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.