Auxins — NEET Biology MCQs with Solutions
Free NEET Biology Auxins MCQs with step-by-step solutions (57 questions). Part of Plant Growth and Development. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Auxins · easy · theory
The first plant hormone to be discovered, isolated from oat coleoptile tips, was:
A. Gibberellin
B. Ethylene
C. Cytokinin
D. Auxin ✓ Correct
Solution: Auxin (IAA) was the first hormone discovered, following Darwin and F.W. Went's coleoptile experiments.
Q2 — Auxins · easy · theory
The suppression of the growth of lateral (axillary) buds by the shoot apex is called:
A. Apical dominance ✓ Correct
B. Vernalisation
C. Senescence
D. Abscission
Solution: Auxin produced at the shoot apex inhibits lateral buds — apical dominance (removed by decapitation).
Q3 — Auxins · medium · theory
Auxins are widely used in agriculture/horticulture to:
A. Break seed dormancy
B. Induce cell division only
C. Promote rooting in stem cuttings and prevent fruit/leaf drop ✓ Correct
D. Cause fruit ripening
Solution: Auxins (e.g. IBA, NAA) promote root initiation in cuttings and prevent premature abscission.
Q4 — Auxins · medium · theory
The natural auxin present in plants is:
A. 2,4-D
B. IBA
C. Indole-3-acetic acid (IAA) ✓ Correct
D. NAA
Solution: IAA is the naturally occurring auxin; NAA, 2,4-D and IBA are synthetic auxins.
Q5 — Auxins · hard · theory
The synthetic auxin 2,4-D is used as a ______ because it kills ______ selectively:
A. Fertiliser; monocots
B. Herbicide (weedicide); dicot weeds ✓ Correct
C. Ripening agent; fruits
D. Rooting hormone; all plants
Solution: 2,4-D selectively kills dicotyledonous weeds without harming mature monocots — a common broad-leaf herbicide.
Q6 — Auxins · hard · theory
Auxin promotes cell elongation according to the acid-growth hypothesis by:
A. Preventing water uptake
B. Blocking proton pumps
C. Thickening the cell wall
D. Loosening the cell wall (increasing its extensibility) ✓ Correct
Solution: Auxin activates wall-loosening (acidification via H⁺ pumping), allowing turgor-driven elongation.
Q7 — Auxins · easy · numerical
A gardener removes the shoot tips of a hedge to make it grow bushier and denser. The physiological basis of this practice is that decapitation:
A. Removes the source of auxin, releasing lateral buds from apical dominance ✓ Correct
B. Stops all cell division in the plant
C. Increases auxin at the axillary buds, promoting their growth
D. Supplies extra cytokinin from the wound
Solution: The shoot apex is the auxin source that suppresses lateral buds; removing it lowers auxin at the buds so they sprout, making the plant bushy.
Q8 — Auxins · easy · numerical
In horticulture, auxins such as IBA and NAA are commonly applied to the cut ends of stem cuttings mainly to:
A. Promote initiation of adventitious roots ✓ Correct
B. Break seed dormancy
C. Induce flowering
D. Delay leaf senescence
Solution: Auxins promote rooting, so IBA/NAA are applied to cuttings to induce adventitious root formation for vegetative propagation.
Q9 — Auxins · easy · numerical
Auxin sprayed on tomato or grape flowers can produce seedless fruits without fertilisation. This induction of fruit development is called:
A. Parthenocarpy ✓ Correct
B. Parthenogenesis
C. Polyembryony
D. Apomixis
Solution: Auxin can induce parthenocarpy — the development of fruit without fertilisation, yielding seedless fruits.
Q10 — Auxins · easy · numerical
A weed-killer that selectively destroys broad-leaved dicot weeds in a lawn of narrow-leaved monocot grasses is the synthetic auxin:
A. 2,4-D ✓ Correct
B. GA3
C. Zeatin
D. Kinetin
Solution: 2,4-D is a synthetic auxin used as a selective herbicide that kills dicot (broad-leaved) weeds while leaving mature monocots relatively unaffected.
Q11 — Auxins · medium · numerical
In the classic Avena coleoptile curvature test, an agar block containing diffused auxin is placed on one side of a decapitated coleoptile in the dark. The coleoptile bends:
A. Away from the side bearing the auxin block, because that side elongates faster ✓ Correct
B. Towards the side bearing the auxin block, because that side elongates faster
C. Away from the block because that side stops growing
D. Straight up with no bending
Solution: Auxin diffusing from the block stimulates greater cell elongation on that side, so the coleoptile curves away from the auxin-bearing side.
Q12 — Auxins · medium · numerical
When a coleoptile tip is illuminated from one side, it bends towards the light. The mechanism established is that auxin:
A. Migrates to the lit side, which elongates more
B. Is destroyed uniformly on both sides
C. Moves upward to the tip and stops elongation
D. Migrates laterally to the shaded side, which then elongates more ✓ Correct
Solution: Unilateral light causes lateral redistribution of auxin to the shaded side, which elongates more, bending the shoot towards the light (positive phototropism).
Q13 — Auxins · medium · numerical
A student states four properties of auxin. Which one is INCORRECT?
A. It induces xylem differentiation and cell division in cambium
B. It promotes abscission of leaves and fruits ✓ Correct
C. It promotes apical dominance
D. It controls tropic responses to light and gravity
Solution: Auxin actually PREVENTS the abscission of young leaves and fruits (it is applied to reduce fruit drop); the other three are genuine auxin actions.
Q14 — Auxins · medium · numerical
Auxin was named from the Greek word "auxein". The choice of this name reflects the effect first observed on coleoptiles, namely:
A. To ripen (promotion of fruit ripening)
B. To grow / increase (promotion of cell elongation) ✓ Correct
C. To divide (promotion of cytokinesis)
D. To close (promotion of stomatal closure)
Solution: Auxin is derived from the Greek "auxein" meaning "to grow", reflecting its role in promoting cell elongation and shoot growth.
Q15 — Auxins · medium · numerical
The first isolated auxin was obtained from human urine before it was found in plants, and later the naturally occurring auxin in plants was identified. The natural plant auxin and that first-isolated compound respectively are:
A. Naphthalene acetic acid and IAA
B. Indole-3-acetic acid (IAA) and auxin-a/auxin-b type indole compounds ✓ Correct
C. Zeatin and kinetin
D. 2,4-D and IBA
Solution: IAA is the principal natural auxin in plants; early auxins were isolated from human urine, and IAA remains the naturally occurring indole auxin.
Q16 — Auxins · medium · numerical
A high concentration of auxin that strongly promotes shoot elongation may simultaneously inhibit root growth. This differential sensitivity means that a concentration optimal for shoots is:
A. Completely ineffective on both organs
B. Always optimal for roots as well
C. Often supra-optimal (inhibitory) for roots ✓ Correct
D. Optimal only when combined with ethylene
Solution: Roots are far more sensitive to auxin than shoots, so a concentration that promotes shoot growth is usually above the optimum for roots and inhibits them.
Q17 — Auxins · medium
Primary precursor of IAA is :-
A. Tyrosine
B. Tryptophan ✓ Correct
C. Phenyl alanine
D. Leucin
Solution: Indole-3-acetic acid (IAA) is synthesised from the amino acid tryptophan.
Q18 — Auxins · medium
The biological activity of IAA is tested by :-
A. Endosperm activity test
B. Chlorophyll preservation test
C. α-amylase test
D. Avena curvature test ✓ Correct
Solution: The bioactivity of auxin (IAA) is assayed by the Avena coleoptile curvature test.
Q19 — Auxins · medium
Plants bend toward the light because -
A. Cells on the shaded side elongate more ✓ Correct
B. Light attracts them
C. They need light for respiration
D. They need light for photosynthesis
Solution: Auxin accumulates on the shaded side, causing those cells to elongate more so the shoot bends toward light (phototropism).
Q20 — Auxins · easy
One of the synthetic auxin is:-
A. NAA ✓ Correct
B. IAA
C. IBA
D. GA
Solution: NAA (naphthalene acetic acid) is a synthetic auxin, whereas IAA is a natural auxin.
Q21 — Auxins · easy
Phototropic curvature is the result of uneven distribution of:
A. Phytochrome
B. Cytokinins
C. Auxin ✓ Correct
D. Gibberellin
Solution: Phototropic curvature results from the unequal (lateral) distribution of auxin, which accumulates on the shaded side causing bending toward light.
Q22 — Auxins · easy
Root development is promoted by:
A. Abscisic acid
B. Gibberellin
C. Auxin ✓ Correct
D. Ethylene
Solution: Auxins promote root initiation and development, which is why they are used in rooting of stem cuttings.
Q23 — Auxins · hard
Dr. F. Went noted that if coleoptile tips were removed and placed on agar for one hour, the agar would produce a bending when placed on one side of freshly-cut coleoptile stumps. Of what significance is this experiment?
A. It demonstrated polar movement of auxins.
B. It made possible the isolation and exact identification of auxin.
C. It is the basis for quantitative determination of small amounts of growth-promoting substances. ✓ Correct
D. It supports the hypothesis that IAA is auxin.
Solution: Went's agar-block coleoptile experiment forms the basis of the Avena curvature bioassay, allowing quantitative determination of small amounts of growth-promoting substances.
Q24 — Auxins · medium
Auxin can be bioassayed by:
A. Potometer
B. Avena coleoptile curvature ✓ Correct
C. Lettuce hypocotyl elongation
D. Hydroponics
Solution: Auxin is bioassayed by the Avena coleoptile curvature test, in which the degree of bending is proportional to auxin concentration.
Q25 — Auxins · easy
The Avena curvature is used for bioassay of:
A. GA3
B. ABA
C. Ethylene
D. IAA ✓ Correct
Solution: The Avena curvature test is the standard bioassay for auxin (IAA).
Q26 — Auxins · medium
Fruit and leaf drop at early stages can be prevented by the application of:
A. Gibberellic acid
B. Auxins ✓ Correct
C. Ethylene
D. Cytokinins
Solution: Auxins prevent premature fruit and leaf drop at early stages while promoting abscission of older parts.
Q27 — Auxins · medium
Removal of shoot tips is a very useful technique to boost the production of tea leaves. This is because:-
A. Auxins prevent leaf drop at early stages
B. Gibberellins prevent bolting and are inactivated
C. Effect of auxins is removed and growth of lateral buds is enhanced. ✓ Correct
D. Gibberellins delay senescence of leaves.
Solution: Removing the shoot tip removes the source of auxin causing apical dominance, so lateral buds grow and produce more leaves.
Q28 — Auxins · easy
The plant hormone used to destroy weeds in a field is:
A. IBA
B. NAA
C. IAA
D. 2,4-D ✓ Correct
Solution: 2,4-D, a synthetic auxin, is a selective herbicide used to kill dicotyledonous weeds.
Q29 — Auxins · medium
Who isolated auxin from the tip of coleoptiles of oat seedlings?
A. Charles Darwin
B. Skoog
C. F.W. Went ✓ Correct
D. Kogl
Solution: F.W. Went recovered/isolated auxin from the tips of oat coleoptiles using his agar-block experiments.
Q30 — Auxins · easy
2,4-D is a:
A. Synthetic auxin ✓ Correct
B. Natural auxin
C. Natural cytokinin
D. Synthetic cytokinin
Solution: 2,4-Dichlorophenoxyacetic acid is a synthetic auxin, widely used as a herbicide.