8.3.3 Plant Hormones and Growth Regulators

Introduction

Plants need to coordinate their growth and development in response to their environment. They achieve this through chemical messengers called plant hormones or phytohormones. These hormones regulate various physiological processes such as growth, development, and responses to stimuli. Plant growth regulators are chemicals that control plant growth and development.

Definition

Plant Hormones (Phytohormones): Plant hormones are chemical messengers produced by plants that regulate various physiological processes such as growth, development, and responses to stimuli.

Plant Growth Regulators: Plant growth regulators are chemicals that control plant growth and development, including both natural hormones and synthetic chemicals.

In simple words: Plant hormones are chemical messengers that control how plants grow and respond to their environment.

Core Concept

• Plants coordinate their growth and development through chemical messengers called plant hormones or phytohormones.
• These hormones regulate various physiological processes:
• Growth
• Development
• Responses to stimuli
• Plant growth regulators include both natural hormones and synthetic chemicals.
• The main plant hormones are:

Detailed Explanation

1. Auxins

Definition: Auxins are plant hormones that promote cell elongation and are involved in various growth processes.

• Site of production: Apical meristems of shoots.
• Functions:
• Promote cell elongation.
• Promote apical dominance.
• Promote root initiation.
• Promote fruit development.
• Involved in phototropism and gravitropism.
• Example: Indole-3-acetic acid (IAA).
• Synthetic auxins: 2,4-D (2,4-dichlorophenoxyacetic acid), NAA (naphthalene acetic acid).
• Uses: Herbicides, rooting hormones.

2. Gibberellins

Definition: Gibberellins are plant hormones that promote stem elongation, seed germination, and flowering.

• Site of production: Apical meristems, seeds, roots.
• Functions:
• Promote stem elongation.
• Promote seed germination.
• Promote flowering.
• Promote fruit development.
• Break seed dormancy.
• Example: Gibberellic acid (GA₃).
• Uses: Increase fruit size, promote seed germination, delay ripening.

3. Cytokinins

Definition: Cytokinins are plant hormones that promote cell division and delay senescence (aging).

• Site of production: Root tips, developing seeds, fruits.
• Functions:
• Promote cell division.
• Delay senescence (aging).
• Promote lateral bud growth.
• Promote nutrient mobilization.
• Example: Kinetin, Zeatin.
• Uses: Tissue culture, delay aging of vegetables.

4. Abscisic Acid (ABA)

Definition: Abscisic acid is a plant hormone that inhibits growth and promotes dormancy and stress responses.

• Site of production: Roots, leaves, seeds.
• Functions:
• Inhibit growth.
• Promote seed dormancy.
• Promote bud dormancy.
• Promote stomatal closure during drought.
• Promote stress responses.
• Example: Abscisic acid.
• Uses: Promote dormancy, stress tolerance.

5. Ethylene

Definition: Ethylene is a gaseous plant hormone that promotes fruit ripening, senescence, and abscission.

• Site of production: Ripening fruits, senescing tissues.
• Functions:
• Promote fruit ripening.
• Promote senescence (aging).
• Promote abscission (leaf and fruit drop).
• Promote stress responses.
• Example: Ethylene gas.
• Uses: Artificial ripening of fruits, promote abscission.

Summary Table of Plant Hormones

Intermediate Level

Auxins in Detail

Gibberellins in Detail

Cytokinins in Detail

Abscisic Acid in Detail

Ethylene in Detail

Summary Table of Plant Hormones

Important Points

• 1. Plants need to coordinate their growth and development in response to their environment.
• 2. They achieve this through chemical messengers called plant hormones or phytohormones.
• 3. These hormones regulate various physiological processes such as growth, development, and responses to stimuli.
• 4. Plant growth regulators are chemicals that control plant growth and development.
• 5. The main plant hormones are:
• - Auxins: Promote cell elongation and are involved in various growth processes.
• - Gibberellins: Promote stem elongation, seed germination, and flowering.
• - Cytokinins: Promote cell division and delay senescence.
• - Abscisic acid: Inhibit growth and promote dormancy and stress responses.
• - Ethylene: Promote fruit ripening, senescence, and abscission.
• 6. Auxins:
• - Site of production: Apical meristems of shoots.
• - Functions: Promote cell elongation, apical dominance, root initiation, fruit development, phototropism, gravitropism.
• - Example: Indole-3-acetic acid (IAA).
• - Synthetic auxins: 2,4-D, NAA.
• - Uses: Herbicides, rooting hormones.
• 7. Gibberellins:
• - Site of production: Apical meristems, seeds, roots.
• - Functions: Promote stem elongation, seed germination, flowering, fruit development, break seed dormancy.
• - Example: Gibberellic acid (GA₃).
• - Uses: Increase fruit size, promote seed germination, delay ripening.
• 8. Cytokinins:
• - Site of production: Root tips, developing seeds, fruits.
• - Functions: Promote cell division, delay senescence, lateral bud growth, nutrient mobilization.
• - Example: Kinetin, Zeatin.
• - Uses: Tissue culture, delay aging of vegetables.
• 9. Abscisic Acid:
• - Site of production: Roots, leaves, seeds.
• - Functions: Inhibit growth, seed dormancy, bud dormancy, stomatal closure, stress responses.
• - Example: Abscisic acid.
• - Uses: Promote dormancy, stress tolerance.
• 10. Ethylene:
• - Site of production: Ripening fruits, senescing tissues.
• - Functions: Fruit ripening, senescence, abscission, stress responses.
• - Example: Ethylene gas.
• - Uses: Artificial ripening of fruits, promote abscission.

Remember

• - Auxins = cell elongation.
• - Gibberellins = stem elongation, seed germination.
• - Cytokinins = cell division.
• - Abscisic acid = dormancy, stress responses.
• - Ethylene = fruit ripening, senescence.
• - Uses: Herbicides, rooting hormones, fruit ripening, tissue culture.

Common Mistakes

• 1. Confusing auxins and gibberellins – Auxins = cell elongation; Gibberellins = stem elongation, seed germination.
• 2. Confusing cytokinins and abscisic acid – Cytokinins = cell division; Abscisic acid = dormancy.
• 3. Thinking ethylene is a solid or liquid – Ethylene is a gas.
• 4. Confusing the functions of abscisic acid – Abscisic acid inhibits growth and promotes dormancy.
• 5. Thinking all hormones promote growth – Some hormones inhibit growth (e.g., abscisic acid).
• 6. Confusing the sites of production – Each hormone has a specific site of production.
• 7. Forgetting the examples – IAA (auxin), GA₃ (gibberellin), Kinetin (cytokinin), ABA (abscisic acid), ethylene.
• 8. Thinking plant hormones are only natural – Plant growth regulators include both natural and synthetic chemicals.
• 9. Confusing the uses of hormones – Herbicides, rooting hormones, fruit ripening, tissue culture.
• 10. Thinking plant hormones are not important – Plant hormones are essential for plant growth and development.
• 11. Confusing the functions of ethylene – Ethylene promotes fruit ripening, senescence, and abscission.
• 12. Thinking all hormones have the same effect – Each hormone has specific functions.

Quick Revision

• Plants need to coordinate their growth and development in response to their environment.
• They achieve this through chemical messengers called plant hormones or phytohormones.
• These hormones regulate various physiological processes such as growth, development, and responses to stimuli.
• Plant growth regulators are chemicals that control plant growth and development.
• The main plant hormones are:
• Auxins: Promote cell elongation and are involved in various growth processes.
• Gibberellins: Promote stem elongation, seed germination, and flowering.
• Cytokinins: Promote cell division and delay senescence.
• Abscisic acid: Inhibit growth and promote dormancy and stress responses.
• Ethylene: Promote fruit ripening, senescence, and abscission.
• Auxins:
• Site of production: Apical meristems of shoots.
• Functions: Promote cell elongation, apical dominance, root initiation, fruit development, phototropism, gravitropism.
• Example: Indole-3-acetic acid (IAA).
• Synthetic auxins: 2,4-D, NAA.
• Uses: Herbicides, rooting hormones.
• Gibberellins:
• Site of production: Apical meristems, seeds, roots.
• Functions: Promote stem elongation, seed germination, flowering, fruit development, break seed dormancy.
• Example: Gibberellic acid (GA₃).
• Uses: Increase fruit size, promote seed germination, delay ripening.
• Cytokinins:
• Site of production: Root tips, developing seeds, fruits.
• Functions: Promote cell division, delay senescence, lateral bud growth, nutrient mobilization.
• Example: Kinetin, Zeatin.
• Uses: Tissue culture, delay aging of vegetables.
• Abscisic Acid:
• Site of production: Roots, leaves, seeds.
• Functions: Inhibit growth, seed dormancy, bud dormancy, stomatal closure, stress responses.
• Example: Abscisic acid.
• Uses: Promote dormancy, stress tolerance.
• Ethylene:
• Site of production: Ripening fruits, senescing tissues.
• Functions: Fruit ripening, senescence, abscission, stress responses.
• Example: Ethylene gas.
• Uses: Artificial ripening of fruits, promote abscission.

60-Second Revision

• Auxins = cell elongation.
• Gibberellins = stem elongation, seed germination.
• Cytokinins = cell division.
• Abscisic acid = dormancy, stress responses.
• Ethylene = fruit ripening, senescence.
• Uses: Herbicides, rooting hormones, fruit ripening, tissue culture.
• Warning: Auxins = cell elongation; Gibberellins = stem elongation, seed germination. Cytokinins = cell division; Abscisic acid = dormancy. Ethylene is a gas. Abscisic acid inhibits growth. Not all hormones promote growth. Each hormone has a specific site of production. Plant growth regulators include both natural and synthetic chemicals. Plant hormones are essential for plant growth and development.