Plant Hormones: Definition, Overview, Types, Function, Auxin

What Are Plant Hormones?

Plant hormones are chemical compounds that regulate the plant development, growth, longevity, and reproductive processes of plants. They are produced in almost all parts of a plant and are transmitted into different areas of the plant for action. The hormones may act synergistically or individually, and their roles could be complementary to each other or even antagonistic.

Classification of Plant Hormones

Plant hormones are grouped into two major groups as per their actions: plant growth promoters and plant growth inhibitors.

Plant Growth Promoters

• Auxins: Stimulate cell elongation, apical dominance, and parthenocarpy.

• Gibberellins: Stimulate stem elongation, bolting, and seed germination.

• Cytokinins: Promote cell division, lateral shoot formation, and chloroplast development.

• Ethylene: Triggers fruit ripening, promotes senescence and induces root growth.

Plant Growth Inhibitors

• Abscisic Acid (ABA): Inhibits seed germination, induces senescence, and regulates abscission.

Types of Plant Hormones & Their Functions

Each of these plant hormones performs some very specific functions that are quite important to the growth and development of plants.

Auxins

• Indole-3-Acetic Acid, IAA: Promotes cell elongation, apical dominance, and parthenocarpy.

• 2,4-Dichlorophenoxyacetic Acid, 2,4-D: Used as a herbicide that kills dicot weeds.

• Naphthalene Acetic Acid, NAA: Rooting of stem cuttings and grafting.

Gibberellins

• Gibberellins GA1, GA2, GA3: Promotes stem elongation, bolting, and seed germination.

• Breaks Seed Dormancy: It stimulates the formation of hydrolytic enzymes in cereal grains.

Cytokinins

• Zeatin: Induces the growth of lateral and adventitious shoots.

• Kinetin: Induces chloroplast formation and delays leaf senescence.

Ethylene

• Fruit Ripening: Advances fruit ripening.

• Senescence: Induces senescence and abscission of leaves and flowers.

Abscisic Acid (ABA)

• Abscission Inducer: Regulates abscission and dormancy.

• Stress Hormone: Improves plant tolerance to water stress through the induction of stomatal closure.

Role of Plant Hormones in Developmental Processes

Plant hormones have a key role in many physiological processes, which include

• Vernalization: The process by which plants require a period of cold temperatures to induce flowering.

• Phototropism: Growth towards light.

• Seed Germination: Transition from dormant seed to active seedling.

• Dormancy: State of inactivity in seed or buds.

Applications of Plant Hormones (Agriculture & Horticulture)

The applications of plant hormones includes:

Auxin

• Stimulate root formation in stem cutting for rapid propagation.

• Herbicide (2,4-D) selectively kills broadleaf weeds.

• Delays abscission, allowing fruits to ripen fully on the plant.

Gibberellins

• Increases fruit size by stimulating cell elongation.

• Promotes enzyme production in barley in the brewing industry.

• Enhances stem internode length, boosting cane yield.

Cytokinins

• Stimulate cell division and shoot formation in plant tissue culture.

• Delay senescence in stored vegetables

Ethylene

• Induces uniform ripening of fruits.

• Ensure uniform flowering for simultaneous fruit harvesting.

Abscisic Acid (ABA)

• Closes stomata to reduce water loss during dry conditions.

• Promotes seed and bud dormancy to survive unfavourable conditions.

Comparison Table — Promoters vs Inhibitors

The difference between growth promoters and growth inhibitors is:

Plant Growth Regulators NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of Plant Hormones and their functions

• Role of Plant hormones in developmental processes

Practice Questions for NEET

Q1. Name the plant growth regulator, which, upon spraying on sugarcane crops, increases the length of the stem, thus increasing the yield of sugarcane crops.

1. Cytokinin

2. Gibberellin

3. Ethylene

4. Abscisic acid

Correct answer: 2) Gibberellin

Explanation:

• The plant growth hormone gibberellin helps in the elongation of stems in crops such as sugarcane.

• When applied to sugarcane plants, it promotes the development of taller stems, resulting in increased yields.

• This hormone is frequently utilized in agriculture since it is essential for encouraging cell elongation.

• Farmers can increase crop yield and have better outcomes with crops that need vertical growth by utilizing gibberellins.

Hence, the correct answer is option 2) Gibberellin.

Q2. The cells in watermelon may increase in size by up to 3,50,000 times. This phenomenon is called

1. Hypotrophy

2. Hypertrophy

3. Hyperplasia

4. Hypoplasia

Correct answer: 2) Hypertrophy

Explanation:

Hypertrophy is the increase in the size of cells leading to the enlargement of a tissue or organ. Hypertrophy typically occurs as a response to increased demand or stress. For instance, muscle hypertrophy takes place when muscle fibres grow larger due to exercise, especially through strength training. While physiological hypertrophy is generally healthy, pathological hypertrophy, such as that occurring in the heart due to high blood pressure, can impair function and lead to complications.

Hence, the correct answer is option 2) Hypertrophy.

Q3. Growth promoting hormone is

1. Auxin

2. Gibberellin

3. Cytokinin

4. All of the above

Correct answer: 4) All of the above

Explanation:

The PGRs can be broadly divided into two groups based on their functions in a living plant body:

• Growth promoting such as mediating cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation.

• Growth inhibiting such as mediating responses to wounds and stresses of biotic and abiotic origin.

Auxin, gibberellin, and cytokinins are growth-promoting hormones.

Hence, the correct answer is option 4) All of the above.

Also Read:

• MCQs on Plant Growth Regulators

• Phases of Plant Growth

• Factors Affecting Plant Growth

Recommended Video on Plant Hormones