Plant Growth and Development 🌱 | Class 11 Notes | NEET | NCERT

Introduction

• Plant growth = Irreversible increase in size (cell division + enlargement + differentiation).
• Development = Sum of growth and differentiation → leads to specific structures/functions.
• Growth is indeterminate in plants (meristems active throughout life).
• Controlled by both intrinsic factors (genes, hormones) and extrinsic factors (light, temperature, water, nutrients, gravity).

Plant Growth Regulators (PGRs)

• Plant hormones are small chemical messengers that regulate growth and development.
• Five major classes: Auxins, Gibberellins, Cytokinins, Ethylene, Abscisic acid (ABA).
• Some promote growth (auxins, gibberellins, cytokinins, ethylene in certain roles).
• Some act as growth inhibitors (ABA, ethylene in senescence).

1. Auxins

• Discovered by F. W. Went (from coleoptile tips of oat seedlings).
• Natural auxin: Indole-3-acetic acid (IAA).
• Synthetic auxins: NAA (naphthalene acetic acid), 2,4-D (2,4-dichlorophenoxyacetic acid).

Functions:

• Promote cell elongation (by loosening cell wall).
• Apical dominance (inhibits lateral buds).
• Root initiation in cuttings (used in vegetative propagation).
• Prevent premature fruit and leaf drop.
• Used as herbicide (2,4-D kills dicot weeds).
• Induce parthenocarpy (seedless fruits).

2. Gibberellins (GA)

• Discovered from fungus Gibberella fujikuroi (rice "foolish seedling disease").
• Over 100 types known (GA₁, GA₂, GA₃… GA₃ most common).

Functions:

• Promote stem elongation (overcome dwarfism).
• Break seed dormancy.
• Induce bolting (elongation of stem before flowering in rosette plants like cabbage, beet).
• Promote flowering in long-day plants.
• Induce parthenocarpy (seedless grapes, apples).
• Mobilize food reserves in germinating seeds (stimulate amylase activity).

3. Cytokinins

• Discovered in tobacco tissue culture (kinetin).
• Naturally occurring: Zeatin (from corn kernels).

Functions:

• Promote cell division (cytokinesis).
• Delay senescence (anti-aging hormone).
• Promote nutrient mobilization.
• Break apical dominance (promote lateral bud growth).
• Influence organ differentiation in tissue culture:
• High auxin : cytokinin → roots.
• Low auxin : cytokinin → shoots.

4. Ethylene

• Gaseous hormone.
• Produced in ripening fruits and senescent tissues.

Functions:

• Promotes fruit ripening (bananas, tomatoes, mangoes).
• Triple response in seedlings: inhibition of elongation, swelling of stem, horizontal growth.
• Promotes senescence and abscission of leaves and flowers.
• Breaks seed and bud dormancy.
• Enhances respiratory climacteric (rise in respiration during ripening).
• Commercial use: Ethephon (releases ethylene) → used for ripening fruits, inducing flowering in pineapple, synchronizing flowering in mango.

5. Abscisic Acid (ABA)

• Also called stress hormone.
• Functions opposite to gibberellins.

Functions:

• Induces dormancy in seeds and buds.
• Promotes abscission (falling of leaves, fruits).
• Inhibits seed germination.
• Promotes stomatal closure during water stress (reduces transpiration).
• Helps plants tolerate stress (drought, salinity, cold).

Growth Promoters vs Inhibitors

Growth Promoters	Growth Inhibitors
Auxins, Gibberellins, Cytokinins (Ethylene in some roles)	Abscisic Acid, Ethylene (in senescence)

Interaction of Hormones

Plant development is regulated by balance between PGRs.

Example:

• Apical dominance → Auxin promotes apical bud, cytokinin promotes lateral buds.
• Seed dormancy vs germination → ABA induces dormancy, GA breaks dormancy.
• Abscission → Auxin prevents, Ethylene + ABA promote.

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