Plant Hormones (Introduction) MCQs Quiz | Class 10

This quiz is designed for Class X Science (Code 086) students, covering Unit II: World of Living. Specifically, it focuses on Plant Hormones (Introduction), including the role of plant hormones and basic examples. Test your understanding by attempting these MCQs, and after submission, you can download a detailed PDF answer sheet for revision.

Understanding Plant Hormones: A Detailed Look

Plant hormones, also known as phytohormones, are naturally occurring organic substances that influence physiological processes in plants. They are produced in very small quantities but have profound effects on growth, development, and responses to environmental stimuli. These chemical messengers coordinate a plant’s development from a tiny seed to a mature plant, influencing everything from cell division to fruit ripening.

Key Roles of Plant Hormones

Phytohormones regulate a wide array of plant processes, ensuring proper growth and adaptation. Their primary roles include:

  • Growth Regulation: Promoting or inhibiting cell division, enlargement, and differentiation.
  • Developmental Processes: Controlling seed germination, flowering, fruit development and ripening, leaf and fruit abscission (shedding), and senescence (aging).
  • Environmental Responses: Mediating responses to light (phototropism), gravity (gravitropism), water stress, pathogen attacks, and nutrient availability.

Major Plant Hormones and Their Basic Examples

1. Auxins

Auxins are primarily known for their role in cell elongation, particularly in shoots. They are produced mainly in the apical meristems (tip of stems) and young leaves.

  • Role:
    • Promote cell elongation in stems.
    • Initiate root growth and development.
    • Establish apical dominance (inhibition of lateral bud growth by the apical bud).
    • Involved in phototropism (growth towards light) and gravitropism (growth in response to gravity).
    • Delay abscission of leaves and fruits at early stages.
  • Example: Indole-3-acetic acid (IAA) is the most common natural auxin. Synthetic auxins like 2,4-D (2,4-Dichlorophenoxyacetic acid) are used as herbicides.

2. Gibberellins (GAs)

Gibberellins are a large group of hormones found in young leaves, seeds, and stem tips. They are crucial for stem elongation and breaking dormancy.

  • Role:
    • Promote stem and internode elongation, leading to increased plant height.
    • Break seed dormancy and promote seed germination.
    • Induce flowering in some plants.
    • Promote fruit development and increase fruit size.
  • Example: Gibberellic acid (GA3) is a widely studied gibberellin.

3. Cytokinins

Cytokinins are primarily involved in cell division and are abundant in regions of active cell division, such as root apices, developing shoot buds, and young fruits.

  • Role:
    • Promote cell division (cytokinesis).
    • Delay senescence (aging) in leaves.
    • Promote the growth of lateral buds, counteracting apical dominance.
    • Help in breaking dormancy of seeds and buds.
  • Example: Zeatin (natural) and Kinetin (synthetic) are common cytokinins.

4. Abscisic Acid (ABA)

Often referred to as the ‘stress hormone’, Abscisic Acid plays a crucial role in plant responses to stress and in promoting dormancy.

  • Role:
    • Induces dormancy in seeds and buds, helping them withstand unfavorable conditions.
    • Promotes the closure of stomata during water stress, reducing water loss.
    • Promotes senescence and abscission of leaves and fruits.
    • Acts as an antagonist to growth-promoting hormones like auxins and gibberellins.
  • Example: Abscisic acid (ABA) itself.

5. Ethylene

Ethylene is a simple gaseous hormone mainly involved in fruit ripening and senescence.

  • Role:
    • Promotes fruit ripening, especially in climacteric fruits (e.g., bananas, apples).
    • Accelerates senescence and abscission of leaves and flowers.
    • Involved in stress responses, such as flooding.
  • Example: Ethene (C2H4) is the only gaseous plant hormone.

Summary Table of Plant Hormones and Functions

Hormone Primary Function(s) Key Examples
Auxins Cell elongation, apical dominance, root initiation, phototropism IAA, 2,4-D (synthetic herbicide)
Gibberellins Stem elongation, seed germination, breaking dormancy GA3
Cytokinins Cell division, delaying senescence, lateral bud growth Zeatin, Kinetin
Abscisic Acid Induces dormancy, stomatal closure, stress response ABA
Ethylene Fruit ripening, senescence, abscission Ethene

Quick Revision Points

  • Plant hormones are chemical messengers regulating growth and development.
  • Auxins cause cell elongation and apical dominance.
  • Gibberellins promote stem growth and germination.
  • Cytokinins stimulate cell division and delay aging.
  • Abscisic Acid (ABA) induces dormancy and stress tolerance.
  • Ethylene is a gaseous hormone that ripens fruits.

Extra Practice Questions

  1. Which plant hormone is responsible for the phenomenon of phototropism, causing shoots to bend towards light?
  2. Name the plant hormone that helps in breaking seed dormancy and promoting rapid stem elongation.
  3. If a plant is treated with a chemical that promotes cell division and lateral bud growth, which class of plant hormones does this chemical likely mimic?
  4. Why is abscisic acid often called the ‘stress hormone’ in plants?
  5. What is the main physiological effect of ethylene on fruits?