Plant hormones (phytohormones) are chemical messengers produced in minute quantities that regulate growth, differentiation, and responses to environment. NEET questions on this topic test precise recall of hormone-function pairings, site of synthesis, and physiological effects — the trap is confusing which hormone does what.
The five classical plant hormones:
Auxin (IAA) — synthesised at shoot apices; promotes cell elongation, apical dominance, root initiation in cuttings. Causes phototropism (Went's experiment). High concentration inhibits root growth but promotes shoot growth. Used as 2,4-D (synthetic auxin) — a selective herbicide.
Gibberellins (GA₃) — produced in young leaves, root tips. Promote stem elongation (bolting in rosette plants), break seed dormancy, induce parthenocarpy, delay senescence. Key distinction from auxin: gibberellins cause internode elongation without cell division.
Cytokinins (kinetin, zeatin) — synthesised in root tips, transported via xylem. Promote cell division (cytokinesis — hence the name), delay leaf senescence (Richmond-Lang effect), promote lateral bud growth (counter apical dominance by auxin).
Abscisic acid (ABA) — "stress hormone." Promotes stomatal closure under drought, induces seed dormancy, inhibits growth. Antagonist to gibberellins in seed germination.
Ethylene (C₂H₄) — gaseous hormone. Promotes fruit ripening, abscission of leaves/flowers, senescence. Produced abundantly in ripening fruits and stressed tissues. Breaks seed and bud dormancy in some species.
High-frequency NEET trap: confusing auxin vs gibberellin effects (both promote growth, but auxin = elongation + apical dominance; gibberellin = internode elongation + dormancy breaking). Another common confusion: ABA promotes dormancy while gibberellin breaks it — they are antagonists.
NCERT Class 11 Biology Chapter 13, page 174 is the primary source for hormone functions and discovery.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
A gardener decapitates (removes the terminal bud of) a hedge plant, releasing its lateral buds from apical dominance. On a second, intact plant, the gardener instead applies cytokinin paste directly to a lateral bud while leaving the terminal bud in place. What is the expected outcome on the second plant, and why?
Answer: B. Removing the terminal bud releases lateral buds because it removes the source of inhibitory auxin (NCERT Class 11 Biology Chapter 13, page 175). Cytokinin achieves a similar release by a different route: applied directly to a lateral bud, it helps overcome apical dominance locally, promoting outgrowth even while the terminal bud (and its auxin supply) remains intact (NCERT Class 11 Biology Chapter 13, page 177). The two treatments release the same bud from dominance by different mechanisms — removing the inhibitor versus counteracting it.
Why A is wrong: This treats auxin's inhibition as absolute and unopposable, but cytokinin's documented action is precisely to counter apical dominance at the site it is applied, even when the terminal bud keeps producing auxin (trap: assuming only removing the auxin source can release a bud).
Why C is wrong: This misattributes the mechanism: the released growth in this scenario comes from cytokinin acting locally on the treated bud, not from cytokinin travelling to and shutting down auxin synthesis at the terminal bud, which is not how the two hormones interact (trap: importing the decapitation mechanism into the cytokinin-application case).
Why D is wrong: Cytokinins are synthesised mainly in root tips but are transported and act throughout the plant, including on shoot lateral buds — restricting their action to roots contradicts their well-documented role in overcoming apical dominance when applied to a lateral bud (trap: confusing site of synthesis with site of action).
Which hormone is called the "stress hormone" in plants and promotes stomatal closure during drought?
Answer: A. Abscisic acid (ABA) is termed the stress hormone; it causes stomatal closure by triggering K⁺ efflux from guard cells during water stress. NCERT Class 11 Biology Chapter 13.
Why B is wrong: B — Gibberellic acid promotes growth and breaks dormancy; it does not close stomata.
Why C is wrong: C — Ethylene is a gaseous hormone involved in fruit ripening and senescence, not stomatal regulation.
Why D is wrong: D — IAA (auxin) promotes cell elongation and phototropism; it is not termed the stress hormone.
The Richmond-Lang effect demonstrates that cytokinins:
Answer: C. Richmond and Lang (1957) showed that kinetin application delays chlorophyll degradation and senescence in detached leaves — the Richmond-Lang effect. NCERT Class 11 Biology Chapter 13.
Why A is wrong: A — Internode elongation in dwarf plants is a gibberellin effect, not cytokinin (trap: similar growth-promotion language).
Why B is wrong: B — Parthenocarpy is induced by auxins and gibberellins, not cytokinins.
Why D is wrong: D — Abscission is promoted by ethylene and ABA; cytokinins delay senescence rather than promote it.
2,4-D is a synthetic auxin used as:
Answer: B. 2,4-dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin that selectively kills broad-leaved (dicot) weeds while monocot crops remain unaffected, due to differential auxin sensitivity. NCERT Class 11 Biology Chapter 13.
Why A is wrong: A — 2,4-D is a herbicide, not a fungicide; fungicides target fungal pathogens, not weeds (trap: confusing agrochemical categories).
Why C is wrong: C — Growth retardants (e.g., CCC, AMO-1618) are anti-gibberellins; 2,4-D kills dicots, it doesn't retard monocot growth.
Why D is wrong: D — Fruit ripening is promoted by ethylene (e.g., ethephon application), not by auxin analogues.
A gardener removes the terminal bud of a hedge plant to promote bushy growth. Which hormonal change explains the increased lateral branching?
Answer: D. Removing the terminal bud eliminates the auxin source that suppresses lateral buds (apical dominance). With reduced basipetal auxin flow, lateral buds are released from inhibition and grow out. NCERT Class 11 Biology Chapter 13.
Why A is wrong: A — Gibberellin promotes internode elongation, not specifically lateral bud release; the key event is removal of auxin-mediated suppression (trap: both are growth promoters).
Why B is wrong: B — ABA in root tips is unrelated to apical dominance; the phenomenon is controlled by shoot-apex auxin concentration.
Why C is wrong: C — Ethylene promotes senescence/abscission; bushy growth from pruning is explained by auxin removal, not ethylene increase.
Which pair correctly represents antagonistic hormone actions in seed germination?
Answer: B. Gibberellic acid induces α-amylase synthesis in the aleurone layer, breaking seed dormancy and promoting germination. ABA maintains dormancy by inhibiting this process. They are classical antagonists in seed germination. NCERT Class 11 Biology Chapter 13.
Why A is wrong: A — Auxin and cytokinin are not the classical antagonistic pair for seed germination; their interplay governs cell division vs elongation (trap: picking any two hormones as antagonists).
Why C is wrong: C — Ethylene can break dormancy in some species but gibberellin PROMOTES germination, it does not inhibit it — this option reverses gibberellin's role.
Why D is wrong: D — Cytokinin promotes cell division but is not the primary germination-promoting hormone; auxin does not inhibit germination in the classical antagonist model.
Ethylene is unique among plant hormones because it is:
Answer: D. Ethylene (C₂H₄) is the only gaseous plant hormone, diffusing freely through intercellular spaces. It is produced in all plant parts (not just roots) and promotes (not inhibits) ripening. NCERT Class 11 Biology Chapter 13.
Why A is wrong: A — Ethylene is synthesised in ripening fruits, senescing tissues, and stressed organs — not only in roots (trap: confusing cytokinin's root-tip synthesis with ethylene).
Why B is wrong: B — Ethylene PROMOTES fruit ripening; it is commercially applied as ethephon to hasten ripening (trap: confusing with ABA, the inhibitor hormone).
Why C is wrong: C — Ethylene acts in both light and dark conditions; photoperiodism involves phytochrome, not ethylene activation.
A researcher applies gibberellic acid to a rosette plant (compact, short internodes). The expected result is:
Answer: A. Gibberellin causes bolting in rosette plants like cabbage — it stimulates rapid internode elongation, converting the compact rosette into a tall, elongated stem. This is one of GA₃'s defining physiological effects. Step 1: identify rosette = compressed internodes; Step 2: GA₃ promotes internode elongation → bolting. NCERT Class 11 Biology Chapter 13.
Why B is wrong: B — Root branching is promoted by auxin (lateral root initiation); gibberellin's primary visible effect is on stem elongation (trap: confusing auxin and gibberellin growth effects).
Why C is wrong: C — Gibberellin can actually promote flowering in long-day plants grown in short-day conditions; it does not inhibit flowering.
Why D is wrong: D — Leaf abscission is promoted by ethylene and ABA; gibberellin promotes growth, not senescence.
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
Given
A NEET question states: "Match the following hormones with their effects" and provides:• Column I: (a) Auxin, (b) Gibberellin, (c) ABA, (d) Ethylene• Column II: (i) Fruit ripening, (ii) Apical dominance, (iii) Seed dormancy, (iv) Bolting
Required
Correct matching of each hormone to its primary physiological effect.
Concept
Each plant hormone has signature effects that NEET tests. The trap is pairing hormones with effects that sound plausible but belong to a different hormone (similar-term-confusion distractor type).
Key associations (in place of formula)
• Auxin → apical dominance, phototropism, cell elongation• Gibberellin → bolting, seed germination, internode elongation• ABA → seed dormancy, stomatal closure, stress response• Ethylene → fruit ripening, abscission, senescence
Substitution (matching)
• (a) Auxin → (ii) Apical dominance• (b) Gibberellin → (iv) Bolting• (c) ABA → (iii) Seed dormancy• (d) Ethylene → (i) Fruit ripening
Verification
Cross-check: Could ABA cause ripening? No — ABA inhibits growth; ripening is ethylene. Could gibberellin cause apical dominance? No — that's auxin's role specifically.
Final answer
(a)-(ii), (b)-(iv), (c)-(iii), (d)-(i)
Common trap
The most common error is swapping gibberellin and auxin: both promote growth, but apical dominance is specifically auxin (not gibberellin). Similarly, students confuse ABA-induced dormancy with ethylene-induced senescence — dormancy is reversible (ABA), while senescence/ripening is progressive and irreversible (ethylene).
Similar NEET-style question
"Assertion: Removal of shoot tips leads to lateral bud growth. Reason: Cytokinins produced in shoot tips suppress lateral buds." — The Assertion is correct (apical dominance removed), but the Reason is incorrect (it's AUXIN from shoot tips that suppresses laterals, not cytokinin; cytokinin actually promotes lateral growth). Answer: A is true, R is false.
Auxin (IAA) — apical dominance, phototropism. Gibberellin — bolting, parthenocarpy. Cytokinin — delay senescence, lateral branching. Abscisic acid — stomatal closure, dormancy. Ethylene — fruit ripening, abscission, triple response.
-- NCERT Class 11 Biology, Chapter 13, p. 174More in Plant Physiology: 5 exam traps and mistakes · 1 question pattern from its other lessons.
12 questions from NEET 2020, 2022, 2023, 2024, 2025, 2026. Answers verified against NTA official keys.
Which of the following plant growth regulators is used as herbicide ?
Auxin is used by gardeners to prepare weed-free lawns. But no damage is caused to grass as auxin
Which hormone promotes internode/petiole elongation in deep water rice?
The gaseous plant growth regulator is used in plants to :
Which of the following is not an inhibitory substance governing seed dormancy ?
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →