The growth of plants is regulated by certain chemical substances, which are synthesized by the plant in very small quantities. These substances are formed in one tissue or organ of the plant and are then transported to other sites where they produce specific effects on growth and development. They are referred to as plant hormones. Plant hormones are organic compounds which are capable of promotion, inhibition or modification of growth. The plant hormones are also known as growth factors, growth hormones, growth substances, growth regulators or phytohormones.
Phytohormones are grouped into 2 main types - growth promoters have a positive effect on a process and thus promote it, whereas the growth inhibitors have a negative effect and cause inhibition. A particular hormone may promote certain processes, inhibit some others and not affect many others. They act synergistically (co-operative and beneficial) or antagonistically (acting in opposition) with one another.
Auxins (in Greek - auxein means to grow) are one of the most important groups of plant hormones because of their many sided role in plants. These substances were also the first growth factors identified as plant hormones. The principal naturally occurring auxin is indole-3 acetic acid - IAA
In the last sixty years, a large number of growth regulators have been isolated from plants.
The first indication of their existence was given by Darwin (1880)
Darwin's Experiment with the Seeding of Canary Grass
Working on canary grass (Phalaris) he found that if a unilateral source of light was given, the coleoptile would bend towards the source of light. He believed that the tip contained a substance which was transmitted to the lower portion where it caused a curve. He also demonstrated that a decapitated coleoptile or a coleoptile which was covered with a tin foil cap failed to respond to unilateral light. But curvature was seen when seedling was buried in fine black sand with tip exposed.
Boysen Jensen (1910 - 1913) of Denmark found that the phototropic response lost by decapitation of the tip could be recovered if the tip was replaced on the stump. He further demonstrated that if a transverse slit was caused in the coleoptile on the dark side and a piece of mica inserted into the slit, no phototropic response took place. On the other hand there was a phototropic response if the slit and the piece of mica were on the illuminated side. He concluded that a substance migrates down the dark side promoting growth curvature towards light.
Discovery of Auxins Experiments of Boysen-Jensen
Went (1928) from Holland demonstrated the existence of a substance in the tip of Oat (Avena) seedling which diffused downwards and promoted growth. He demonstrated that when the tip of Oat coleoptile was removed, growth stopped. When the freshly cut coleoptile tip was placed on an agar block, for a few hours so as to allow the growth substances to diffuse into, and the agar block was then placed on the cut end of the coleoptile, growth occurred.
Oat Coleoptile Experiment
If the agar block was placed laterally on the cut end of the coleoptile, only that side of the coleoptile elongated resulting in a curvature. The side of the coleoptile that received the growth substance elongated faster and caused the curvature towards the opposite side. He called this substance Auxin from the Greek word 'auxein' to grow.
Hypothesis for Effect of Unilateral Illumination on Distribution of Auxin in a Coleopite
It is interesting to note that the first auxin was isolated from human urine of persons suffering from pellagra.
Auxins control several plant growth processes.
Auxins promote elongation and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all directions.
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.
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
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.
Auxin Spray Prevents Premature Fruit Abscission and Increase in Yield.a) Auxin Sprayed; b) Auxin not Sprayed
Auxin induces parthenocarpy, i.e., the formation of seedless fruits without the act of fertilisation.
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.
Higher concentration of auxins inhibit the growth and exert toxic effect on plants. In nature auxin inhibits the growth of lateral buds.
Auxins of various types are used now-a days for certain useful aspects of agricultural practices directly related to economic gains and food production.
Many synthetic auxins are used as selective weed killers and herbicides. 2, 4 - D (2, 4 - dichloro phenoxy acetic acid) is used to destroy broad leaved weeds. It does not affect mature monocotyledonous plants.
Destruction of Weeds by 2,4-D Spray
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.
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.
Auxins such as 2, 4 - D, IAA, IBA have been used successfully to prevent premature fruit drop in apples, pears and oranges.
NAA and IBA treatment induces parthenocarpy of fruits in tomatoes, ladies finger and brinjal.
Some Common Auxins
Dormancy of seeds can be broken by auxin application.
The methyl ester of NAA is used to prevent the sprouting of potato tubers.
Auxins improve the quality of vegetable crops by inhibiting flower formation in some plants.
Auxins such as IBA, increase the sweetening of fruits in many plants.
Synthetic auxins have been misused by people. Large scale application of defoliants used in Vietnam to expose the forests, exterminated the wild relatives of economically useful plants such as Citrus.
These are naturally occurring in plants. The best known and universally present natural auxin is Indole - 3 - acetic acid. Other natural auxins are Indole - 3 - pyuruvic acid, Indole - 3 - ethanol, Indole - 3 - acetaldehyde.
These are the chemicals synthesized by chemists that cause various physiological actions similar to IAA. Some of the synthetic auxins are Indole - 3 - butyric acid (IBA), b
- napthalene acetic acid (NAA) and 2,4 - dichlorophenoxy acetic acid (2,4 - D).
Auxin precursors - They are compounds which can be converted into auxins.
These are chemicals which inhibit the action of auxins.
Examples: 2, 3, 5 triiodobenzoic acid (TIBA) and napthylthalmic acid (NPA).
Free auxins - are auxins which can be easily extracted. They are active.
Bound auxins - are auxins which are difficult to extract. They are inactive.