently translocated to other organs of the plant.

Movements of auxin

It has been shown that the movement of auxin is polar and takes place only in one direction from the tip to the base and that it can occur against the concentration gradient. In mature differentiated tissues, however, both polar and non-polar movements may occur.




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Physiological Functions of auxin

IAA is the principal natural auxin which is found in all plants studied so far. The usual sites of auxin synthesis are meristems and enlarging tissues. Some important physiological functions of IAA are as follows:

Cell elongation

The primary physiological function performed by IAA is the elongation and growth of stems and roots and enlargement of many fruits. The cell elongation is promoted by the following matter:
i) Increase in osmotic solutes.
ii) Decrease in wall pressure.
iii) Increase in permeability of cytoplasm to water.
iv) Increase in wall synthesis.

The action of IAA Auxin in regulating growth seems to be associated with nucleic acid metabolism. Shoot growth is stimulated by 10-6– 10-7 M auxin. Root elongation, on the other hand, is much more sensitive with maximum stimulation of growth at 10—9-10—10 M auxin, and reserve at higher concentrations.

Apical dominance

Apical bud has a strong influence on the growth of lateral buds, and it suppresses their growth. This phenomenon is known as apical dominance. When apical bud is removed, lateral buds start developing into branches/The process can be reversed if Auxin is applied to the decapitated apex.




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Root initiation and Growth

If IAA Auxin is applied to the tips of roots, their elongation is retarded and there is a noticeable increase in the number of branch roots.

Abscission

Abscission refers to detachment of plant organs such as leaves, flowers and fruits. The region of separation is anatomically diverse and is said to be abscission zone. The auxin gradient across the abscission zone controls abscission. The natural abscission takes place due to decrease in auxin content. It gradually decreases and attains the lowest level when the abscission layer begins to form. Auxin synthesized by the leaf prevents abscission. But when the leaf becomes old, its auxin producing capacity decreases and this leads to abscission. If auxin is supplied to the plant at this stage, abscission can be prevented.

Callus formation and cell division

Auxin is responsible for initiating and promoting cell division in tissues like cambium. Wounding causes formation of callus due to proliferation of parenchyma cells. The formation of callus is linked with the cambial activity which is stimulated by the auxin. In tissue cultures, cell division is entirely dependent on the presence of IAA in the medium, and the amount of callus tissue formed is related to the concentration of IAA applied.

Flower initiation and sex expression

Auxin, in general, slows down flowering. Auxin concentration intensely affects sex expression. Application of auxin to cucurbits reduces i number of male flowers and increases the female flowers.




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Use of Auxin in agriculture

A large number of synthetic auxin are being used in agriculture. The application of auxin increases the number of female flowers and decreases the male flowers in cucurbits. This results in a corresponding increase in the number of fruits. Maturity and ripening of fruits (such as mango) can be greatly hastened by spraying proper auxin is used to prevent sprouting of potatoes, and onion bulbs can be stored for a longer time by spraying auxin on to the plants before harvest.