Plant Hormones

Plant Hormones

Table of Contents

Introduction of Plant Hormones

Plants may not have brains, but they still have a way to control their growth and development. This magic is achieved through tiny chemical messengers called plant hormones, also known as phytohormones. These hormones, produced in miniscule amounts throughout the plant, act like signals, telling different parts of the plant what to do.

Types of plant hormone. growth promoters and growth inhibitors include auxins, gibberelins, cytokinins, ethylene, abscicic acid

Importance of Plant Hormones

Plant hormones are essential for a plant’s entire life cycle, influencing everything from seed germination to flowering and fruit production. They play a crucial role in:

  • Growth: Stem elongation, root development, and overall plant size are all regulated by plant hormones.
  • Development: Hormones determine how a plant forms its various organs, like leaves, flowers, and fruits.
  • Response to stimuli: Plant hormones help plants react to their environment, such as bending towards sunlight or closing their stomata (tiny pores on leaves) during drought.

Types of Plant Hormones


Plants rely on chemical messengers called plant hormones to regulate growth. One key player is auxin, influencing everything from how tall a plant grows to how its roots develop.

Mechanism of Phototropism in Plants

  • Cell Expansion: Auxin loosens cell walls, letting them expand for a taller plant.
  • Cell Division: Auxin can trigger cell division, creating new cells for overall growth.
  • Root Development: Applying synthetic auxin to cuttings helps them root faster.
  • Fruit Growth: Auxin aids in proper fruit development.
  • Tropisms: Auxin is involved in plant responses to light (phototropism) and gravity.

Phototropism light. Plant grows in the direction of sun, with auxin hormone. Maximize energy absorption for photosynthesis. Illustration vector

Auxin and the Main Shoot

Auxin is produced at the growing tip (apical meristem) and travels down the stem. This unequal distribution creates apical dominance, where the main stem grows more than side shoots. The higher auxin concentration at the top suppresses side bud growth, keeping the plant tall and straight.


Natural and Synthetic Auxins

Indole-3-acetic acid (IAA) is the most common natural auxin. Synthetic auxins mimic natural auxins and are used in gardening and agriculture for tasks like:

  • Encouraging root growth
  • Preventing fruit drop
  • Controlling weeds

Gibberellins and Cytokinins

Plants may not have complex nervous systems, but they orchestrate growth and development through amazing chemical messengers called plant hormones. Two key players are gibberellins and cytokinins.

Gibberellins: The Stretch Inducers

Gibberellins - Location, Structure, Functions and Uses

  • Definition: Plant hormones that promote stem elongation by stimulating cell growth and division.
  • Functions: Gibberellins also influence seed germination, leaf expansion, fruit development, and flowering.

Cytokinins: The Cell Division Crew

vector illustration of cytokinin Structure .

  • Definition: Another class of plant hormones that encourage cell division, vital for new tissues and organs.
  • Functions: Cytokinins also delay leaf aging and promote bud formation.

Gibberellins and Cytokinins in Action

  • Seed Germination: Gibberellins break seed dormancy, allowing germination. Cytokinins might also be involved in this process.
  • Stem Elongation: Gibberellins are the key players in making stems grow taller.
  • Flowering: The interplay between these hormones can influence flowering. In some plants, gibberellins promote flowering, while in others, they inhibit it. Cytokinins can also play a role.

Natural and Synthetic Plant Hormones

  • Gibberellins: Over 130 natural forms exist in plants. Synthetic gibberellins like GA3 are used in agriculture to boost fruit growth and crop yield.
  • Cytokinins: Zeatin is a natural cytokinin. Synthetic ones like kinetin are used in plant tissue culture to promote cell division and bud formation.

Abscisic Acid and Ethylene

Plants rely on chemical messengers called hormones to call the shots. Two key players are abscisic acid (ABA) and ethylene.

ABA (stress hormone)

Molecular formula of abscisic acid. Chemical formula of abscisic acid

Produced from carotenoids, ABA helps plants weather tough times like drought and cold. It also promotes seed dormancy and leaf shedding.

Ethylene (gaseous hormone)

3d chemical structure of ethylene formula molecule. Scientific vector illustration isolated on white background.

Made by all plant parts, ethylene influences seed germination, fruit ripening, and responses to injury.

Big Roles:

  • Seed Dormancy: ABA puts seeds on hold until conditions are right for growth, while ethylene can sometimes break this dormancy.
  • Stress Response: ABA is essential for drought tolerance by closing stomata (plant pores) to conserve water. Ethylene’s role in stress response varies.
  • Fruit Ripening: Ethylene takes center stage, triggering changes that make fruits sweeter, softer, and more attractive to seed-dispersers. ABA can slow this process.

Natural and Synthetic:

    • ABA occurs naturally in plants and can be synthetically produced to aid crops in dry conditions.
    • Ethylene is naturally produced throughout a plant’s life and synthetically used to ripen fruits after harvest.


In conclusion, plant hormones orchestrate the complex dance of growth and development in the botanical world. Their interactions and precise regulation ensure that plants can adapt to changing environmental conditions and thrive in diverse ecosystems. As our understanding of plant hormones deepens, so too does our ability to harness their power for sustainable agriculture and environmental stewardship.


The five main plant hormones are auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene.

The collective term for the chemical messengers that regulate various aspects of plant growth, development, and responses to environmental stimuli is “plant hormones” or “phytohormones.”

Plant hormones are chemical compounds produced naturally by plants that regulate growth, development, and responses to environmental stimuli. They control processes such as seed germination, root and shoot growth, flowering, fruit ripening, and responses to stress. Plant hormones are extensively used in agriculture for purposes such as stimulating rooting of cuttings, promoting uniform fruit set, and enhancing stress tolerance in crops.

An example of a plant hormone is auxin. Auxins are primarily responsible for promoting cell elongation, influencing the direction of growth, stimulating root development, and regulating the balance between shoot and root growth.

Hormones are chemical messengers produced by organisms that regulate various physiological processes, including growth, development, metabolism, and response to stimuli. In plants, hormones play a critical role in coordinating growth, development, and responses to environmental changes.


  1. Which plant hormone is primarily responsible for promoting cell elongation in shoots?

    • a) Gibberellins
    • b) Cytokinins
    • c) Auxins
    • d) Abscisic Acid

    Answer: c) Auxins

  2. Which hormone is known as the stress hormone in plants?

    • a) Gibberellins
    • b) Cytokinins
    • c) Abscisic Acid
    • d) Ethylene

    Answer: c) Abscisic Acid

  3. Which hormone promotes seed germination by breaking dormancy?

    • a) Cytokinins
    • b) Gibberellins
    • c) Auxins
    • d) Brassinosteroids

    Answer: b) Gibberellins

  4. Which hormone is involved in promoting lateral bud growth and delaying leaf senescence?

    • a) Auxins
    • b) Cytokinins
    • c) Ethylene
    • d) Brassinosteroids

    Answer: b) Cytokinins

  5. Which hormone regulates fruit ripening and leaf senescence?

    • a) Auxins
    • b) Gibberellins
    • c) Ethylene
    • d) Brassinosteroids

    Answer: c) Ethylene

  6. Which hormone is used in agriculture to stimulate rooting of cuttings?

    • a) Cytokinins
    • b) Gibberellins
    • c) Auxins
    • d) Abscisic Acid

    Answer: c) Auxins

  7. What is the function of gibberellins in seed germination?

    • a) Inhibiting seed germination
    • b) Promoting seed dormancy
    • c) Breaking seed dormancy
    • d) Inducing stomatal closure

    Answer: c) Breaking seed dormancy

  8. Which hormone is involved in promoting stem elongation in young seedlings?

    • a) Cytokinins
    • b) Auxins
    • c) Ethylene
    • d) Brassinosteroids

    Answer: b) Auxins

  9. What is the primary function of cytokinins in plants?

    • a) Promoting cell elongation
    • b) Stimulating lateral bud growth
    • c) Inhibiting leaf senescence
    • d) Regulating stomatal closure

    Answer: b) Stimulating lateral bud growth

  10. Which hormone is responsible for regulating the balance between shoot and root growth?

    • a) Auxins
    • b) Gibberellins
    • c) Cytokinins
    • d) Abscisic Acid

    Answer: a) Auxins

  11. What role does abscisic acid play in response to environmental stresses?

    • a) Promoting stomatal opening
    • b) Inhibiting seed germination
    • c) Stimulating lateral bud growth
    • d) Inducing fruit ripening

    Answer: b) Inhibiting seed germination

  12. Which hormone is involved in the shedding of leaves, flowers, and fruits?

    • a) Auxins
    • b) Ethylene
    • c) Cytokinins
    • d) Gibberellins

    Answer: b) Ethylene

  13. What is the primary function of brassinosteroids in plants?

    • a) Promoting leaf senescence
    • b) Inducing stomatal closure
    • c) Regulating vascular differentiation
    • d) Inhibiting seed germination

    Answer: c) Regulating vascular differentiation

  14. Which hormone influences the direction of growth, causing the plant to bend towards light?

    • a) Gibberellins
    • b) Cytokinins
    • c) Auxins
    • d) Ethylene

    Answer: c) Auxins

  15. What is the primary function of gibberellins in stem elongation?

    • a) Promoting leaf growth
    • b) Stimulating lateral bud growth
    • c) Inhibiting cell division
    • d) Promoting cell elongation

    Answer: d) Promoting cell elongation

  16. Which hormone is involved in promoting cell division and differentiation?

    • a) Ethylene
    • b) Auxins
    • c) Cytokinins
    • d) Abscisic Acid

    Answer: c) Cytokinins

  17. What is the primary role of ethylene in fruit ripening?

    • a) Inhibiting fruit ripening
    • b) Promoting fruit ripening
    • c) Delaying fruit ripening
    • d) Inducing fruit dormancy

    Answer: b) Promoting fruit ripening

  18. Which hormone is primarily responsible for regulating stomatal closure during drought stress?

    • a) Auxins
    • b) Cytokinins
    • c) Ethylene
    • d) Abscisic Acid

    Answer: d) Abscisic Acid

  19. What effect does cytokinin have on leaf senescence?

    • a) Stimulating leaf senescence
    • b) Inhibiting leaf senescence
    • c) No effect on leaf senescence
    • d) Delaying leaf senescence

    Answer: d) Delaying leaf senescence

  20. Which hormone is commonly used in agriculture to induce uniform fruit set?

    • a) Gibberellins
    • b) Auxins
    • c) Cytokinins
    • d) Ethylene

    Answer: a) Gibberellins

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