Authors: Shilpa Devi and Arvind Nagar
Hormones are organic substances produced in small, often minute, quantities in one part of an organism’s body and then transported to another part, where they bring about physiological or developmental responses. The activity of hormones results from their capacity to stimulate certain physiological processes and to inhibit others. Term hormone is derived from greek word ‘hormao’ which means to urge on or to stimulate.
How hormones act?
Action is influenced both by the hormone and the tissue that receives the message. In animals, hormones are usually produced at definite sites, usually organs. In plants, hormones are not produced in specialized tissues but, instead, in tissues that also carry out other, usually more obvious functions. Plant hormones are called as phytohormones. These are also known as Plant growth regulators due to their mode of action as they act to promote or inhibit plant growth.
There are five major kinds of plant hormones:
- Abscisic acid (ABA)
ACTIVE SITE OF AUXIN SYNTHESIS : Shoot tip region.
KEY FEATURES OF AUXINS: Polar translocation i.e. basipetal.
- Delay in abscission & differentiation of xylem elements.
- Root initiation.
- Apical dominance.
- Variable behaviour of root & shoot growth.
- Examples of auxins:- IAA,IBA,NAA,2,4-D, 2,4,5-T.
ANTI-AUXINS These are compound which compete with the auxins for the reactive site, e.g. TIBA,4-chlorophenoxy isobutyric acid, 2,6 dichlorophenoxy acetic acid. The molecular structure of anti-auxins closely related to that of auxins. Weak auxins e.g. phenylbutyric acid) behave similarly.
- Indole -3-acetic acid (IAA)
- Molecular formula: C10 H9 O2 N. It is an almost universal natural auxin of plants.
- IAA is found in almost all plant parts but their presence in roots is not yet clear.
Cytokinins comprise another group of naturally occurring growth hormones in plants. A cytokinin is a plant hormone that, in combination with auxin, stimulates cell division and differentiation in plants. Most cytokinins are produced in the root apical meristems and transported throughout the plant. Developing fruits are important sites of cytokinin synthesis. All naturally occurring cytokinins are purines that appear to be derivatives of or have molecule side chains similar to those of adenine. Cytokinins promote growth of lateral buds into branches though along with auxin and ethylene, they also play a role in apical dominance (the suppression of lateral bud growth).
Auxin & Cytokinin relationship :
- When the apical meristem of a plant is intact, auxin from the apical bud will inhibit the growth of lateral buds.
- When the apical bud is removed, cytokinins are able to produce the growth of lateral buds into branches.
- When the apical bud is removed and auxin is added to the cut surface, axillary bud outgrowth is suppressed.
- Cytokinins inhibit formation of lateral roots, while auxins promote their formation.
- As a consequence of these relationships, the balance between cytokinins and auxin, along with other factors, determines the appearance of a mature plant.
- In addition, the application of cytokinins to leaves detached from a plant retards their yellowing. They function as anti-aging hormones.
KEY FEATURES OF CYTOKININS:-
- Initiation of cell division.
- Delay of senescence.
- Counteract apical dominance.
- Flowering induction in short day plants
SYNTHETIC CYTOKININS: 6-benzyladenine, 1-benzyladenine and 6-aminopurine.
- Gibberellins are named after the fungus Gibberella fujikuroi, which causes rice plants, on which it is parasitic, to grow abnormally tall . Japanese plant pathologist Eiichi Kurosawa investigated Bakane (“foolish seedling”) disease in the 1920s.
- All are acidic and are usually abbreviated as GA (gibberellic acid), with a different subscript (GA1, GA2,and so forth) to distinguish each.
- Gibberellins, which are synthesized in the apical portions of stems and roots, have important effects on stem elongation.
- The elongation effect is enhanced if auxin is also present.
- The application of gibberellins to dwarf mutants is known to restore the normal growth and development in many plants.
KEY FEATURES OF GIBBERELLINS:-
- Induction of flowering in long day plants.
- Breaking of dormancy.
- Prevention of genetic & physiological dwarfism.
- Increase of amylase activity.
SITE OF SYNTHESIS :- Young leaves, root tips & immature seeds.
- Abscisic acid, a naturally occurring plant hormone, appears to be synthesized mainly in mature green leaves,fruits and root caps.
- Abscisic acid probably induces the formation of winter buds—dormant buds that survive through the winter—by suppressing growth.
- Like ethylene, it may also suppress growth of dormant lateral buds. It appears that abscisic acid, by suppressing growth and elongation of buds, can counteract some of the effects of gibberellins (which stimulate growth and elongation of buds).
- It also promotes senescence by counteracting auxin (which tends to retard senescence).
- Abscisic acid plays a role in seed dormancy and is antagonistic to gibberellins during germination.
- It is also important in controlling the opening and closing of stomata.
- Abscisic acid occurs in all groups of plants and apparently has been functioning as a growth-regulating substance since early in the evolution of the plant kingdom.
- Like other plant hormones, abscisic acid probably will prove to have valuable commercial applications when its mode of action is better understood.
OTHER MISCELLANEOUS COMPOUNDS
Florigen: Responsible for flowering initiation in plants. Synthesized in older leaves Wound hormone: A hormone whose activity increases in response to wound or injury. Example:- Traumatic acid.
PRACTICAL APPLICATIONS IN VEGETABLES
- Seed Germination: GA@ of 40-100ppm,IAA@ 10-15ppm as seed treatment.
- Flower Drop : NAA @ 10-20ppm in form of foliar spray.
- For fruit set & yield : PCPA,2,4-D @ of 50ppm & 0.5ppm are used.
- Earliness & fruit ripening: 1000ppm Ethephon( CEPA)-applied before harvest.
- Seed Germination: IAA @ 10-15ppm,NOA(Naphthoxyacetic acid) @ 25-100ppm as seed treatment or foliar spray.
- Flowering,fruit set: Cycocel @ 250-500ppm as foliar spray.
- Flowering & sex expression: Cycocel @ 250-500ppm,CEPA @ 100-500ppm,MH (Maleic hydrazide) @ 50-150ppm,2,3,5- tri-iodobenzoic acid(TIBA)@ 25-50ppm.
- Flower Drop : NAA@ 10-20ppm is applied as foliar spray.
- Fruit Set : Tricontanol, Mixtallol @ 2ppm as foliar spray.
- Fruit Ripening : CEPA(Ethephon) @ 1000ppm as pre-harvest application. CUCUMBER For induction of male flowers on gynoecious lines are as follows :
- 3 applications of GA3 at 1000ppm, sprayed on at fortnight intervals, commencing when the plants have 2-3 leaves
- As above, but using GA4/GA7 at 50ppm
- A single application of silver nitrate (AgNO3)solution (600 mg/ltr) before the first flower open. For induction of female flower :
- 2 sprays of 100ppm MH, 100ppm of NAA and 200ppm of ethrel at 2 leaf stage and again at 4 leaf stage.
- Induction of female flowers: TIBA @ 25-50ppm and GA3 @ 25,50 and 100ppm respectively.
- Breaking tuber dormancy 1% thiourea and 1ppm GA3 solution for 1 hour.
- A pre harvest spray of MH controls sprouting.
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