Q.________ is called 'Agent Orange'.
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Auxins — The Plant's Growth Director
Imagine a plant growing toward sunlight. Somewhere inside, there must be a signal that tells the shaded side to stretch longer, bending the stem toward the light. That signal is auxin.
Auxins are the first plant hormones ever discovered, and they are the master regulators of cell elongation. The most common natural auxin is Indole-3-Acetic Acid (IAA). It is produced primarily in the shoot apical meristem — the growing tip of the stem — and moves downward, controlling how the plant grows and responds to its environment.
What auxins actually do
The core action of auxin is simple: it makes plant cells expand lengthwise. But because it moves directionally (from tip to base, and away from light), it creates patterns of growth that shape the entire plant.
Apical dominance is a classic example. The terminal bud produces auxin, which travels down the stem and suppresses the growth of lateral (side) buds. This keeps the plant growing tall and straight. If you cut off the tip, auxin production stops, and the side buds finally get their chance — the plant becomes bushier. Gardeners and farmers exploit this when they prune.
Rooting works differently. At low concentrations, auxin actually promotes root formation. That is why commercial rooting powders contain synthetic auxins like IBA (Indole-3-Butyric Acid) — dip a cutting in it, and roots form faster.
Prevention of abscission (leaf and fruit drop) is another role. Auxin keeps the abscission layer — a zone of weak cells at the base of the leaf stalk — from forming. As long as enough auxin flows from the leaf into the stem, the leaf stays attached. When auxin levels drop (due to age or stress), the abscission layer develops and the leaf falls.
Auxin (IAA) at low concentration promotes cell elongation and root initiation. The same auxin (IAA) at high concentration inhibits lateral bud growth and can even inhibit root growth. Concentration, not just presence, decides the outcome.
The mechanism — how auxin makes cells grow
Auxin activates a proton pump in the cell membrane, pumping H⁺ ions into the cell wall. This acidifies the wall, activating enzymes called expansins that loosen the cellulose fibres. The cell wall becomes more flexible, and water enters by osmosis, inflating the cell. This is called the acid growth hypothesis. …
'Agent Orange' is the well-known code name for a herbicide mixture used as a defoliant, made up of two synthetic auxin-type plant growth regulators. …
Agent Orange is the code name for a herbicide made of the synthetic auxins 2,4-D and 2,4,5-T, used at high concentration to kill broad-leaved plants by disrupting their growth.
2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) are synthetic auxin analogues. At low concentrations synthetic auxins promote normal growth responses, but at high concentrations they cause uncontrolled, abnormal growth that kills the plant - so they are widely used as selective herbicides against broad-leaved dicot weeds (monocots like cereals are relatively less sensitive). A roughly 1:1 mixture of 2,4-D and 2,4,5-T, code-named 'Agent Orange' (after the orange identification stripe on its storage drums), was used as a defoliant.
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- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is the primary precursor of IAA?(a) Gibberellin(b) Ethylene(c) Phenylalanine(d) Tryptophan
›Reveal solutionSolution
IAA (auxin) is naturally synthesised in plants from the amino acid tryptophan.
Auxins are a class of plant growth hormones, the most important naturally occurring one being indole-3-acetic acid (IAA). IAA is biosynthesised in plant tissues (especially shoot and root apices) from the amino acid tryptophan through a series of enzymatic steps. Auxins promote cell elongation, apical dominance, rooting, and various tropic responses.
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- CBSE 2026Set ANNUAL1 markQ.Write True/False: Auxin promotes flowering.
›Reveal solutionSolution
This statement is False — Auxin's textbook functions are growth promotion (cell elongation), apical dominance, root initiation, and parthenocarpy; flowering induction is not one of its characteristic roles.
Auxins (like IAA) are plant growth hormones with well-defined functions: they promote cell elongation in shoots and coleoptiles, help initiate rooting in stem cuttings, maintain apical dominance (suppression of lateral bud growth by the apical bud), induce parthenocarpic (seedless) fruit development, and prevent premature fruit/leaf drop at appropriate concentrations, besides being used commercially as a herbicide at high concentration. Promotion of flowering is instead most closely associated with gibberellins (which can induce bolting and flowering in many long-day/biennial plants) and the flowering si …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is a natural plant hormone ?(a) NAA(b) 2, 4-D(c) IAA(d) Ethylene glycol
›Reveal solutionSolution
IAA is a natural plant hormone, so the answer is (C).
Auxins occur as natural and synthetic types:
- Natural auxin: IAA (indole-3-acetic acid), produced by the plant itself in growing shoot and root tips. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which plant hormone is responsible for cell elongation ?(a) Cytokinin(b) Auxin(c) Abscisic acid(d) Ethylene
›Reveal solutionSolution
Auxin is responsible for cell elongation, so the answer is (B).
Roles of the main plant hormones:
- Auxin — promotes cell elongation, apical dominance, phototropism and root initiation.
- Cytokinin — promotes cell division and delays ageing.
- Abscisic acid — a growth inhibitor; promotes dormancy and stomatal closure. …
- CBSE 2025Set ANNUAL1 markMCQQ.Auxins promote(a) Cell growth and development(b) Apical dominance(c) Cambial activity(d) All of these
›Reveal solutionSolution
Auxin is a multifunctional plant hormone that promotes cell growth, enforces apical dominance, and stimulates cambial activity — so all three listed effects are correct.
Auxins (e.g., IAA) are synthesized mainly in shoot and root apices and young leaves. Their documented physiological roles include:
- Promoting cell elongation and overall growth and development, by loosening the cell wall (acid growth theory) and stimulating cell division/differentiation.
- Maintaining apical dominance — auxin from the growing shoot tip suppresses the growth of lateral (axillary) buds; removing the tip (decapitation) releases lateral buds from this inhibition. …
- CBSE 2025Set botany-sz1 markMCQQ.A hormone which helps to initiate rooting in stem cuttings is :(a) Auxins(b) Gibberellins(c) Cytokinins(d) Ethylene
›Reveal solutionSolution
Auxins are the hormone group used to initiate rooting in stem cuttings, a widely used horticultural application of this growth regulator.
Among the five major classes of plant hormones:
- Auxins promote cell elongation, apical dominance, and — importantly — induce the formation of adventitious roots. Synthetic auxins such as indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA) are widely applied as 'rooting hormones' to stem cuttings in horticulture and nurseries to promote rapid and profuse root development before the cutting is planted.
- Gibberellins (B) promote stem elongation, seed germination and flowering; they are not associated with root initiation in cuttings. …
- CBSE 2025Set ANNUAL1 markMCQQ.Which hormone was isolated by F.W. Went from tips of coleoptiles of oat seedlings?(a) Auxin(b) Gibberellins(c) Cytokinins(d) Ethylene
›Reveal solutionSolution
F.W. Went's classic oat-coleoptile experiment led to the discovery and isolation of the plant hormone auxin.
F.W. Went's experiments (1928) built on earlier observations by Charles and Francis Darwin on phototropic bending of grass coleoptiles. Went cut off the tips of oat coleoptiles, placed them on agar blocks to let the growth-promoting substance diffuse into the agar, and then placed the agar blocks asymmetrically on decapitated coleoptile stumps. The stumps bent away from the side bearing the agar block, showing that a chemical substance from the tip promoted cell elongation. This substance was isolate …
- CBSE 2024Set ANNUAL1 markQ.What is the full form of IAA?
›Reveal solutionSolution
IAA stands for Indole-3-Acetic Acid, a naturally occurring auxin (plant growth-promoting hormone).
IAA is the principal natural auxin of plants.
Indole-3-Acetic Acid (IAA) is the most common naturally occurring auxin in plants. It is synthesized mainly in shoot apices, young leaves and developing seeds, and it promotes cell elongation, apical dominance, root initiation, and various tropic (phototropic and geotr …
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following auxin has been isolated from plants ?(a) Naphthalene acetic acid (NAA)(b) 2,4-dichlorophenoxy acetic acid (2,4-D)(c) Indole acetic acid (IAA)(d) None of these
›Reveal solutionSolution
IAA (indole-3-acetic acid) and indole butyric acid (IBA) are naturally-occurring auxins isolated from plants; NAA and 2,4-D are synthetic auxin analogues.
Auxins were first isolated from human urine by F.W. Went, and later, Indole-3-acetic acid (IAA) was identified as a naturally occurring auxin produced by plants, mainly in the shoot and root apices, from where it migrates to the regions of its action. Indole butyric acid (IBA), also a natural auxin, was later isolated from plants too. In contrast, 2,4-Dichloroph …
- CBSE 2023Set ANNUAL1 markMCQQ.________ is called 'Agent Orange'.(a) ABA(b) Auxin(c) NAA(d) 2, 4 - D and 2, 4, 5 - T
›Reveal solutionSolution
Agent Orange is the code name for a herbicide made of the synthetic auxins 2,4-D and 2,4,5-T, used at high concentration to kill broad-leaved plants by disrupting their growth.
2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) are synthetic auxin analogues. At low concentrations synthetic auxins promote normal growth responses, but at high concentrations they cause uncontrolled, abnormal growth that kills the plant - so they are widely used as selective herbicides against broad-leaved dicot weeds (monocots like cereals are relatively less sensitive). A roughly 1:1 mixture of 2,4-D and 2,4,5-T, code-named 'Agent Orange' (after the orange identification stripe on its storage drums), was used as a defoliant.
…
- CBSE 2022Set ANNUAL1 markQ.Which is the primary precursor of IAA in plants?
›Reveal solutionSolution
Tryptophan (the amino acid tryptophan) is the primary precursor of IAA (indole-3-acetic acid).
IAA, the principal natural auxin, is biosynthesised in plants mainly from the amino acid tryptophan.
Indole-3-acetic acid (IAA) is the major naturally occurring auxin in plants, involved in cell elongation, apical dominance, root initiation and tropic responses. It is synthesised biochemically in actively growing tissues (such as shoot apices and …
- CBSE 2022Set ANNUAL1 markMCQQ.One of the synthetic auxin is:(a) IAA(b) IBA(c) GA(d) NAA
›Reveal solutionSolution
NAA (Naphthalene Acetic Acid) is a synthetic (man-made) auxin, unlike IAA and IBA which occur naturally in plants.
Auxins are a group of plant growth hormones that promote cell elongation, apical dominance, and root initiation. They are classified based on their source:
- Natural auxins: Indole-3-acetic acid (IAA) and Indole-3-butyric acid (IBA) are synthesised by the plant itself, mainly in the shoot apex and young leaves. …
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