🌿 Plant Physiology – Growth and Development in Plants (Part 3)

🌱 Plant Growth Regulators (Phytohormones)

Plant growth is regulated by various phytohormones, which are chemical substances that influence the growth and development of plants. These hormones are produced in one part of the plant and transported to other parts where they trigger specific physiological responses. There are five major types of phytohormones: Auxins, Gibberellins, Cytokinins, Abscisic Acid (ABA), and Ethylene.

1. Auxins – The Growth Promoters

Auxins are a group of plant hormones that primarily regulate cell elongation, particularly in roots and stems. They also play a significant role in the formation of lateral roots and the development of fruit.

Role in Phototropism: Auxins promote growth on the shaded side of the plant, causing the plant to bend towards light.
Apical Dominance: Auxins produced at the tip of the plant (apical meristem) inhibit the growth of lateral buds, maintaining the plant's upward growth.

2. Gibberellins – The Growth Promoters

Gibberellins are another important group of growth hormones that are primarily involved in promoting stem elongation and seed germination.

Stem Elongation: Gibberellins promote cell division and elongation, helping plants grow taller.
Seed Germination: Gibberellins activate enzymes that break down stored food in seeds, aiding in the germination process.

3. Cytokinins – The Growth Regulators

Cytokinins are plant hormones that promote cell division in plant roots and shoots. They also delay senescence (aging) of plant tissues.

Cell Division: Cytokinins stimulate the division of meristematic cells in the shoot and root tips.
Delaying Senescence: Cytokinins help maintain the vitality of plant tissues by delaying the aging process, especially in leaves and flowers.

4. Abscisic Acid (ABA) – The Growth Inhibitor

Abscisic Acid is often called the "stress hormone" because it helps plants respond to environmental stress, such as drought and cold.

Stomatal Closure: ABA plays a key role in reducing water loss by causing the stomata to close during drought conditions.
Seed Dormancy: ABA helps to maintain seed dormancy and prevents premature germination until conditions are favorable.

5. Ethylene – The Ripening Hormone

Ethylene is a gaseous plant hormone that regulates various aspects of plant development, including fruit ripening, leaf abscission, and flower wilting.

Fruit Ripening: Ethylene is responsible for the ripening of fruits like bananas, tomatoes, and apples. It accelerates the breakdown of starches into sugars, making the fruit sweeter.
Leaf Abscission: Ethylene triggers the shedding of leaves in response to changes in the seasons.

🌿 Growth of Plants – The Stages of Plant Development

Plant growth and development are characterized by various stages that begin from germination and continue until the plant reaches maturity.

1. Germination – The Beginning of Plant Life

Germination is the process by which a seed develops into a seedling. It requires favorable conditions such as water, oxygen, and the right temperature. The seed absorbs water, and the embryo inside begins to grow, pushing through the seed coat.

Seed Dormancy: Many seeds undergo a period of dormancy, during which they are inactive and do not germinate until favorable conditions are present.
Radicle Emergence: The first part of the plant to emerge from the seed is the radicle, which develops into the root system.
Shoot Development: After the radicle, the shoot (stem and leaves) starts to grow upwards.

2. Vegetative Growth – The Phase of Cell Division and Elongation

In the vegetative phase, the plant grows primarily by cell division and elongation. During this phase, the plant develops its roots, stems, and leaves.

Apical Meristem: Located at the tips of roots and shoots, the apical meristems are responsible for primary growth.
Lateral Meristems: These meristems lead to the secondary growth of the plant, increasing the girth of stems and roots.

3. Reproductive Growth – The Flowering Phase

Once the plant has completed its vegetative growth, it enters the reproductive phase, where it produces flowers, seeds, and fruits for reproduction.

Flowering: The plant forms flowers, which are essential for pollination and fertilization.
Pollination: The transfer of pollen from male to female parts of the flower, either through wind, water, or pollinators (like bees), is crucial for fertilization.
Fertilization: The fusion of male and female gametes leads to the formation of seeds and fruits.

⚡ NEET PYQ Alerts – Plant Growth and Development

Which hormone is responsible for the elongation of stems and leaves?
- A) Gibberellins
- B) Cytokinins
- C) Ethylene
- D) Abscisic Acid
- Answer: A) Gibberellins
Which of the following hormones promotes the process of fruit ripening?
- A) Cytokinins
- B) Ethylene
- C) Abscisic Acid
- D) Auxins
- Answer: B) Ethylene
Which hormone is primarily involved in the delay of leaf senescence?
- A) Cytokinins
- B) Gibberellins
- C) Ethylene
- D) Auxins
- Answer: A) Cytokinins

🌿 Conclusion and What’s Next?

In Part 3, we explored the major plant growth regulators (phytohormones) and their roles in promoting and inhibiting plant growth. We also discussed the stages of plant growth, from germination to reproductive development.

In Part 4, we will dive into the plant responses to environmental factors such as light, temperature, and gravity, and how these factors influence plant behavior.

Stay tuned for more details in the next blog!

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