Plant Growth & Hormones: Types, Functions & Applications

What Are Plant Hormones?

Plant hormones (or phytohormones) are chemical messengers that control growth, development, and responses to the environment. Unlike animals, plants do not have a nervous system, so hormones coordinate all processes.

Key Characteristics of Plant Hormones:

• Produced in small amounts but have big effects.
• Can move from one part of the plant to another (e.g., roots to shoots).
• Work together (synergistically) or against each other (antagonistically).
• Affect cell division, elongation, differentiation, and senescence (aging).

Types of Plant Hormones

There are five major plant hormones you must know:

1. Auxins (IAA – Indole-3-Acetic Acid)

Function: Promotes cell elongation, root initiation, and apical dominance (prevents side branches from growing). Sources: Young leaves, shoot tips, seeds. Examples:

• Used in rooting powders for cuttings.
• Weed killers (e.g., 2,4-D) mimic auxins and kill broad-leaved weeds.

graph LR
    A["Auxin (IAA)"] --> B["Stimulates cell elongation"]
    A --> C["Promotes root growth"]
    A --> D["Inhibits lateral buds (apical dominance)"]
    A --> E["Used in tissue culture"]

2. Gibberellins (GA)

Function: Stimulates stem elongation, seed germination, and fruit growth. Sources: Young leaves, roots, seeds. Examples:

• Used to increase fruit size (e.g., grapes, apples).
• Helps dwarf plants grow taller.

graph LR
    A["Gibberellins (GA)"] --> B["Stimulates stem growth"]
    A --> C["Breaks seed dormancy"]
    A --> D["Promotes flowering"]
    A --> E["Used in agriculture for larger fruits"]

Chemical structure of GA₃ (Gibberellic Acid) (Image: Ben Mills, Public domain, via Wikimedia Commons)

3. Cytokinins (CK)

Function: Promotes cell division, delay senescence (aging), and lateral bud growth. Sources: Root tips, developing seeds. Examples:

• Used in tissue culture to grow plantlets.
• Keeps cut flowers fresh longer.

graph LR
    A["Cytokinins (CK)"] --> B["Stimulates cell division"]
    A --> C["Delays leaf aging"]
    A --> D["Promotes shoot formation"]
    A --> E["Used in micropropagation"]

4. Abscisic Acid (ABA)

Function: Controls stress responses (drought, cold), seed dormancy, and stomatal closure. Sources: Leaves, roots, mature fruits. Examples:

• Helps plants survive drought by closing stomata.
• Prevents premature germination in seeds.

graph LR
    A["Abscisic Acid (ABA)"] --> B["Closes stomata (water conservation)"]
    A --> C["Induces seed dormancy"]
    A --> D["Helps plants survive stress"]
    A --> E["Used in drought-resistant crops"]

Chemical structure of ABA (Image: Michael D. Turnbull, CC0, via Wikimedia Commons)

5. Ethylene (C₂H₄)

Function: Controls fruit ripening, leaf abscission (falling), and senescence (aging). Sources: Ripening fruits, aging leaves. Examples:

• Used to ripen bananas artificially.
• Helps detach leaves/fruits at the right time.

graph LR
    A["Ethylene (C₂H₄)"] --> B["Speeds up fruit ripening"]
    A --> C["Causes leaf fall"]
    A --> D["Used in post-harvest fruit storage"]
    A --> E["Helps in flower senescence"]

How Plant Hormones Work Together

Hormones do not work alone—they interact to regulate growth.

Applications of Plant Hormones

1. Agriculture:

   • Weed killers (2,4-D mimics auxin).
   • Rooting powders (auxin for cuttings).
   • Dwarf plant breeding (reducing gibberellin).

2. Food Industry:

   • Artificial ripening (ethylene for bananas, tomatoes).
   • Seedless fruits (auxin + gibberellin in seedless watermelons).

3. Biotechnology:

   • Tissue culture (auxin + cytokinin for plant cloning).
   • Stress-resistant crops (ABA for drought tolerance).

4. Horticulture:

   • Flowering control (gibberellin for pineapples).
   • Delaying senescence (cytokinin for cut flowers).

Solved Example: Apical Dominance

Question: Why does a plant grow taller when its shoot tip is intact, but grows bushier when the tip is cut? Answer:

• The shoot tip produces auxin, which inhibits lateral buds (apical dominance).
• If the tip is removed, auxin levels drop → lateral buds grow → bushier plant.

How auxin controls shoot growth (Image: Sten Porse, CC BY-SA 3.0, via Wikimedia Commons)

NEB Board-Style Questions

Short Answer (5 marks each)

1. Define auxin and explain its role in phototropism.

   • Auxin is a plant hormone that promotes cell elongation.
   • In phototropism, auxin moves to the shaded side → cells elongate → plant bends toward light.

2. How does gibberellin help in seed germination?

   • Gibberellin breaks seed dormancy by stimulating enzyme production (e.g., amylase) that converts starch to sugar for growth.

3. What is the role of ethylene in fruit ripening?

   • Ethylene triggers ripening by activating enzymes (e.g., cellulase, pectinase) that soften fruit and change color.

Long Answer (10 marks)

4. Describe the interaction between auxin and cytokinin in tissue culture.

   • Auxin promotes root growth; cytokinin promotes shoot growth.
   • High auxin:low cytokinin → More roots.
   • High cytokinin:low auxin → More shoots.
   • Balanced ratio → Whole plantlets.

5. Explain how abscisic acid helps plants survive drought.

   • ABA is produced in roots under drought → signals stomata to close → reduces water loss.
   • Also induces seed dormancy to wait for better conditions.

Exam Tip

✅ Memorize:

• Functions of each hormone (e.g., auxin = elongation, ABA = stress).
• Sources (e.g., ethylene from fruits, cytokinin from roots).
• Applications (e.g., 2,4-D = weed killer, GA = dwarf plant treatment).

✅ Compare & Contrast:

• Auxin vs. Gibberellin (both promote growth but in different ways).
• Cytokinin vs. ABA (one delays aging, the other induces dormancy).

✅ Diagrams:

• Draw apical dominance, stomatal closure (ABA), and fruit ripening (ethylene).
• Label hormone sources (e.g., shoot tip for auxin, root for cytokinin).

✅ Real-World Links:

• Farmers use ethylene to ripen bananas.
• Gardeners use auxin rooting powder for cuttings.
• Scientists modify ABA levels for drought-resistant crops.

Final Note: Plant hormones are like a plant’s command center—they control everything from roots to flowers! Master their functions, interactions, and applications for full marks in NEB exams. 🌱📚