Plant Hormones

Plant hormones:

• Plant growth regulatory

Growth hormones or phytohormones are the organic substance which are synthesized in minute quantity. In one part of the body and transported to another part where they influence physiological process.

A group of plant hormones including auxins, cytokines, Gibberellins, Ethylene, Abscisic acid are know to regulate growth. The term phyto hormones are those chemical substance which are synthesized by plant & are naturally found.

On the other hand there are several manufacuter chemicals which upon resemble the hormones in physiological action & even in molecular structure are called growth regulators not the hormones.

So there are five measure types of growth regulators.

1. Auxin: Associated with cell elongation, apical dominance.
2. Gibberellins: Cell difference.
3. Cytokinin: Cell division, lateral bud.
4. Ethylene: Ripening of fruit.
5. Abscisic acid: Ageing, closing of stomata, Senescence.

The growth regulator may different way effect either in different way either by inhibiting or promoting the process. Based on their function it is 2 types:

i) Growth Promoting hormones:

They are involved in growth promoting activity such as:

• Cell division
• Enlargement
• Tropic growth
• Flowering and fruit
• Seed formation

These are called Growth Promoter.

ii) Growth inhibiting

They are involved in growth inhibiting activity.

• Dormancy
• Abscission: Falling / shedding of plant parts such as flowers, fruit or leaf from the plant.

* Bioassay: It is used to test the function of Growth hormones & measure Concentration required to produce to effect in a living plant & are quantitative in nature.

1) Auxins → Auxein → To Grow (Greek word)

• Discovery: Charles Darwin & Francis Darwin. They worked in coleoptile of Canary Grass (shoot tip).
• F.W. Went - worked on coleoptile of oat seedling (Avena sativa).
• Auxins are best described as growth hormones they donot increase the rate of cell division but increase the height by cell elongation especially at the shoot tip.
• The term auxin is applied to Indole-3-acetic acid (IAA) & to all natural & synthetic compounds having similar structure & growth regulating properties.
• Indole-acetic acid (IAA) is the principle naturally occurring auxin present in plants.
• Auxins are present in the shoot tip & young leaves from where they are transported to the base of tip by diffusion.
• Natural auxins: IAA & Indole-butyric acid (IBA).
• Synthetic auxins: Napthalene acetic acid (NAA) & 2,4-chlorophenoxyacetic acid are synthetic auxine that have many uses in agriculture & horticulture (2,4-D). 2,4-dichlorophenoxyacetic acid is a selective weed killer.

Misuse

During Vietnam war 2,4-D & 2,4,5-T mix was used as spray to defoliant by us to expose forest also called Agent Orange. But it removed many wild relatives of number of economically useful plant like Citnes.

Function of Auxins:

1. Root initiation in stem cutting.
2. It promotes flowering. Ex. Pineapple.
3. It cause cell elongation i.e it brings enlargement of shoot & root tip behind the apical meristem.
4. It induces parthenocarpy i.e development of ovule into fruit without fertilization. Ex tomatoes (seedless).
5. It includes fruit growth i.e enlargement of many fruits by stimulating cell wall to stretch in more than on direction.
6. Apical dominance: It allows the growth of apical or terminal bud & inhibit the growth of lateral buds. On removal of apical bud generally lateral buds develop into branches but this process can be reversed by application of auxins on the decapitated shoot tip.

Horticultural value of auxins:

1. They are used as weed killers such as 2,4-D.
2. It is used for fruiting & root intiation such as NAA & IBA. It induces fruits especially in absence of pollination Ex:- Litchi + pinapple.
3. It inhibit sprouting of potato & hence is good for storage.
4. It inhibit or stimulate the growth of lateral buds giving a branch appearence.

2) Gibberellins:

Gibberellins are acidic in nature. It was named after a fungus called Gibberella fujikuroi (now called Fusarium moniliforme). This fungus caused a disease in the rice seeding called foolish seeding disease or 'bakanae' in Japan.

• Kurosawa in 1926 isolated the fungal extract & purified the active substance as Gibberellins.
• Gibberellins are present in greater concⁿ in immature seeds.
• They are found in root, shoot apices, buds, young apical leaves & young embryo. They are transported to diffrent parts of plant through xylem & phloem.

Functions of Gibberellins:

1. Internode elongation: They cause stem elongation & leaf expansion but no effect on roots. Ex Sugarcane when sprayed with Gibberlins increase the yield.
2. They can induce parthenocarpy (formation of seedless fruit). Ex: Pome, apple & pear.
3. It causes the reversal of dwarfism in many genetically dwarf plants.
4. It causes breaking of seed dormancy by stimulating enzyme production.
5. They delay senescence which help farmer to leave fruits on tree longer.
6. Gibberellins changes reaction expression in plants. Ex: Cannabis.
7. It Promotes flowering in long day plants during non-inductive period. It enhances malting process in Brewing industry.
8. Bolting is the elongation of reduced stem ex: cabbage. internodal length is very short giving rossettle appearance.

Uses of Gibberellins:

1. Gibberllins is used to promote seedless fruit in horticulture.
2. Numbers & size of fruit can be enhanced in Grapes, tomato by the application of Gibberlline.
3. It is widely used to break dormancy of seed & bud.
4. Ripening of fruits can be delayed with the help of Gibberellins. It is helpful in storage of fruit.

3) Cytokinin (Skoog & his workers discovered)

• The cytokinin are plant growth substance which acts primarily on cell division & have little or no effect on extension growth.
• These hormones are basic in nature.
• The first cytokinine was discovered by Miller et al from degraded Sample of DNA of yeast & Coconut milk & so called Kinetin.
• In 1963, the first naturally occuring cytokinins was chemically isolated & crystallized by Letham from the milky endosperm of Corn (Zea mays) and called as Zeatin.
• It produces in area of rapid cell division or actively growing region like root apices, developing shoot bud.

Function of Cytokinins:

1. The most important function of cytokinins is the promotion of cell division.
2. It causes enlargement of cell in leaf disc & cotyledons.
3. The Cytokinin can induce the formation of interfascicular cambium in plants.
4. Cytokinins play a vital role in morphogenesis in plants i.e help in the formation of callus.
5. External application of cytokinin from the growth of lateral buds even if the apical but is intack.
6. It can break the dormancy of many seeds & also promotes their germination.
7. Delay of Senescence (Richmond-Lang effect): Senescence is the phenomenon in which the mature leaves loose their pigment chlorophyll, turn yellow as protein is degraded so it shed off from the plants. So the application of cytokinin the process of senescence fast no of days.
8. It provides resistance to plants under high temp.
9. It also help volute translocation in phloem.

Uses of Cytokinins:

1. It promote cell differentiation in cultured cell (plant tissue culture).
2. They reduce apical dominance.
3. They help to break the dormancy of some seed also.
4. It delay leaf senescence b. in plant i.e It delay the process of ageing in leaves by controlling protein synthesis & mobilisation of resources.

4) Ethylene:

It has been recently known that the ripening is due to a gaseous plant growth regulator called ethylene.

Ethylene is produced in large amount by tissue under going senescence & by Ripening fruit. It escapes from their surface in the gaseous form.

Ethylene is a natural product of metabolism in plant.

5. Abscisic acid (ABA):

It is a growth inhibitor & plays an important role in plant development by regulating the growth. It causes abscission or separation so called abscisic acid. It interact with many growth promoters like auxin, Gibberllins & cytoplasm. It regulates bud dormancy & abscission.