Growth regulators: auxins, gibberellins, cytokinins, ethylene, abscisic acid

1. Introduction

1. Plant growth regulators (PGRs) are chemical substances that influence plant growth and development.
2. These include auxins, gibberellins, cytokinins, ethylene, and abscisic acid (ABA).
3. They are produced in specific parts of the plant and transported to other regions to regulate growth.
4. These regulators are categorized into growth promoters (auxins, gibberellins, cytokinins) and growth inhibitors (ethylene, abscisic acid).

2. Auxins

1. Auxins are plant hormones that promote cell elongation and regulate various growth processes.
2. First discovered by Charles Darwin in phototropism experiments.
3. The main natural auxin is Indole-3-acetic acid (IAA).
4. They are mainly produced in the apical meristem of shoots and young leaves.
5. They promote cell elongation, root initiation, and fruit development.
6. They inhibit lateral bud growth, leading to apical dominance.
7. Artificial auxins like 2,4-D are used as herbicides.

3. Gibberellins

1. Gibberellins (GAs) are growth regulators that promote stem elongation, seed germination, and flowering.
2. First discovered in Gibberella fujikuroi, a fungus causing "foolish seedling disease" in rice.
3. The most common gibberellins are GA1, GA3, and GA4.
4. They stimulate stem elongation by breaking dormancy in seeds and buds.
5. Gibberellins promote bolting in rosette plants like cabbage.
6. They induce parthenocarpy (fruit development without fertilization).
7. Used in agriculture to increase grape and sugarcane yield.

4. Cytokinins

1. Cytokinins are plant hormones that promote cell division and shoot formation.
2. First discovered in coconut milk and later identified as kinetin.
3. The most common cytokinin is zeatin, found in maize.
4. They delay senescence (aging) in leaves.
5. Promote axillary bud growth and break apical dominance.
6. Used in tissue culture to promote shoot development.

5. Ethylene

1. Ethylene is the only plant hormone that exists as a gas.
2. It promotes fruit ripening (e.g., bananas, mangoes, tomatoes).
3. Induces flowering in pineapples.
4. Stimulates leaf abscission (shedding of leaves).
5. It helps in the breaking of seed and bud dormancy.
6. Used in agriculture for artificial fruit ripening using ethrel.

6. Abscisic Acid (ABA)

1. Abscisic acid (ABA) is a growth inhibitor.
2. It induces seed dormancy and prevents premature germination.
3. It helps plants withstand water stress by closing stomata.
4. Also known as the "stress hormone" of plants.
5. Involved in leaf senescence and fruit abscission.

7. Comparison of Growth Regulators

8. Importance of Plant Growth Regulators

1. Used in agriculture to improve crop yield and fruit quality.
2. Play a crucial role in seed germination and dormancy.
3. Help plants adapt to environmental stress (e.g., drought).
4. Essential for plant development, flowering, and ripening.

9. Conclusion

1. Plant growth regulators control various aspects of plant life.
2. Auxins, gibberellins, and cytokinins are growth promoters.
3. Ethylene and ABA regulate ripening, dormancy, and stress responses.
4. Understanding these hormones helps improve crop production and plant management.