Plant Growth Regulators - 2 | Agriculture Optional Notes for UPSC PDF Download

Types of Plant Growth Regulators

Plant growth hormones or regulators are of the following types:

• Plant Growth Promoters
• Plant Growth Inhibitors

Plant Growth Promoters

Auxins

The first phytohormone to be identified is auxin, and it was Charles Darwin, the biologist, who made this discovery.

Auxins are among the most crucial plant hormones, with the primary naturally occurring auxin being indole-3 acetic acid (IAA) and related compounds. The term "auxin" is derived from the Greek word for "to grow."

These plant growth regulators are typically synthesized at the stems and root tips and are then transported to various parts of the plant. They can originate from both natural and synthetic sources. Indole-3-acetic acid and indole butyric acid are examples of auxins derived from natural plant sources, while naphthalene acetic acid and 2,4-dichlorophenoxyacetic acid are obtained from synthetic sources.

Functions of  Auxins:

• Facilitate flowering in plants
• Used in the process of plant propagation.
• Used by gardeners to keep lawns free from weeds.
• Involved in the initiation of roots in stem cuttings.
• Prevention of dropping of leaves and fruits at early stages.
• Regulate xylem differentiation and assists in cell division.
• Auxins are widely used as herbicides to kill dicot weeds.
• Used to produce fruit without preceding fertilization.
• Promote natural detachment (abscission) of older leaves and fruits.
• Apical dominance may occur in which the growth of lateral buds is inhibited by the growth of apical buds. In such cases, the shoot caps may be removed.
• These are produced by the apex of root and shoot.

Gibberellins

Gibberellins constitute a diverse chemical family characterized by the ent-gibberellane structure. The initial gibberellin discovered was gibberellic acid, but today, there are over 100 different types of gibberellins. These compounds are primarily extracted from a range of organisms, including fungi and higher plants.
They are acidic and are denoted as follows – GA₁, GA₂, GA₃ etc.

Functions of Gibberellins 

• Delay senescence in fruits.
• Involved in leaf expansion.
• Break bud and seed dormancy.
• Promote bolting in cabbages and beet.
• Facilitate elongation of fruits such as apples and enhance their shape.
• Used by the brewing industry to accelerate the malting process.
• Used as the spraying agent to increase the yield of sugarcane by elongation of the stem.
• In young conifers, utilized to fasten the maturity period and facilitate early seed production
• Helps in increasing the crop yield by increasing the height in plants such as sugarcane and increase the axis length in plants such as grape stalks.
• Gibberellins are acidic in nature.
• It also delays senescence.

Cytokinins

Gibberellins are synthesized in regions where cell division is active, primarily in the roots and shoots of plants. They play a role in various plant growth processes such as the production of new leaves, lateral shoot growth, and the development of chloroplasts in leaves. Gibberellins also help overcome apical dominance, which is the suppression of lateral bud growth by the terminal bud, and they delay the aging of leaves.

Functions of Cytokinins

• Break bud and seed dormancy.
• Promotes the growth of the lateral bud.
• Promotes cell division and apical dominance.
• They are used to keep flowers fresh for a longer time.
• Used in tissue culture to induce cell division in mature tissues.
• Facilitate adventitious shoot formation and lateral shoot growth.
• Promotes nutrient mobilization that in turn assists delaying leaf senescence.
• Helps in delaying the process of ageing (senescence) in fresh leaf crops like cabbage and lettuce.
• Involved in the formation of new leaves and chloroplast organelles within the plant cell.
• Used to induce the development of shoot and roots along with auxin, depending on the ratio.

Plant Growth Inhibitors

Abscisic acid

Abscisic acid is a growth inhibitor that was first identified in the 1960s and was initially referred to as "dormin." Later, another compound known as abscisin-II was discovered, and these compounds are commonly referred to as abscisic acid. This growth inhibitor is synthesized within various parts of the plant, including the stem, leaves, fruits, and seeds. Abscisic acid often acts as an antagonist to gibberellic acid. It is also known as the "stress hormone" because it enhances the plant's tolerance to various forms of stress.

Functions of Abscisic acid

• Stimulates closing of stomata in the epidermis.
• Helps in the maturation and development of seeds.
• Inhibits plant metabolism and seed germination.
• It is involved in regulating abscission and dormancy.
• It is widely used as a spraying agent on trees to regulate dropping of fruits.
• Induces seed-dormancy and aids in withstanding desiccation and various undesired growth factors.

Ethylene

Ethylene is a basic gaseous plant growth regulator produced by various plant organs, including maturing fruits and aging tissues. It is an unsaturated hydrocarbon characterized by double covalent bonds between carbon atoms, some of which are adjacent.

Ethylene functions both as a promoter and an inhibitor of plant growth. It is generated by maturing fruits and aging tissues.

Functions of Ethylene

Ethylene is the most widely utilized plant growth regulator due to its role in regulating numerous physiological processes:

• It induces flowering in mango trees.
• It promotes the sprouting of potato tubers.
• It breaks the dormancy of seeds and buds.
• It enhances the respiration rate during the ripening of fruits.
• It is applied to rubber trees to stimulate latex flow.
• It facilitates the senescence and abscission of both flowers and leaves.
• It is used to stimulate the ripening of fruits, such as tomatoes and citrus fruits.
• It affects the horizontal growth of seedlings and the swelling of the axis in dicot seedlings.
• It increases the formation and growth of root hairs, thereby aiding plants in expanding their surface area for absorption.

This highlights the significant role of plant hormones or growth regulators in the growth and development of plants.