Q30. Which of the following plant growth regulators is used as a herbicide?
[RE-NEET 2026]
(A) 2,4-D
(B) Kinetin
(C) Abscisic acid
(D) Gibberellin
Answer: (A) 2,4-D
Explanation: 2,4-dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin widely applied as a selective herbicide, particularly against broad-leaved weeds. At excessive concentrations, it causes uncontrolled growth in susceptible plants. Kinetin is a cytokinin, whereas abscisic acid and gibberellin serve different regulatory functions.
Q29. Which plant growth regulator promotes internode elongation before flowering in cabbage?
[RE-NEET 2026]
(A) Abscisic acid
(B) Gibberellin
(C) Indole-3-butyric acid
(D) Ethephon
Answer: (B) Gibberellin
Explanation: Gibberellins promote bolting, the rapid elongation of internodes immediately before flowering in rosette plants such as cabbage. This effect makes the flowering shoot grow taller. Abscisic acid mainly helps regulate dormancy and stress, while indole-3-butyric acid belongs to the auxin group.
Q28. Consider the following statements about plant hormones:
I. They coordinate growth, development and environmental responses.
II. They are always synthesised away from their site of action and simply diffuse there.
Which statement is correct?
[SSC CHSL Tier-I, 30 November 2025, Shift 1]
(A) Only I
(B) Only II
(C) Both I and II
(D) Neither I nor II
Answer: (A) Only I
Explanation: Statement I correctly describes phytohormones as chemical regulators of growth, development and responses to environmental conditions. Statement II is too absolute: hormones may act near their synthesis sites or travel through regulated transport pathways. Movement is not invariably simple diffusion from distant sites.
Q27. How does auxin cause a plant shoot to bend towards light?
[SSC CHSL Tier-I, 22 November 2025, Shift 3]
(A) It makes all shoot cells elongate equally
(B) It lengthens cells on the illuminated side
(C) It inhibits elongation on the shaded side
(D) It stimulates greater cell elongation on the shaded side
Answer: (D) It stimulates greater cell elongation on the shaded side
Explanation: With illumination from one side, auxin redistributes towards the shaded side of a young shoot. Cells there elongate more rapidly than those on the illuminated side. The unequal growth bends the shoot towards light, a response called positive phototropism.
Q26. Which meristematic tissue generally has high auxin concentrations?
[RRB Group D, 3 December 2025, Shift 3]
(A) Lateral meristem
(B) Intercalary meristem
(C) Apical meristem
(D) Xylem
Answer: (C) Apical meristem
Explanation: Auxin is synthesized prominently in young growing regions, especially shoot apices and developing leaves. Therefore the apical meristem region is associated with high auxin activity. This hormone regulates elongation and apical dominance, whereas xylem is primarily a conducting tissue rather than a meristem.
Q25. Which one among the following is not a plant hormone?
[UPSC CDS-I, 13 April 2025]
(A) Gibberellins
(B) Abscisic acid
(C) Auxins
(D) Thyroxine
Answer: (D) Thyroxine
Explanation: Thyroxine is produced by the thyroid gland in vertebrates and regulates metabolic activities. The other three are plant growth regulators: gibberellins encourage elongation and germination, auxins regulate differential growth, and abscisic acid coordinates dormancy and water-stress responses.
Q24. Spraying sugarcane with which plant growth regulator increases stem length and thereby can increase yield?
[NEET 2024]
(A) Auxin
(B) Gibberellin
(C) Cytokinin
(D) Abscisic acid
Answer: (B) Gibberellin
Explanation: Gibberellins stimulate elongation of stem internodes. Sugarcane is cultivated chiefly for its elongated, sugar-containing stems, so additional internode growth can increase harvestable stem biomass. Cytokinins mainly encourage cell division, while abscisic acid is strongly associated with dormancy and stress responses.
Q23. Auxin-based weedkillers can make lawns weed-free without substantially damaging grass because such auxins generally:
[NEET 2024]
(A) Promote apical dominance only
(B) Cause shedding of mature leaves only
(C) Do not significantly affect mature monocotyledonous grasses at selective doses
(D) Increase cell division in all grasses
Answer: (C) Do not significantly affect mature monocotyledonous grasses at selective doses
Explanation: Synthetic auxin herbicides such as 2,4-D are commonly used against broad-leaved dicot weeds. Established monocot grasses generally tolerate recommended selective doses better, so lawns can remain intact. Selectivity is dose- and species-dependent; it does not mean auxins are completely inactive in grass.
Q22. Which one among the following hormones inhibits growth activity in plants?
[UPSC CDS-II, 1 September 2024]
(A) Auxins
(B) Cytokinins
(C) Abscisic acid
(D) Gibberellins
Answer: (C) Abscisic acid
Explanation: Abscisic acid is commonly classified as a growth-inhibiting plant hormone. It promotes seed dormancy and, during water shortage, helps guard cells close stomata to limit transpiration. Auxins, gibberellins and cytokinins generally promote particular growth or developmental processes.
Q21. Which hormone promotes internode and petiole elongation in deep-water rice?
[NEET 2023]
(A) Kinetin
(B) Ethylene
(C) 2,4-D
(D) GA3
Answer: (B) Ethylene
Explanation: Submergence traps ethylene within deep-water rice tissues. Elevated ethylene signalling promotes rapid elongation of internodes and petioles, helping the growing shoot rise towards or above the water surface. This is a specialised flooding adaptation and differs from ethylene’s familiar role in fruit ripening.
Q20. Spraying which phytohormone on juvenile conifers helps hasten maturity and early seed production?
[NEET 2023]
(A) Gibberellic acid
(B) Zeatin
(C) Abscisic acid
(D) Indole-3-butyric acid
Answer: (A) Gibberellic acid
Explanation: Gibberellic acid can hasten the transition to reproductive maturity in some juvenile conifers and may promote earlier cone and seed formation. It is valuable in forestry seed production. Zeatin is a cytokinin, abscisic acid favours dormancy, and indole-3-butyric acid commonly promotes rooting.
Q19. Which plant growth regulator is an adenine derivative?
[NEET 2022, Phase 2]
(A) Abscisic acid
(B) Auxin
(C) Cytokinin
(D) Ethylene
Answer: (C) Cytokinin
Explanation: Many cytokinins, including kinetin and zeatin, are derivatives of the purine base adenine. Their principal effects include promoting cell division and delaying leaf senescence. Auxins such as IAA are indole-related compounds, while ethylene is a simple gaseous molecule.
Q18. Which phytohormone can increase cucumber yield by promoting female flower formation?
[NEET 2022, Phase 1]
(A) Abscisic acid
(B) Gibberellin
(C) Ethylene
(D) Cytokinin
Answer: (C) Ethylene
Explanation: Ethylene promotes femaleness in cucurbits such as cucumber. Increasing the proportion of female flowers can improve fruit set because fruits develop from female flowers. Ethephon is often used as an ethylene-releasing compound, but its application must suit the crop and conditions.
Q17. A student wants to increase cell division in plants. Which plant hormone should be recommended?
[UPSC CDS-I 2021]
(A) Abscisic acid
(B) Gibberellins
(C) Cytokinin
(D) Auxin
Answer: (C) Cytokinin
Explanation: Cytokinins are especially associated with cytokinesis and cell proliferation. They are frequently used with auxins in plant tissue culture to control callus growth and organ formation. Auxins primarily influence cell elongation and rooting, while gibberellins favour elongation and seed germination.
Q16. Which plant hormone helps in the growth of plant stems?
[RRB NTPC CBT-I, 14 March 2021, Shift 2]
(A) Abscisic acid
(B) Cytokinin
(C) Ethylene
(D) Gibberellin
Answer: (D) Gibberellin
Explanation: Gibberellins are well known for promoting internode elongation and increasing plant height. They also help some seeds overcome dormancy and can induce bolting in rosette plants. Abscisic acid has largely opposing effects on germination, while ethylene regulates diverse growth responses.
Q15. Which plant hormone regulates growth particularly by stimulating cell elongation in stems?
[SSC CGL 2020 Tier-I, held 23 August 2021, Shift 1]
(A) Ethylene
(B) Auxin
(C) Gibberellin
(D) Cytokinin
Answer: (B) Auxin
Explanation: Auxins such as indole-3-acetic acid stimulate elongation of young shoot cells through processes including cell-wall loosening. They also help establish apical dominance and directional growth responses. Gibberellins promote stem elongation too, but the question specifically identifies the classic auxin function.
Q14. Which of the following is not a plant hormone?
[RRB NTPC CBT-I, 7 March 2021, Shift 2]
(A) Gibberellins
(B) Prolactin
(C) Ethylene
(D) Abscisic acid
Answer: (B) Prolactin
Explanation: Prolactin is secreted mainly by the anterior pituitary in animals and is important for lactation. Gibberellins, ethylene and abscisic acid are plant hormones. Ethylene regulates ripening, gibberellins stimulate elongation and germination, and abscisic acid helps maintain dormancy.
Q13. Leaf fall generally becomes more likely when which hormonal change occurs in the leaf?
[UPPSC GIC Lecturer Biology 2021]
(A) The amount of auxin decreases
(B) The amount of auxin increases
(C) The amount of abscisic acid decreases
(D) The amount of gibberellic acid decreases
Answer: (A) The amount of auxin decreases
Explanation: Auxin supplied by a healthy leaf helps maintain its attachment to the stem. As a leaf ages, lower auxin supply can make the abscission zone more responsive to ethylene, allowing separation. Thus declining auxin is associated with leaf fall.
Q12. Which combination of hormones can help induce flowering in pineapple throughout the year?
[NEET 2019]
(A) Gibberellin and abscisic acid
(B) Cytokinin and abscisic acid
(C) Auxin and ethylene
(D) Gibberellin and cytokinin
Answer: (C) Auxin and ethylene
Explanation: Ethylene is an important inducer of flowering in pineapple, and auxin-based treatments may also help trigger flowering through ethylene-related responses. Controlled flowering allows growers to coordinate crop production. Gibberellins and abscisic acid are not the standard combination identified here.
Q11. Sub-apical elongation in plants is induced mainly by which hormone?
[RPSC Senior Teacher Grade II Science, 1 November 2018]
(A) Auxins
(B) Gibberellins
(C) Cytokinin
(D) Ethylene
Answer: (B) Gibberellins
Explanation: Gibberellins strongly stimulate elongation of the sub-apical region and stem internodes. They are especially important in bolting and the growth of certain dwarf varieties. Auxin promotes cell elongation in shoots but gibberellin is the intended regulator for the described sub-apical response.
Q10. Which naturally occurring plant hormone is commonly called the stress hormone?
[RPSC Senior Teacher Grade II Science, 1 July 2017]
(A) Auxin
(B) Cytokinin
(C) Gibberellin
(D) Abscisic acid
Answer: (D) Abscisic acid
Explanation: Abscisic acid is called the stress hormone because its signalling becomes prominent during drought and other environmental challenges. It helps close stomata, conserve water and maintain seed dormancy. Its action supports survival when growth would be unfavourable.
Q9.
Q8. The Avena curvature test is used for the bioassay of which substance?
[NEET 2016, Phase 1]
(A) IAA
(B) Ethylene
(C) ABA
(D) GA3
Answer: (A) IAA
Explanation: The Avena curvature test uses the bending response of an oat coleoptile to measure auxin activity. Indole-3-acetic acid causes unequal elongation when applied asymmetrically, making curvature a biological indicator of its concentration. It is a classic experiment in plant hormone research.
Q7. Which one of the following is a natural auxin?
[WBCS Preliminary Examination 2015]
(A) NAA
(B) IAA
(C) 2,4-D
(D) IBA
Answer: (B) IAA
Explanation: Indole-3-acetic acid (IAA) is the best-known natural auxin found widely in plants. NAA and 2,4-D are synthetic auxin analogues. IBA can also occur naturally, so the wording is imperfect scientifically; IAA was the answer designated in this examination.
Q6. Which of the following is a plant hormone?
[Chhattisgarh PSC Preliminary Examination 2014]
(A) Insulin
(B) Thyroxine
(C) Estrogen
(D) Cytokinin
(E) None of the above
Answer: (D) Cytokinin
Explanation: Cytokinin is a plant growth regulator that promotes cell division and can delay leaf ageing. Insulin, thyroxine and estrogen are animal hormones with metabolic or reproductive roles. Plant hormones operate at low concentrations and coordinate development and responses to surroundings.
Q5. Which hormone stimulates plant cells to grow so that the plant bends towards light?
[UPSC NDA-I 2013]
(A) Cytokinin
(B) Auxin
(C) Gibberellin
(D) Abscisic acid
Answer: (B) Auxin
Explanation: Auxin accumulates preferentially on the shaded side of a young shoot illuminated from one direction. It stimulates greater cell elongation there, producing curvature towards the light. This directional growth response is phototropism and improves light capture for photosynthesis.
Q4. Which plant hormone exists in gaseous form?
[Chhattisgarh PSC Preliminary Examination 2013]
(A) Abscisic acid
(B) Ethylene
(C) Gibberellins
(D) Auxins
(E) None of these
Answer: (B) Ethylene
Explanation: Ethylene is a gaseous plant hormone at ordinary temperatures and can diffuse through plant tissues and the surrounding air. It regulates ripening, organ abscission and several developmental responses. The other listed hormones are not gases under normal conditions.
Q3. Seed dormancy is regulated chiefly by which of the following?
[SSC MTS 2013]
(A) Abscisic acid
(B) Gibberellic acid
(C) Indole acetic acid
(D) Ethylene
Answer: (A) Abscisic acid
Explanation: Abscisic acid helps establish and maintain seed dormancy, preventing germination when conditions are unsuitable. Gibberellins generally act antagonistically by promoting dormancy release and seed germination. The balance between these hormonal pathways determines whether many seeds remain dormant or begin growing.
Q2. Which phytohormone is mainly responsible for fruit ripening?
[UPPSC Mains 2009 (also UPPSC Prelims 2006)]
(A) Cytokinin
(B) Abscisic acid
(C) Ethylene
(D) None of the above
Answer: (C) Ethylene
Explanation: Ethylene initiates and coordinates many changes during ripening, including pigment development, softening and aroma formation in climacteric fruits. The gas can act on nearby fruits, making synchronized commercial ripening possible under controlled conditions. Cytokinins and abscisic acid have different principal roles.
Q1. Seed dormancy is due mainly to which of the following?
[AIPMT 2002]
(A) Ethylene
(B) Abscisic acid
(C) IAA
(D) Starch
Answer: (B) Abscisic acid
Explanation: Abscisic acid suppresses premature germination and supports dormancy during seed development. Gibberellins tend to counter this effect when conditions favour germination. Ethylene may help overcome dormancy in some species, while starch is a stored carbohydrate rather than a plant hormone.