Plant Hormones is part of Biology. Below are notes on 8 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Abscisic Acid (ABA): Biosynthesis, Action, and Physiological Effects

  • Abscisic Acid (ABA) is primarily a plant growth inhibitor.
  • It acts as a stress hormone, increasing plant tolerance to various stresses.
  • ABA mediates stomatal closure, particularly in response to water stress, to reduce transpiration.
  • It promotes dormancy in seeds and buds, inhibiting seed germination and bud development.
  • ABA plays a role in promoting senescence (aging) and abscission (shedding) of leaves and fruits.

Auxins:

Biosynthesis,

Transport,

and

Physiological Effects

  • Definition: Small, simple molecules of diverse chemical composition that regulate plant growth and development. Auxins are a type of Plant Growth Regulator (PGR).
  • Chemical Nature/Biosynthesis:
    • Belong to indole compounds.
    • Main natural auxin: Indole-3-acetic acid (IAA).
    • Precursor for IAA biosynthesis: Amino acid Tryptophan.
    • Synthetic auxins: NAA (naphthalene acetic acid), IBA (indole butyric acid), 2,4-D (2,4-dichlorophenoxyacetic acid).
  • Transport:
    • Polar transport: Unidirectional, primarily from the shoot apex downwards.
  • Physiological Effects:
    • Cell Elongation: Primarily responsible for cell elongation in stems and coleoptiles.
    • Phototropism: Auxin accumulates on the darker side of a shoot, causing cells there to elongate more rapidly, leading to bending towards light.
    • Apical Dominance: High concentrations of auxin produced by the apical bud inhibit the growth of lateral (axillary) buds. Removal of the apical bud promotes the growth of lateral buds.
    • Root Initiation: Promotes root initiation in stem cuttings.
    • Parthenocarpy: Induces fruit development without fertilization, leading to seedless fruits (e.g., seedless tomatoes) when sprayed on flowers.
    • Root Nodule Formation: Involved in the formation of root nodules (e.g., in leguminous plants).
    • Xylem Differentiation: Promotes xylem differentiation.
    • Abscission: Delays abscission (falling) of young leaves and fruits.
    • Herbicides: Synthetic auxins like 2,4D are widely used as selective herbicides to kill dicot weeds, while not affecting monocot crops.

Commercial Applications of Plant Hormones in Agriculture and Horticulture

  • TIVA: A growth hormone developed to reduce flower dropping in pulses, enhancing production.
  • Potassium permanganate (KMnO₄): Used as an 'Ethylene scrubber' in fruit storage to delay ripening by absorbing ethylene.
  • Ethrel: A chemical used to hasten fruit ripening, as it releases ethylene.
  • Pheromones: Are ectohormones, external chemical signals affecting the behavior of other individuals of the same species.

Cytokinins: Biosynthesis, Transport, and Physiological Effects

  • Cytokinins are a class of plant hormones.
  • Their primary function is to stimulate cell division (cytokinesis) in plants.
  • They also play a crucial role in delaying senescence (aging) in plant organs.
  • The effect of cytokinins in delaying senescence is specifically known as the RichmondLang effect.
  • Cytokinins are distinct from animal hormones (e.g., Insulin, Thyroxin, Oestrogen, Melatonin, Gastrin, Cholecystokinin) and mineral elements (e.g., Calcium).

Ethylene: Biosynthesis, Perception, and Physiological Effects

  • Ethylene is a simple gaseous plant hormone.
  • Biosynthesis: Synthesized from Methionine. Key intermediate is 1-aminocyclopropane-1carboxylic acid (ACC).
  • Key Enzymes in Biosynthesis: ACC synthase (rate-limiting step) and ACC oxidase.
  • Perception: Perceived by specific receptor proteins, initiating a signal transduction pathway.
  • Physiological Effects:
    • Promotes fruit ripening, especially in climacteric fruits (e.g., apples, bananas, tomatoes).
    • Initiates and accelerates senescence (aging) of leaves and flowers.
    • Promotes abscission (shedding) of leaves, flowers, and fruits.
    • Induces the "triple response" in etiolated seedlings: inhibited hypocotyl elongation, radial swelling of hypocotyl, and exaggerated apical hook.
    • Enhances root hair formation.
    • Promotes adventitious root formation.
    • Involved in responses to various stresses (e.g., wounding, flooding).
  • Synthetic Analogues/Releasing Compounds: Ethephon (Ethaphon) is a commercially used compound that releases ethylene and is applied to accelerate fruit ripening.

Gibberellins: Biosynthesis, Metabolism, and Physiological Effects

  • Gibberellins (GAs) are responsible for breaking seed dormancy and promoting germination.

Hormonal Regulation of Plant Growth and Development (e.g., flowering, senescence, dormancy)

  • Plant Hormones (Phytohormones): Chemical messengers that regulate plant growth and development.
  • Auxins:
    • Promote cell enlargement, apical dominance.
    • Initiate rooting in stem cuttings.
    • Prevent fruit/leaf drop at early stages.
    • Promote flowering in pineapples.
    • Used as herbicides (e.g., 2,4-D).
  • Gibberellins (GAs):
    • Promote stem elongation (e.g., bolting in rosette plants).
    • Delay senescence.
    • Break seed dormancy.
    • Promote malting process in brewing.
    • Hastens maturity in juvenile conifers.
  • Cytokinins:
    • Promote cell division.
    • Delay leaf senescence (RichmondLang effect).
    • Help break apical dominance.
    • Promote chloroplast development.
  • Abscisic Acid (ABA):
    • "Stress hormone": increases tolerance to environmental stresses.
    • Promotes seed and bud dormancy.
    • Induces stomatal closure.
    • Promotes senescence and abscission of leaves/fruits.
    • Antagonistic to Gibberellins.
  • Ethylene:
    • Gaseous hormone.
    • Promotes fruit ripening (especially climacteric fruits).
    • Promotes senescence and abscission.
    • Breaks seed/bud dormancy.
    • Promotes root growth and root hair formation.
  • Hormonal Regulation of Plant Processes:
    • Flowering:
      • Photoperiodism: Plant's response to relative lengths of day and night.
      • Short-day plants (SDP): Flower when day length < critical photoperiod.
      • Long-day plants (LDP): Flower when day length > critical photoperiod.
      • Day-neutral plants (DNP): Flowering independent of photoperiod.
      • Vernalisation: Requirement of a cold period for flowering in some plants.
      • Gibberellins can induce flowering in some LDPs under non-inductive conditions.
    • Senescence:
      • Promoted by Ethylene, ABA.
      • Delayed by Auxins, Cytokinins, Gibberellins.
    • Dormancy (Seed/Bud):
      • Induced by ABA.
      • Broken by Gibberellins, Ethylene, chilling, light.
  • Phytochrome: A biological proteinaceous pigment that perceives red and far-red light. Crucial for photoperiodism, seed germination, and de-etiolation. It is NOT a photosynthetic pigment.

Plant Hormones in Stress Response and Adaptation

  • Abscisic Acid (ABA) is widely known as the primary plant stress hormone.
  • ABA mediates plant responses to various abiotic stresses including drought, salinity, cold, and heat.
  • During drought, ABA triggers stomatal closure to reduce transpirational water loss.
  • It also plays a role in inducing seed dormancy and bud dormancy, which are crucial for adaptation to unfavorable environmental conditions.

21. Plant Reproduction

Practice MCQs: Plant Hormones

Abscisic Acid (ABA): Biosynthesis, Action, and Physiological Effects

  1. Which one among the following hormones inhibits the growth activity in plants ?

    Asked in: UPSC CDS 2 2024

    1. Auxins
    2. Cytokinins
    3. Abscisic acid
    4. Gibberellins
    Show answer

    Answer: C. Abscisic acid

    Plant hormones regulate various aspects of plant growth, development, and responses to environmental stimuli.

  2. Stomatal closure in response to stress is mediated by

    Asked in: Agri. Officer 2020

    1. IAA
    2. ABA
    3. IBA
    4. 2,4-D
    Show answer

    Answer: B. ABA

    Abscisic acid (ABA) is the primary plant hormone responsible for mediating stomatal closure during stress.

  3. Which hormone act as plant growth inhibitors ?

    Asked in: School Lecturer (Sanskrit Edu.) 2020

    1. Abscisic acid
    2. Cytokinins
    3. Gibberellins
    4. Auxins
    Show answer

    Answer: A. Abscisic acid

    This question assesses knowledge of plant physiology, specifically the functions of different plant hormones.

  4. Which of the following hormone inhibits growth in plants?

    Asked in: School Lecturer (Sanskrit Edu.) 2022 3 2022

    1. Abscisic acid
    2. Gibberellic acid
    3. Auxin
    4. Ethylene
    Show answer

    Answer: A. Abscisic acid

    Abscisic acid (ABA) is a plant hormone that generally inhibits growth and promotes dormancy. Gibberellic acid, auxin, and ethylene, while having complex roles, can also stimulate or be involved in growth processes.

  5. Which of the following is a growth inhibitor hormone in plants? - Question not attempted

    Asked in: School Lecturer (Sanskrit Edu.) 2024

    1. Gibberellin
    2. Cytokinins
    3. Auxin
    4. Abscisic Acid
    Show answer

    Answer: D. Abscisic Acid

    Plant hormones are broadly classified into growth promoters (like auxins, gibberellins, and cytokinins) and growth inhibitors (primarily abscisic acid and ethylene, though ethylene's role is complex and can be both promoting and inhibiting).

Auxins: Biosynthesis, Transport, and Physiological Effects

  1. The plant growth regulators are small, simple molecules of diverse chemical composition. They are

    Asked in: UPSC CDS 1 2017

    1. carbohydrates, fats and proteins
    2. indole compounds, adenine derivatives, carotenoids and terpenes
    3. fatty acids, glucose and vitamins
    4. vitamin C, vitamin D and glucose
    Show answer

    Answer: B. indole compounds, adenine derivatives, carotenoids and terpenes

    Plant growth regulators (PGRs), also known as phytohormones, are organic compounds other than nutrients which in small amounts promote, inhibit or otherwise modify any physiological process in plants. The main classes of PGRs include Auxins (indole compounds, e.g., IAA), Cytokinins (adenine derivatives, e.g., kinetin), Gibberellins (terpenes, e.g., GA3), Abscisic Acid (carotenoid derivatives/ terpenes, e.g., ABA), and Ethylene (a gaseous hormone). These chemical classifications align with the options provided.

  2. The growing plant bends towards the source of light only on one side. Identify the best explanation for this phenomenon.

    Asked in: KPSC KAS Prelims 2017

    1. Light stimulates the cells on the illuminated side to increase in length
    2. Shoot apices are attracted by light
    3. They need light for photosynthesis
    4. Auxin accumulates towards darker side to induce greater cell elongation
    Show answer

    Answer: D. Auxin accumulates towards darker side to induce greater cell elongation

    Phototropism is the growth of a plant towards or away from a light stimulus. Positive phototropism (growth towards light) in shoots is mediated by the hormone auxin, which promotes differential cell elongation.

  3. Hormone secreted in root tip during the formation of nodule is

    Asked in: TNPSC CS Prelims 2017

    1. Auxin
    2. Cytokinin
    3. Zymase
    4. Lipase
    Show answer

    Answer: A. Auxin

    Both auxins and cytokinins play crucial roles in legume nodule formation. Auxins, often synthesized in root tips, are involved in initiating cell division and promoting the formation of root hair structures and infection threads, which are early steps in nodule development triggered by symbiotic bacteria (Rhizobia).

  4. It is possible to produce seedless tomato fruits by

    Asked in: UPPCS Prelims 1996

    1. applying trace elements in tomato fields
    2. spraying mineral solution on plants
    3. spraying hormones on flowers
    4. applying fertilizers containing radioactive ele‐ ments
    Show answer

    Answer: C. spraying hormones on flowers

    - The development of fruits typically requires fertilization (pollination leading to seed formation). However, some fruits can develop without fertilization, a process called parthenocarpy. - Parthenocarpy can be naturally occurring or artificially induced by applying plant hormones like auxins or gibberellins to the flowers. These hormones stimulate fruit set and growth in the absence of pollination, resulting in seedless fruits. - Trace elements, mineral solutions, or radioactive elements in fertilizers are not direct methods for inducing seedless fruit formation.

  5. Which of the following statement is true regarding apical dominance in plants ?

    Asked in: ASO 2025

    1. Root and shoot in their apex are dormant
    2. Removal of apical buds promote lateral bud growth
    3. Decrease in cytokinin increases apical domin‐ ance
    4. Auxin stimulates lateral bud formation
    Show answer

    Answer: B. Removal of apical buds promote lateral bud growth

    Apical dominance is the suppression of growth of lateral buds by the apical bud. This is primarily mediated by auxin produced in the apical bud. Removing the apical bud eliminates the source of auxin, thereby releasing the lateral buds from inhibition and promoting their growth.

  6. Plant hormone which is primarily responsible for cell elongation?

    Asked in: lower devision clerk 2025

    1. Auxin
    2. Cytokinin
    3. Gibberellin
    4. Abscisic acid
    Show answer

    Answer: A. Auxin

    Auxins are the primary plant hormones responsible for stimulating cell elongation, a fundamental process in plant growth and development.

Commercial Applications of Plant Hormones in Agriculture and Horticulture

  1. Scientists of the Chandra Shekhar Azad University of Agriculture and Technology have developed a substance that can reduce the percentage of flower droppings in pulses so as to enhance pulse production. Which is that substance ?

    Asked in: UPPSC Upper Mains 2007

    1. An insecticide called TIVA
    2. A fertilizer called TIVA
    3. A nutrient mixture called TIVA
    4. A growth hormone called TIVA
    Show answer

    Answer: C. A nutrient mixture called TIVA

    Flower dropping in pulses is a significant factor limiting yield. Providing specific nutrients at critical growth stages can help the plant overcome physiological stress and retain a higher percentage of flowers, leading to more pods and increased production. TIVA is formulated as a nutrient supplement for this purpose.

  2. Which of the following is used as ‘Ethylene scrubber’ in storage of fruits ?

    Asked in: Food Safety Officer 2019

    1. Potassium permanganate
    2. Calcium chloride
    3. Sodium chloride
    4. Citric acid
    Show answer

    Answer: A. Potassium permanganate

    Potassium permanganate is used as an ethylene scrubber to slow down the ripening and spoilage of stored fruits.

  3. Which of the following chemicals is used to hasten ripening of fruits ?

    Asked in: RO ARO pre 2023

    1. Malic hydrazide
    2. Ethrel
    3. Cycocel
    4. Indole-3-Butyric acid
    Show answer

    Answer: B. Ethrel

    Ethrel (Ethephon) is used to hasten the ripening of fruits by releasing ethylene.

  4. Pheromones are example of

    Asked in: Lower Subordinate Service Examination 2021

    1. Growth hormones
    2. Endohormones
    3. Ectohormones
    4. None of the above
    Show answer

    Answer: C. Ectohormones

    Pheromones are external chemical signals that mediate social and reproductive behaviors between individuals of the same species. They are secreted externally and affect the behavior of other individuals, thus classifying them as ectohormones.

Cytokinins: Biosynthesis, Transport, and Physiological Effects

  1. A student was doing an experiment on increasing the cell division among plants. She asked her supervisor to suggest the specific plant hormone for the same. Had you been her supervisor, which plant hormone would you suggest ?

    Asked in: UPSC CDS 1 2021

    1. Abscisic acid
    2. Gibberellins
    3. Cytokinin
    4. Auxin
    Show answer

    Answer: C. Cytokinin

    Cytokinins are a class of plant hormones that promote cell division (cytokinesis) in plant roots and shoots. They are involved in cell growth, differentiation, and delay of senescence.

  2. Which of the following is a plant hormone?

    Asked in: CGPSC Prelims Exam 2014 · OPSC CS Prelims 2015

    1. Insulin
    2. Thyroxin
    3. Oestrogen
    4. Cytokinin
    Show answer

    Answer: D. Cytokinin

    Plant hormones, also known as phytohormones, are naturally occurring chemical messengers that regulate plant growth and development. Examples include auxins, cytokinins, gibberellins, abscisic acid, and ethylene. Insulin, Thyroxin, and Oestrogen are all hormones found in animals and regulate various physiological processes in animal bodies. Cytokinin is a recognized class of plant hormones primarily involved in cell division and differentiation.

  3. Which of the following is not a plant hormone ?

    Asked in: HPPSC HPAS Prelims 2010 · UPPSC Upper Mains 2011

    1. Cytokinin
    2. Auxin
    3. Calcium
    4. Gibberellin
    Show answer

    Answer: C. Calcium

    Cytokinin, Auxin, and Gibberellin are major classes of plant hormones (also called phytohormones). They are organic substances produced by plants that regulate various aspects of growth and development.

  4. Which one of the following is a plant hormone ?

    Asked in: UPSC CISF AC EXE 2021 · RO ARO Mains 2021

    1. Oestrogen
    2. Cytokinin
    3. Insulin
    4. Thyroxin
    Show answer

    Answer: B. Cytokinin

    Cytokinins are a group of plant hormones that promote cell division (cytokinesis) in plant roots and shoots. They are involved in various processes such as cell growth, differentiation, apical dominance, and delaying senescence (aging) of leaves.

  5. Which of the following is a plant hormone?

    Asked in: WBCS Main Paper 4 2021 · Combined (Group C) Exam 2023

    1. Insulin
    2. Thyroxin
    3. Oestrogen
    4. Cytokinin
    Show answer

    Answer: D. Cytokinin

    Plant hormones are chemical messengers that regulate various aspects of plant growth, development, and physiological processes. Major classes of plant hormones include auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Cytokinins are involved in cell division, differentiation, and delaying senescence.

  6. Effects of kinetin on delaying senescence is called as

    Asked in: Agri. Officer 2020

    1. War bug effects
    2. Mothers effects
    3. Richmond-Lang effect
    4. Osborne effect
    Show answer

    Answer: C. Richmond-Lang effect

    The delay of senescence by kinetin in plants is known as the Richmond-Lang effect.

  7. The class of hormones that stimulates cell division in plants is :-

    Asked in: Assistant Engineer (AE) Exam 2013

    1. Cytokinins
    2. Abscisic Acid
    3. Hydroxyindole acetic acid
    4. Ethylene
    Show answer

    Answer: A. Cytokinins

    Cytokinins are plant hormones responsible for stimulating cell division.

  8. Which of the following is a plant hormone?

    Asked in: mineral dev officier 2025 · Exam 2023

    1. Insulin
    2. Thyroxin
    3. Cytokinin
    4. Oestrogen
    Show answer

    Answer: C. Cytokinin

    Executive Officer and Tax & Revenue Inspector Plant hormones, also known as phytohormones, are chemical signals that regulate growth and development in plants. Cytokinins are a major class of plant hormones.

  9. Which of the following is a plant hormone ?

    Asked in: Lower Subordinate Services Examination 2024

    1. Insulin
    2. Thyroxin
    3. Oestrogen
    4. Cytokinin
    Show answer

    Answer: D. Cytokinin

    Plant hormones (phytohormones) are signaling molecules produced within plants that occur in extremely low amounts. They regulate growth and development. Cytokinins are a class of plant growth substances that promote cell division (cytokinesis).

Ethylene: Biosynthesis, Perception, and Physiological Effects

  1. Which hormone is found in gaseous form?

    Asked in: CGPSC Prelims Exam 2013

    1. Abscisic acid
    2. Ethylene
    3. Gibberellins
    4. Auxins
    Show answer

    Answer: B. Ethylene

    Ethylene is primarily known for its role in fruit ripening, leaf abscission, and senescence.

  2. Which of the following plant hormone is responsible for senescence ?

    Asked in: MPSC CS Prelims 2022

    1. Auxin
    2. Cytokinin
    3. Ethylene
    4. Abscisic Acid
    Show answer

    Answer: C. Ethylene

    Plant hormones regulate various aspects of plant growth and development. Ethylene is primarily known for its effects related to ripening, senescence, and responses to stress.

  3. Which one of the following chemicals helps in fruit ripening?

    Asked in: UPPCS Prelims 2005

    1. Ethaphon
    2. Atrazine
    3. Isoproturan
    4. Malathion
    Show answer

    Answer: A. Ethaphon

    Ethylene is the primary gaseous plant hormone responsible for fruit ripening.

  4. Which of the following phyto hormone is mainly responsible for fruit ripening ?

    Asked in: UPPSC Upper Mains 2009

    1. Cytokinin
    2. Abscissic Acid
    3. Ethylene
    4. None of the above
    Show answer

    Answer: C. Ethylene

    - Phytohormones are plant hormones that regulate growth and development. - Ethylene is a gaseous phytohormone that plays a significant role in various plant processes, including fruit ripening, senescence (aging), and abscission (shedding of leaves, flowers, or fruits). - Exposure to ethylene accelerates the ripening process in many fruits, leading to changes in color, texture, aroma, and sugar content.

Gibberellins: Biosynthesis, Metabolism, and Physiological Effects

  1. Which hormone is responsible for breaking the dormancy of a seed?

    Asked in: WBPSC CS Prelims 2023

    1. Auxin
    2. Dormin
    3. Cytokinin
    4. Gibberellin
    Show answer

    Answer: D. Gibberellin

    Gibberellins (GAs) are a group of plant hormones that play a crucial role in various growth and development processes in plants, including breaking

Hormonal Regulation of Plant Growth and Development (e.g., flowering, senescence, dormancy)

  1. Which of the following statement is true about phytochrome?

    Asked in: ASO 2025

    1. It is a biological proteinaceous pigment
    2. The site of phytochrome production is stem
    3. It is a photosynthetic pigment
    4. Phytochromes produce fluorescence
    Show answer

    Answer: A. It is a biological proteinaceous pigment

    Phytochrome is a biological proteinaceous pigment that senses light and regulates plant growth and development.

Plant Hormones in Stress Response and Adaptation

  1. Stress hormone of the plant is

    Asked in: law officier 2025 · Agri. Officer 2020

    1. ABA
    2. CK
    3. IAA
    4. GA
    Show answer

    Answer: A. ABA

    Abscisic acid (ABA) is the primary plant hormone that mediates responses to stress. C. Rumani × Neelam D. Alphanso × Neelum Ans: C Mango breeding programs aim to develop new varieties with improved traits such as disease resistance, better fruit quality, regular bearing, and desirable sensory characteristics. Understanding the parentage of important commercial varieties is crucial for breeding and genetic studies.

  2. "Arka Harit" is a variety of –

    Asked in: Agri. Officer 2020

    1. Tomato
    2. Pea
    3. Bitter gourd
    4. Brinjal
    Show answer

    Answer: C. Bitter gourd

    Horticultural research institutions develop new varieties of crops to enhance yield, improve nutritional value, and increase resistance to pests and diseases. Knowing the names of these improved varieties and the crop they belong to is important for agricultural students and professionals.

  3. What term is used for the production of seeds without fertilization ?

    Asked in: Co operative Supervisor & Environment Supervisor (Group C) Exam 2023

    1. Apomixis
    2. Parthenocarpy
    3. Polyembryony
    4. None of these
    Show answer

    Answer: A. Apomixis

    Apomixis allows for the propagation of desirable traits in plants without the variation that results from sexual reproduction. This is particularly useful in agriculture for maintaining hybrid vigor and specific genetic characteristics.

  4. Nucellar seedlings arising from polyembryonic seeds of mango are considered as

    Asked in: Food Safety Officer 2022

    1. Vigorous root-stock
    2. Clonal root-stock
    3. Seedling root-stock
    4. Dwafing root-stock
    Show answer

    Answer: B. Clonal root-stock

    Polyembryony is a phenomenon where a single seed produces multiple embryos. In mangoes, nucellar embryos are adventitious embryos formed from the nucellus tissue surrounding the embryo sac. These are a form of asexual reproduction and thus produce genetic clones of the parent tree.