Plant Growth Regulators

Control And Coordination of Class 10

The growth regulators are the important chemicals affecting growth.

Growth hormones (phytohormones) are the natural growth substances which are produced in any part of the plant and ara transferred to another part and there they influence the growth of plant. The growth regulators consist of auxins, gibberellins, cytokinins, ethylene and abscisic acid. Except abscisic acid, ethylene the three are called Growth hormones and ethylene, abscisic acid are growth inhibitor.

AUXINS:

It is a type ofhormone, or growth substance, found in plants. Planthormonesare molecules produced by plants to accelerate, inhibit or modify its growth. In plants, growth occurs during cell division, cell elongation and cell differentiation. The hormones can affect any or all of these different processes in plants.The first discovered plant hormone was identified as indole acetic acid (I.A.A.).

DISCOVERY Auxins were first discovered to be chemical substances and as hormones by F.W. Went in 1928.

F. W. Went coined the term auxin (Greek Auxein: to grow) to the chemical influence responsible for the phototropic response.

Auxins were first of all discovered from human urine.

Chemistry of Auxins

Auxins are chemically Indole Acetic Acid (IAA).

IAA is a true natural auxin of plants.

Synthetic Auxins :

A number of synthetic compounds related to auxins have been produced. Some of these are indole-3-butyric acid (IBA), a and b naphthalene acetic acid (NAA), phenyl-acetic acid (PAA), 2, 4 dichlorophenoxy acetic acid (2, 4-D) etc.

Occurrence

The auxins are widely distributed throughout the plant body, but their greatest amount is found in actively growing regions and the lowest amount is found in the non-growing regions. However, the concentration of auxin found at root tip is much lower than the concentration found at the shoot tip. In general, the auxins are located mainly in young and actively growing regions, such as, root apex, shoot apex, lateral meristems and enlarging leaves.

PHYSIOLOGICAL EFFECTS OF AUXINS

Cell elongation : They induce cell elongation.

Apical dominance : In higher plants the apical bud is far more active than the lateral buds. For certain period, the growth of the lateral buds remains suppressed. This phenomenon is called apical dominance or bud inhibition. This property of auxin is used in prolonging the dormancy period of potato tubers.

Functions of auxins:

Auxins control several plant growth processes.

Cell elongation

Auxins promote elongation and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all directions.

Reactivation of cambium

Auxins promote cell division in vascular cambium. The reactivation of cambium in the growing season is due to the moving of IAA from the developing shoot buds.

Apical Dominance

Auxins induce apical dominance, where the apical bud suppressed the growth of lateral buds.

Possible Involvement of Plant Growth Substances in Apical Dominance in Presence of Apical Bud

Possible Involvement of Plant Growth Substances in Apical Dominance after Removal of Apical Bud.

Inhibition of Abscission:

Formation of an abscission layer at the base of petiole or pedicel results in shedding of leaves, flowers or fruits. But auxins inhibit abscission, as they prevent the formation of abscission layer.

illustration of usage of auxin spray

Auxin Spray Prevents Premature Fruit Abscission and Increase in Yield.a) Auxin Sprayed; b) Auxin not Sprayed.

Parthenocarpy:

Auxin induces parthenocarpy, i.e., the formation of seedless fruits without the act of fertilisation.

illustration of parthenocarpy

Root Formation:

IAA stimulates cell division in the pericylce leading to the formation of lateral and adventitious roots. Auxin stimulates formation of new roots, but inhibits the root growth.

Weedicides:

Many synthetic auxins are used as selective weed killers and herbicides. 2, 4 - D (2, 4 - dichlorophenoxy acetic acid) is used to destroy broad leaved weeds. It does not affect mature monocotyledonous plants.

Plant growth regulators

Plant growth regulators

Destruction of Weeds by 2,4-D Spray

Rooting:

Naphthalene acetic acid (NAA), indole butyric acid (IBA) and indole acetic acid (IAA) are used to induce rooting of cuttings of woody plants like guava.

Flowering:

Foliar spray of NAA and 2, 4 - D induces flowering in litchi and pineapple. Percentage of ball setting in cotton plants increases when NAA or IBA are applied.

Pre-mature fruit drop:

Auxins such as 2, 4 - D, IAA, IBA have been used successfully to prevent premature fruit drop in apples, pears and oranges.

Parthenocarpy:

NAA and IBA treatment induces parthenocarpy of fruits in tomatoes, ladies finger and brinjal.

Dormancy:

Dormancy of seeds can be broken by auxin application.

Storage:

The methyl ester of NAA is used to prevent the sprouting of potato tubers.

Vegetable crops:

Auxins improve the quality of vegetable crops by inhibiting flower formation in some plants.

Plant growth regulators

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