PHYTOHORMONES//PLANT GROWTH HORMONE//PLANT GROWTH REGULATORS// PLANT HORMONE///

 

PLANT HORMONES (PYTOHORMONES)

 

Phytohormones are also known as Plant Growth Hormones or Growth Hormones or Plant Growth Regulators (PGRs).

Phtytohormones are organic substances which are synthesiszed in minute quantities in one part of the plant body and transported to another part where they influence specific physiological process.

Plant hormones control all the physiological and cellular activities like cell division, enlargement, flowering, seed formation, dormancy and abscission.

A group of plant hormones including Auxins, Gibberellins, Cytokinins,  Ethylene and Abscisic acid are presently known as growth regulators.

Out of these Auxins, Gibberellins and Cytokinins have growth promoting effects, where as Ethylene and Abscisic acid (ABA) have growth inhibitory effects.  

 Basically PGR are divided into two main groups-

 (A). Plant growth promoters- Auxins, Gibberellins and Cytokinins.

 (B). Plant growth inhibitors- Ethylene and Abscisic acid (ABA).

Ethylene could fit in both groups but largely it is an Inhibitor.

1.    Auxins:

Auxin, the first plant hormones was discovered by Frits Went as a growth promoting chemical in the tip of Oat (Avena sativa) coleoptiles

The  chemical isolated by went promoted the elongation of the coleoptile it was eventually  named Auxin ( derived from the Greek word auxein, meaning to Increase).

The naturally occurring Auxins are Indole-3-acetic acid(IAA), 4-chloro-indole acetic acid (4-chloro IAA) and Phenyl acetic acid (PAA).

The synthetic auxins are Indole -3-butyric Acid (IBA),  Phenoxy acetic Acid, 2,4-dicholorophenoxy acetic acid (2,4-D),etc.

The auxins are widely distributed throughout the plant body, but the greatest amount is found in actively growing regions (root and shoot apex) and the lowest amount is found in the non- growing regions.

Physiological effects of Auxins:

a.     Auxins promotes apical dominance and suppress the growth of lateral branches/ Auxillary buds.

b.     Auxin prevents leaf, flower, fruit dehiscence/ abscission.

c.      Auxin induces parthenocarpic fruit (Cucumber, apples, Tomatoes and watermelon) development.

d.     Auxin induces more root growth, at lower concentration primary root development while at higher concentration promote secondary and tertiary root development.

e.      Auxin at low concentration promote Phloem development but at higher concentration promote both xylem and phloem development.

f.       Auxin  induce acidification of Apoplastic space that induce cell elongation.

 

 

 

 

 

 

2.        Gibberellins:

Gibberellins were discovered in 1926 by a Japanese  plant pathologist Kurosawa, while working on ‘foolish seedling disease of rice’, caused by the fungus  Gibberella  fujikuroi ( present name is  Fusarium fujikuroi).

There are more than 100 gibbrellins (GA₁, GA₂, GA₃….) that are known. GA₃ is the most important gibberellins. It is also called as Gibberellic acid.

All plant organ contains gibbrellins at varying levels, but the richest sources and probable sites of synthesis were found to be the fruits,seeds buds,young leaves and root tips.

 

 

Physiological effects of Gibberellins:

a.     It induces stem elongation.

b.     It induces seed germination.

c.      Promotes bolting (formation of internode before flowering).

d.     Determine the sex of flower.

e.      Induces parthenocarpic fruit development.

f.       It stimulates pollen grain development.

 

3.    Cytokinins:

Cytokinins (Kinin) are plant growth substances that typically stimulate cell division (Cytokinesis).

It is first time discovered by skoog  in yeast.

Zeatin was first natural cytokinin.

Young roots, immature fruits and seeds, and the nurse tissues ( e.g. liquid endosperm) are rich in cytokinins.

Immature fruit of maize, banana, apple, and coconut ( milky endosperm) are especially rich sources of cytokinins.

 

Physiological effects of cytokinins:

a.     Accerelate cell division.

b.     It reduces apical dominance and promote more growth of auxillary bud/ lateral bud.

c.      It is antagonistic to Auxin.

d.     Cytokinins delay senescence.

e.      Cytokinins promotes chloroplast and leaf development.

f.       Cytokinins promotes movement of nutrient (photosynthate) from leaf to another part.

 

 

4.    Ethylene:

It is a simple gaseous plant growth regulator and synthesized from the amino acid Methionine.

Ethylene is present in almost every part of the plant, i.e., roots, leaves, flowers, seed, fruits, etc.

Its synthesis increase in tissue undergoing Senescence or ripening due to this property it is also known as fruit ripening hormones.

Ethylene biosynthesis is increased by stress conditions such as drought, flooding, chilling or mechanical wounding.

Physiological effects of Ethylene:

a.     Ethylene promotes fruit ripening.

b.     Ethylene break seed and bud dormancy.

c.      Stimulates rapid elongation of petioles and internodes.

d.     Ethylene promotes flowering in pineapple.

e.      Ethylene promotes senescence and abscission of leave and flowers.

 

 

 

 

 

5.    Abscisic Acid (ABA):

It is a growth- inhibiting hormone.  ABA is slightly acidic growth hormone.

Its production is stimulated under stress (unfavourable conditions such as drought, water lodging, excessive temperature) etc. So it is also called ‘stress hormone’.

It acts antagonistically to Gibberellic acid.

Physiological effects of ABA:

a.     ABA stimulates the closure of stomata.

b.     It inhibits metabolism of plants.

c.      It has major role in regulating abscission and dormancy of buds and seed.

d.     It promotes senescence in leaves.

e.      Increases resistance of plant to cold.