Plant Growth Hormones And Their Functions

Introduction

Plant requires light, water, oxygen and nutrition for proper growth and development. All these are the extrinsic factors. Plant also needs some intrinsic compounds to signal, regulate and control their growth and development. These intrinsic compounds are known as plant growth regulators or plant growth hormones or phytohormones. Plant growth hormones are organic chemical substances, which are either produced naturally in plants or synthesized in laboratories. They control and modify the growth, development and movement of plants.

Table of Contents

Types of plant growth

Primary and secondary growth

The primary growth of plant is the mitotic division of meristematic cell, which are present at apices of root and shoot. This process increases the length of plant body. The secondary growth of plant is the increase in diameter of plant body by division of secondary meristem.

Unlimited growth

Plants can grow unlimited throughout their life due to the presence of meristems. When plant grows continuously from its germination stage to death is known as unlimited growth.

Limited growth

In this stage plant growth is stopped after attaining a certain size.

Vegetative growth

It is the growth of stems, leaves and roots of plants. It is the period between germination and flowering. In this phase plants accumulate resources required for flowering and reproduction.

Reproductive growth

In this stage flowering is occurred.

Plant growth hormones

Plant growth regulators can be of different chemical composition like gases (ethylene), terpenes (gibberellic acid) and carotenoid derivatives (abscisic acid). Based on their action they can be classified in two types as follows,

Plant growth promoters

These are the hormones which help in the plant growth processes like cell division, cell enlargement, flowering, fruiting and seed formation. Examples, Auxins, Gibberellins, Cytokinin.

Auxins

Auxin is the first plant growth hormone, discovered by the Charles Darwin. Darwin observed that the coleoptile (protective sheath) in canary grass grows towards the sunlight. This phenomenon is known as phototropism. Auxin was isolated by scientist F.W. Went. Auxin is derived from the Greek word ‘auxein’ which means ‘to grow’.

These hormones naturally produced at points of stems and roots of plants from where they are transported to other parts of plants. Auxins can be obtained from both natural and synthetic sources. Natural sources of auxins are Indole-3-acetic acid and Indole butyric acid. Auxins obtained from synthetic sources are naphthalene acetic acid and 2,4-dichlorophenoxyacetic acid.

Functions of auxins

Promote flowering in plants.
Helps to initiate roots in stem cutting.
Prevent dropping of fruits and leaves in early stage.
Helps in plant propagation.
Regulate xylem differentiation and help in cell division.
Can be used as herbicide to kill weeds.
Promote natural detachment of older fruits and leaves.
It used for production of fruits prior fertilization (this process is known as parthenocarpy)

Gibberellins

Gibberellins is the chemical family based on the ent-gibberellin structure. Gibberellic acid is the first discovered gibberellin. Today there are more than 100 gibberellins exist, which obtained from variety of organism from fungi to higher plants. They are denoted as GA₁, GA₂, GA₃, etc.

Functions of gibberellins

They delay the ageing (senescence) of fruits.
Increase axis length of plants.
They help in expansion of leaf.
They help to elongate and improve shape of fruits.
They help to promote bolting in cabbages and beet. Bolting is the sudden growth of a plant just before flowering.
They help to increase malting process in brewing industry.
They help to increase sugarcane yield by lengthening stem.
They help to break bud and seed dormancy.

Cytokinin

Natural cytokinin exist in the region where cell division occurs, like apices of root and shoot. Skoog and Miller discovered the chemical substance kinetin (cytokinin). Cytokinin concentration is high in young parts of plants.

Functions of cytokinin

Cytokinin helps in formation of new leaves and chloroplasts.
It promotes lateral shoot growth and adventitious shoot formation.
It helps in cell division and to overcome apical dominance.
It promotes nutrient mobilisation, which helps to delay leaf senescence.

Plant growth inhibitors

These are the hormones which inhibits growth of plant and promote dormancy and abscission in plants. Example, Abscisic acid, Ethylene.

Abscisic acid

It is synthesized in stem, leaves, fruits, and seeds of the plants. Abscisic acid is a growth inhibitor and antagonist the gibberellic acid action. It is also known as the stress hormone. It helps plants to increase tolerance in stress.

Functions of abscisic acid

Regulates abscission and dormancy.
Inhibits the plant growth, metabolism and seed germination.
Stimulates closure of stomata in epidermis.
Increases tolerance of plants in various types of stress.
Induce seed dormancy, which helps plants to withstand in desiccation and other unfavorable growth factors.
Helps in development and maturation of seeds.

Ethylene

It is a simple gaseous plant growth inhibitor, chemically it is an unsaturated hydrocarbon having double covalent bonds between and adjacent to carbon atoms. Ripening fruits and ageing tissues synthesize ethylene. It enhances respiration rate during the ripening of fruits, this phenomenon called as ‘respiratory climatic’.

Functions of ethylene

Increase root growth and root hair formation, which helps plants to increase their absorption surface area.
Breaks seed and bud dormancy and initiate germination.
Used to promote sprouting of potato tubers.
Used to induce flowering in mangoes.
Used in rubber trees to stimulate the latex flow.
Promotes abscission and senescence, mostly in leaves and flowers.
It affects horizontal growth of seedlings and swellings of the axis in dicot seedlings.

Conclusion

Plant growth regulators are important compounds for growth and development of plants. They play pivotal roles in various physiological processes like cell division, elongation and cell differentiation. Plant growth regulators used in agriculture and horticulture. They helps to enhance crop yield, quality and stress tolerance.

Frequently asked question

What is apical dominance?

Apical dominance is the phenomenon where growth of lateral buds is inhibited by the apical buds. In this condition vertical growth of plant surpasses lateral growth of plant. Apical dominance is controlled by the auxins. Removal of apical bud allows the growth of lateral bud.

What is abscission?

It is the natural detachment of leaf, fruits, flowers and seeds of plants. Abscission is the word made from Latin word ab- away and scindere- to cut.

What is parthenocarpy?

Parthenocarpy is the production of fruits without prior fertilization of ovules. It can be natural or artificial production of fruits, which makes fruits seedless.

What are the 5 plant growth regulators?

There are 5 plant growth regulators; auxin, gibberellin, cytokinin, abscisic acid and ethylene.

Which is the most commonly used plant growth regulators (PGR)?

Ethylene is the most commonly used plant growth regulators in agriculture and horticulture as it regulates so many physiological processes.

Plant Growth Hormones and Their Functions