Plant Growth and Regulators short notes NEET/Class 11

 

plant growth and regulators NEET short notes

Plants cannot move from place to place like humans, but they still grow, bend, flower, and ripen fruits at the right time.
This happens because of special chemicals called Plant Growth Regulators (PGRs) or plant hormones.

These hormones control:
✅ Growth
✅ Cell division
✅ Flowering
✅ Fruit ripening
✅ Dormancy
✅ Stress responses

There are five main plant hormones:

• Auxin

• Cytokinin

• Gibberellin

• Abscisic Acid (ABA)

• Ethylene

Let’s understand each one in very simple words.

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🌱 1. Auxin – The Growth Hormone

What is it?

Auxin is an indole derivative hormone mainly produced in shoot tips.

Precursor

Tryptophan

Bioassay

Avena curvature test

Types

Natural

• IAA (Indole Acetic Acid)

• IBA (Indole Butyric Acid)

Synthetic

• NAA

• 2,4-D

• 2,4,5-T

Functions

Auxin mainly helps in growth and elongation.

✔ Initiates rooting in stem cuttings
✔ Promotes flowering (example: pineapple)
✔ Prevents early leaf & fruit drop
✔ Helps in cell division
✔ Xylem differentiation
✔ Induces parthenocarpy (seedless tomato)
✔ Used in tea plantations & hedge making
✔ Used as herbicide

👉 2,4-D kills dicot weeds but does not affect monocots

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🌿 2. Cytokinin – The Cell Division Hormone

What is it?

Cytokinin is an adenine derivative hormone that promotes cell division.

Precursor

Adenine

Discovery

• Skoog → Tobacco callus

• Miller → Kinetin from DNA

• First natural cytokinin → Zeatin (from maize)

Functions

✔ Promotes cell division
✔ Increases number of leaves
✔ Increases chloroplasts & chlorophyll
✔ Delays leaf ageing (senescence)
✔ Mobilizes nutrients
✔ Breaks apical dominance (helps lateral buds grow)

👉 Makes plants look fresh and green for longer

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🌳 3. Gibberellin – The Height Increasing Hormone

What is it?

A terpene derivative hormone responsible for stem elongation.

Precursor

Acetyl CoA

Discovery

Kurosawa (rice seedlings)

Bioassay

Dwarf maize test

Functions

✔ Increases plant height
✔ Stem elongation
✔ Helps in seed germination
✔ Used in malting industry
✔ Increases grape stalk length
✔ Increases fruit retention
✔ Used in sugarcane to increase yield
✔ Causes bolting (beet, cabbage)

👉 Think of GA as “plant height booster”

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🌵 4. Abscisic Acid (ABA) – The Stress Hormone

What is it?

ABA is a growth-inhibiting hormone that works during stress.

Derived from

Carotenoids

Bioassay

Stomatal closure

Functions

✔ Inhibits seed germination
✔ Causes seed dormancy
✔ Induces leaf fall (abscission)
✔ Closes stomata
✔ Reduces transpiration
✔ Helps plant survive drought

👉 Antagonistic to Gibberellin

👉 Think of ABA as “plant brake hormone”

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🍎 5. Ethylene – The Ripening Hormone

What is it?

Only gaseous plant hormone

Bioassay

Triple response:

1. Vertical growth stops

2. Horizontal growth increases

3. Apical hook formation

Discovery

Cousins (1910)

Functions

✔ Fruit ripening (banana, mango, tomato, apple)
✔ Promotes flowering in pineapple
✔ Increases root hairs
✔ Promotes femaleness in flowers
✔ Helps maturation
✔ Used commercially for artificial ripening

👉 Both promoter and inhibitor, but mostly inhibitory

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📏 How is Plant Growth Measured?

Growth can be measured by:

• Fresh weight

• Dry weight

• Length

• Area

• Volume

• Cell number

At cellular level → increase in protoplasm

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🌿 Phases of Plant Growth

1. Meristematic Phase

• Rapid cell division

• Dense cytoplasm

• Large nucleus

2. Elongation Phase

• Cell enlargement

• Vacuole formation

• Cell wall deposition

3. Maturation Phase

• Cell differentiation

• Wall thickening

• Specialized functions

• Plant Growth: Types, Rate, Conditions & Development

  Plants grow throughout their life.
  Unlike humans, their growth never completely stops.

  But have you ever wondered:

  👉 How do we measure growth?
  👉 Why do some plants grow faster than others?
  👉 What controls plant growth?

  Let’s understand everything in very simple words.

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  📈 What is Growth Rate?

  Growth rate = Increase in growth per unit time

  It tells us how fast a plant is growing.

  Example:
  If a plant grows 5 cm in 5 days, its growth rate is 1 cm/day.

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  🌿 Types of Growth

  There are two main types of growth in plants:

  1️⃣ Arithmetic Growth (Linear Growth)

  What happens?

  • Growth is uniform and constant

  • Cells divide slowly

  • One daughter cell becomes mature

  • Other daughter cell continues dividing

  So increase in size happens at the same rate

  Graph

  Straight line graph

  Growth ↑
  Time →

  Formula

  Growth = Final − Initial

  Example

  Root or shoot elongation at constant speed

  👉 Easy memory tip:
  Arithmetic = Straight line growth

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  2️⃣ Geometric Growth (Exponential Growth)

  What happens?

  • Growth is very fast

  • All cells keep dividing

  • Growth increases exponentially

  This is the natural growth pattern of living organisms

  Graph

  S-shaped curve (Sigmoid curve)

  It has 3 phases:

  🌱 Phases of Geometric Growth

  1. Lag Phase

  • Slow growth

  • Cells preparing for division

  2. Log (Exponential) Phase

  • Very fast growth

  • Maximum cell division

  3. Stationary Phase

  • Growth slows/stops

  • Due to:

    • Nutrient deficiency

    • Accumulation of toxic substances

  👉 Easy memory tip:
  Lag → Log → Stationary

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  📊 Types of Growth Rate Measurements

  ✅ Absolute Growth Rate (AGR)

  Formula

  Growth = Final − Initial

  Meaning

  Total increase in size

  Example:
  10 cm → 15 cm
  AGR = 5 cm

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  ✅ Relative Growth Rate (RGR)

  Meaning

  Growth per unit initial size

  Shows efficiency of growth

  Important note:
  👉 Even if absolute growth is same, relative growth can differ.

  Example:

  • Plant A: 10 → 20 (10 increase)

  • Plant B: 100 → 110 (10 increase)

  Both same AGR
  But Plant A grows faster relatively

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  🌿 Conditions Required for Growth

  Growth needs both external and internal factors.

  ---

  🌍 External Factors

  ✔ Water
  ✔ Oxygen (O₂)
  ✔ Nutrients
  ✔ Temperature

  Without these → growth slows or stops

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  🔬 Internal Factors

  ✔ DNA (genetic control)
  ✔ Plant Growth Regulators (PGRs/hormones)

  These decide:

  • When to grow

  • How much to grow

  • When to flower or rest

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  🌱 What is Development?

  Development includes:

  👉 All changes from seed germination to death (senescence)

  So development =
  Growth + differentiation + maturation + ageing

  It is the complete life cycle of a plant.

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  🌿 What is Plasticity?

  Plants show plasticity, meaning:

  👉 They can change their structure according to environment

  This helps them survive better.

  Causes of plasticity:

  ✔ Phase of life
  ✔ Environmental conditions

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  🌱 Heterophylly (Different Types of Leaves)

  Because of plasticity, plants sometimes form different types of leaves on the same plant.

  This is called Heterophylly.

  Examples:

  • Cotton

  • Coriander

  • Larkspur

  • Buttercup (due to water conditions)

  👉 Leaf shape changes depending on:

  • Topic	Key Idea
    Arithmetic growth	Linear, constant
    Geometric growth	Exponential, sigmoid curve
    AGR	Final − Initial
    RGR	Growth per initial size
    External factors	Water, O₂, nutrients, temperature
    Internal factors	DNA, hormones
    Development	Germination → death
    Plasticity	Structure changes with environment

  
  

  Hormone	Main Role	Easy Memory Trick
  Auxin	Rooting & elongation	Growth
  Cytokinin	Cell division	Youth hormone
  Gibberellin	Height increase	Tall plants
  ABA	Dormancy & stress	Brake
  Ethylene	Ripening	Fruit gas