Plant Growth Regulator (PGR)

Plant growth regulators are the chemical substances which govern all the factors of development and growth within plants. Some other names used to refer to it are phytohormones and plant growth hormones.

There are five groups of plant-growth-regulating 

compounds: auxin, gibberellin (ga), cytokinin, ethylene, and abscisic acid (aba). for the most part, each group contains both naturally occurring hormones and synthetic substances.

Based on their actions, plant growth regulators are broadly classified into two major groups:

Plant growth promoters (Auxins, Gibberellins, and Cytokinins )

Plant growth inhibitors (Abscisic acid and Ethylene)

Ethylene can be grouped either into the promoters or into the plant inhibitors.

Growth substance and their functions

1) Auxin

The first phytohormone to be discovered is the Auxin and it was discovered by the biologist Charles Darwin.Auxins are one of the most important plant hormones. Auxins are a powerful growth hormone produced naturally by plants. 

Uses

They are found in shoot and root tips and promote cell division, stem and root growth.

Promote apical dominance

Helps in Xylem differentiation

It prevent Abscission in young part of plants

Examples

Indole-3-Acetic Acid (IAA) - Prevents premature fruit drop

IAA is the main auxin in plants, regulating growth and developmental processes such as cell division and elongation, tissue differentiation, apical dominance, and responses to light, gravity, and pathogens.

Indole butyric acid (IBA) - Mainly used for Root formation

Naphthalene acetic acid (NAA) - Fruit thinner, prevent fruit drop

2,4-D (2,4-dichlorophenoxyacetic acid) is a herbicide in the phenoxy or phenoxyacetic acid family that is used postemergence for selective control of broadleaf weeds.

It kills only Dicot weeds

Dicamba - It is active herbicide

2) Gibberellin 

It Promotes Malting 

Delay in Senescence

it increase the Stem length ( particularly in sugarcane increases upto 20t/ha)

Increase fruit size 

Increase the stalk length in Grapes

It break Seed dormancy 

Bolting ( internodal elongation followed by flowering)

Examples

GA³

Gibberella fujikuroi is a fungal plant pathogen which produce Gibbersllin.It causes bakanae disease in rice seedlings. Another name is foolish seedling disease.

3) Cytokinin

Promotes cell division (cytokinesis)

Break apical dominance

Promote lateral bud growth

Richmond long effect ( delays the aging)

Examples

Kinatin ( herring sperm DNA extract)

Zeatin ( Maize grain extract)

4) Abscisic acid (ABA)

Abscission in flower and fruits

Dormancy inducer

stomata closer( Decrease the transpiration rate - prevents water loss)

also called as Anti gibberellins and stress hormone( resistance from cold and stress).

5) Ethylene

It act as both inhibitor and promoter

Promotes -  intiation of Germination of peanut, Breaks Bud dormancy (helps to grow)

Inhibitior - increases the Abscission(fall of fruits & leaves),  Senescence(aging)

Ripening of fruits

Isodiametric growth of stem & roots

Avaliable in two forms

Ethrel - it is liquid form of ethylene and used for banana fruit ripening

Ethephone - it is gaseous form of ethylene and used for early ripen of sugarcane, wheat, coffee, tabacco and rice.