Q.Plant growth substances (PGS) have innumerable practical applications. Name the PGS you should use to
a. Increase yield of sugar cane.
b. Promote lateral shoot growth.
c. Cause sprouting of potato tuber.
d. Inhibit seed germination.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Plant Growth Regulators
Plant Growth Regulators: The Plant's Chemical Messengers
Plants can't run away from trouble. They can't chase food or find a mate. So how do they decide when to grow, when to flower, when to drop leaves, or when to defend themselves? They use chemical signals — tiny molecules that act like a postal system inside the plant, carrying instructions from one cell to another.
These chemical signals are called Plant Growth Regulators (PGRs). Think of them as the plant's hormones, but with a twist: unlike animal hormones that mostly travel through blood, plant regulators can work right where they're made, move to nearby cells, or travel through the plant's vascular system. They work in tiny amounts — parts per million or even parts per billion — but their effects are dramatic.
The Two Big Families
All PGRs fall into two broad groups based on what they do:
Growth Promoters — these encourage cell division, cell enlargement, flowering, fruit formation. They make the plant grow bigger and develop faster.
Growth Inhibitors — these slow down growth, promote dormancy, help the plant survive stress, and cause aging and ripening.
The balance between promoters and inhibitors — not the absolute amount of any one regulator — determines what the plant actually does. A seed stays dormant when inhibitors dominate; it germinates when promoters take over.
The Five Classical Regulators
Indian exam syllabi (CBSE, ICSE, state boards) focus on five major PGRs. Here they are, grouped by function:
| Regulator | Type | Where it's made | Key effects |
|---|---|---|---|
| Auxin | Promoter | Shoot tips, young leaves | Cell elongation, apical dominance, root initiation |
| Gibberellin | Promoter | Young tissues, seeds | Stem elongation, seed germination, fruit growth |
| Cytokinin | Promoter | Root tips | Cell division, delay aging, shoot formation |
| Abscisic acid | Inhibitor | Leaves, fruits | Stomatal closure, seed dormancy, stress response |
| Ethylene | Inhibitor (mostly) | Ripening fruits, aging tissues | Fruit ripening, leaf fall, senescence |
Ethylene is the only gaseous PGR. It's a simple hydrocarbon (C₂H₄) — two carbons, four hydrogens — yet it controls when your banana turns yellow and your apple gets sweet.
How They Work Together: A Real Example
Take a seed buried in soil. It's full of abscisic acid — the inhibitor that keeps it dormant. The seed waits. When enough water soaks in and temperature rises, the balance shifts: gibberellins are produced, which break down the stored food and push the shoot upward. Auxin then takes over at the tip, making cells elongate and guiding the shoot toward light. Meanwhile, cytokinins from the growing root tip encourage cell division in the shoot. Later, when fruit forms, ethylene triggers ripening and eventually leaf fall.
No single regulator works alone. They interact, oppose each other, and amplify each other's effects.
Why This Matters for Exams
You will be asked to: …
a. To increase the yield of sugarcane, gibberellin is sprayed on the crop; it lengthens the stem, where the plant stores its sugar, increasing yield by as much as 20 tonnes per acre.
b. Cytokinin promotes lateral shoot growth, along with producing new leaves, new chloroplasts and adventitious shoots.
c. Ethylene causes sprouting of potato tubers, since it is known to break bud dormancy and initiate sprouting in stored tubers. …
Four different practical uses map to four different growth regulators: gibberellin for sugarcane yield, cytokinin for lateral shoots, ethylene for potato sprouting, and abscisic acid for inhibiting germination.
- Increase yield of sugarcane — Gibberellin is the regulator sprayed on a sugarcane crop; it increases the length of the stem, and because the crop stores its sugar in the stem, this stem elongation raises sugar yield substantially, by as much as 20 tonnes per acre.
- Promote lateral shoot growth — Cytokinin is the regulator responsible for this. It is synthesised where rapid cell division occurs — root apices, developing shoot buds and young fruits — and among its effects are producing new leaves, chloroplasts and lateral shoot growth, as well as helping to overcome apical dominance so lateral buds can grow out. …
Method: Matching a Practical Application to the Correct PGR
Same signature-table technique as matching tasks to hormones, but here each application is phrased as a real-world outcome — trace the outcome back to the one hormone known for it.
Steps
Step 1: Identify the crop or outcome named in each sub-question.
Sugarcane yield; lateral shoot growth; potato sprouting; inhibiting germination.
Step 2: Recall the one PGR most specifically tied to that named outcome. …
- AP EAPCET 2026Set ap-2026-05-20-AN1 markMCQQ.Match the following lists List-I: A. Breaking of dormancy B. Plants with rosette has, it can be promoted bolting by C. Weed free lawns can be developed by D. Turgidity of cells helps in extension growth along with List-II: I. Gibberellins II. Oxygen III. Sprouting of potato tubers by Ethylene IV. 2, 4-D (A) A-III, B-I, C-IV, D-II (B) A-II, B-IV, C-I, D-III (C) A-II, B-III, C-I, D-IV (D) A-I, B-II, C-IV, D-III
›Reveal solutionSolution
This matches four classic plant growth-regulator applications — breaking dormancy (ethylene/potato sprouting), bolting of rosette plants (gibberellins), weed-free lawns (2,4-D), and turgor-aided extension growth (oxygen) — to their list-II descriptions.
Concept and Intuition
Plant growth regulators have well-documented practical applications taught alongside their physiological roles. Ethylene is used, among other things, to break the dormancy of seeds and buds and to hasten the sprouting of potato tubers — precisely the description given for breaking dormancy here. Gibberellins are classically used to induce bolting (rapid internode elongation before flowering) in rosette-habit plants such as cabbage, which otherwise stay low and compact. 2,4-D, a synthetic auxin, is a selective herbicide that kills broadleaf (dicot) weeds while sparing grasses (monocots), making it the standard tool for developing weed-free lawns. Finally, cell extension (elongation) growth depends on turgor pressure pushing against a loosened cell wall, and this process is supported by the cell's respiratory activity, which requires oxygen to generate the ATP needed for the associated metabolic work.
Step-by-Step Solution
- Match A ("breaking of dormancy") to the ethylene/potato-tuber-sprouting description (III). …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.Match the following List-I | List-II A) Kinetin | I) Terpenes B) ABA | II) Carotenoids C) GA3 | III) Indole compound D) IAA | IV) Adenine derivatives (A) A-IV, B-III, C-I, D-II (B) A-IV, B-I, C-II, D-III (C) A-IV, B-II, C-I, D-III (D) A-IV, B-III, C-II, D-I
›Reveal solutionSolution
Matching plant hormones to their chemical class: Kinetin = adenine derivative, ABA = carotenoid-derived, GA3 = terpene, IAA = indole compound — giving A-IV, B-II, C-I, D-III.
Concept and Intuition
Each class of plant hormone has a distinct biosynthetic/chemical origin, which is a common basis for classification questions: auxins are indole-ring compounds (built on the tryptophan/indole skeleton), gibberellins are diterpenoids (isoprenoid/terpene-derived), cytokinins (like kinetin) are adenine (purine) derivatives, and abscisic acid is produced via oxidative cleavage of carotenoid pigments in plastids.
Step-by-Step Solution
- Kinetin (A): a synthetic cytokinin built on a modified adenine ring → Adenine derivatives (IV).
- ABA (B): synthesised from a carotenoid precursor (via xanthoxin) rather than directly through the terpenoid (mevalonate) route used for GA → Carotenoids (II).
- GA3 (C): gibberellins are diterpenoids, built from isoprenoid/terpene units → Terpenes (I). …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Assertion (A) : Transport of plant growth regulators are always strictly polarized and unidirection. Reason (R) : Organic and mineral nutrients undergo multi directional transport. Identify the correct option from the following (A) (A) and (R) are true. (R) is correct explanation for (A) (B) (A) and (R) are true. But (R) is not correct explanation for (A) (C) (A) is true, but (R) is false (D) (A) is false, but (R) is true
›Reveal solutionSolution
Not all plant hormones move unidirectionally (only auxin is strictly polar), so the Assertion is false; nutrient transport genuinely is multidirectional, so the Reason is true. The answer is (D).
Concept and Intuition
Auxin is the classic example of polar transport — it moves basipetally (tip-to-base) through parenchyma cells using energy-dependent, unidirectional carrier proteins, regardless of the direction of gravity. But this is a special, auxin-specific property, not a universal rule for all plant growth regulators: gibberellins, cytokinins, and ABA are transported through both the xylem and phloem in whichever direction the mass flow of sap is going (acropetally and basipetally), i.e., non-polarly. Meanwhile, organic nutrients (sugars, amino acids) and minerals move via the phloem/xylem according to source-sink relationships, which change with the plant's developmental stage and organ demand — hence genuinely multidirectional.
Step-by-Step Solution
- Evaluate the Assertion: transport of PGRs is always strictly polarized and unidirectional. Since only auxin is truly polar while other PGRs are transported non-polarly, this generalization is FALSE. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Choose the correct statements among the following (A) ABA plays an important role in stratification. (B) Decapitation result in growth of lateral buds. (C) Kinetin naturally occurs in the plant. (D) Vernalization promotes flowering by high temperature.
›Reveal solutionSolution
Decapitation removes the apical bud's auxin source, releasing lateral buds from apical-dominance suppression so they grow — a solid, well-known fact, unlike the false statements about kinetin's natural occurrence or vernalization needing high temperature. Answer: (B).
Concept and Intuition
Apical dominance is the phenomenon where the growing apical (terminal) bud of a shoot suppresses the growth of lateral (axillary) buds below it, primarily through auxin produced at the tip moving down and inhibiting lateral bud outgrowth. If the shoot apex is removed (decapitation), the source of this inhibitory auxin is eliminated, and the lateral buds — no longer suppressed — begin to grow (this is exactly the principle exploited in horticultural pruning/pinching to make plants bushier). By contrast, kinetin was the first cytokinin discovered, isolated from autoclaved (degraded) herring sperm DNA in the lab — it is not a naturally occurring plant hormone; the naturally occurring cytokinin in plants is zeatin (first isolated from corn/maize). Vernalization refers to the promotion of flowering by exposure to low (cold) temperature during a specific developmental stage, not high temperature.
Step-by-Step Solution
- Statement (A), ABA and stratification: ABA is the hormone responsible for inducing/maintaining dormancy; stratification (cold treatment) is used to overcome that dormancy, so framing ABA as itself playing a promoting role in stratification is not the standard, unambiguous fact tested here. …
- AP EAPCET 2025Set ap-2025-05-20-FN1 markMCQQ.Match the following List - I / List - II A. 2, 4-D - I. Brewing Industry B. GA3 - II. Respiratory climactic C. Cytokinin - III. Delay of leaf senescence D. Ethylene - IV. Dicotyledon weed killer (A) A-IV, B-I, C-III, D-II (B) A-IV, B-III, C-II, D-I (C) A-I, B-II, C-III, D-IV (D) A-I, B-III, C-II, D-IV
›Reveal solutionSolution
Matching each plant hormone to its classic application/effect gives 2,4-D–dicot weed killer, GA₃–brewing industry, cytokinin–delays senescence, ethylene–respiratory climacteric, i.e. A-IV, B-I, C-III, D-II, option (A).
Concept and Intuition
Each major plant hormone has a signature textbook application: 2,4-D is a synthetic auxin herbicide selective for broadleaf (dicot) weeds; gibberellins (GA₃) are used industrially to speed up seed germination/malting in the brewing industry; cytokinins are best known for retarding senescence (keeping tissues youthful); ethylene is the ripening hormone responsible for the climacteric respiratory burst in fruits.
Step-by-Step Solution
- A) 2,4-D — a synthetic auxin widely used as a selective herbicide that kills dicotyledonous weeds without harming monocot crops → IV) Dicotyledon weed killer.
- B) GA₃ — used commercially to accelerate malting (converting barley starch to sugars) in the brewing industry → I. …
- AP EAPCET 2024Set ap-2024-05-16-AN1 markMCQQ.Study the Lists and find correct pairs I. Terpene - GA3 II. Adenine derivative - Kinetin III. Carotenoid - ABA IV. Purine derivative - Ethylene (A) III, IV (B) II, III (C) I, II, III (D) I, III, IV
›Reveal solutionSolution
The question asks to match plant hormones with their chemical classes; the correct pairs are II (adenine derivative → kinetin) and III (carotenoid → ABA), so the answer is option (B).
The key concept here is chemical classification of plant growth regulators. Each hormone belongs to a specific biosynthetic family — knowing this lets you spot mismatches. For example, gibberellins are terpenoids, not terpenes (a subtle but important distinction), and ethylene is a simple hydrocarbon, not a purine derivative. Let’s check each pair.
-
I. Terpene – GA3
Gibberellic acid (GA3) is a diterpenoid, which is derived from the terpenoid pathway. However, “terpene” usually refers to simple hydrocarbons like monoterpenes; gibberellins are more complex terpenoids. In standard plant physiology, gibberellins are classified as terpenoids, not simply terpenes. This pair is incorrect because the term “terpene” is too narrow.
-
II. Adenine derivative – Kinetin
Kinetin is a synthetic cytokinin, and cytokinins are adenine derivatives (a purine base). This pair is correct.
-
III. Carotenoid – ABA …
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- AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQQ.Choose the correct combinations from the following A. Induce sprouting of Potato tubers – Ethylene B. Kill the dicot weeds - Cytokinin C. Increase in stalk length of grapes – Gibberellins D. Induce apical dominance – Auxins (A) A C D (B) B D C (C) B C D (D) A B C
›Reveal solutionSolution
Ethylene breaks potato-tuber dormancy, gibberellins elongate grape stalks, and auxins cause apical dominance — all correct; only "cytokinin kills dicot weeds" is wrong (that is a synthetic auxin's job, e.g. 2,4-D).
Concept and Intuition
Each plant hormone has a signature set of physiological effects that are frequently tested: ethylene governs ripening/senescence/dormancy-breaking; gibberellins govern elongation growth; auxins govern tropisms, apical dominance, and (as synthetic analogues) selective herbicide action; cytokinins govern cell division and delay senescence — none of which includes selective weed-killing.
Step-by-Step Solution
- A. Induce sprouting of potato tubers – Ethylene: ethylene is documented to break the dormancy of buds/seeds and specifically to induce sprouting in potato tubers — correct.
- B. Kill the dicot weeds – Cytokinin: the actual agent used to selectively kill dicot weeds (while sparing monocot crops like cereals) is a synthetic auxin, 2,4-D — cytokinin plays no such role, making this combination wrong.
- C. Increase in stalk length of grapes – Gibberellins: gibberellins are commercially sprayed on grapes specifically to increase the length of the fruit stalk (and fruit size) — correct. …
- AP EAPCET 2021Set ap-2021-09-03-FN1 markMCQQ.Match the following? Column - I:(a) Francis Darwin(b) F.W. Went(c) E. Kurosawa(d) F. Skoog. Column - II:(i) Coconut milk(ii) Rice seedling(iii) Coleoptiles of canary grass(iv) Oats seedlings. Column - III: (p) Auxin (q) Phototropism (r) Kinetin (s) Gibberellic acid (A) (a – iii – q), (b – iv – p), (c – ii – s), (d – i – r) (B) (a – iv – p), (b – iii – r), (c – i – q), (d – ii – s) (C) (a – ii – s), (b – i – q), (c – iii – p), (d – iv – r) (D) (a – i – r), (b – ii – s), (c – iv – p), (d – iii – q)
›Reveal solutionSolution
This tests the classic history of phytohormone discovery — matching each scientist to their experimental material and the hormone/phenomenon they are credited with.
Concept and Intuition
Each major plant hormone has a well-known discovery story tied to a specific plant material:
- Francis Darwin (with Charles Darwin) studied the bending of canary-grass (Phalaris) coleoptiles toward light — the classic phototropism experiments that eventually led to the discovery of auxin.
- F.W. Went extracted the growth-promoting substance from the tips of oat (Avena) coleoptiles and named it auxin, using the now-famous agar-block diffusion technique.
- E. Kurosawa studied "bakanae" (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi, which led to the discovery of gibberellic acid.
- F. Skoog and co-workers discovered the cell-division-promoting substance kinetin; coconut milk (liquid endosperm) was found to be a rich natural source of cytokinin-like activity used in their tissue-culture work.
Step-by-Step Solution
- Match Francis Darwin → coleoptiles of canary grass (iii) → phototropism (q).
- Match F.W. Went → oat seedlings (iv) → Auxin (p). …
- AP EAPCET 2021Set ap-2021-09-07-FN1 markMCQQ.Select incorrect pair with hormone and function? (A) Auxin induces parthenocarpy (B) Gibberellin induces bolting (C) Ethylene respiratory climactic (D) Cytokinin induces apical dominance
›Reveal solutionSolution
Cytokinins promote lateral bud growth and counteract apical dominance — the opposite of what option (D) claims — making it the incorrect hormone–function pair.
Concept and Intuition
Each plant hormone has well-established signature effects: auxins induce seedless fruit development (parthenocarpy) and also maintain apical dominance by suppressing lateral buds; gibberellins cause bolting (rapid internode elongation before flowering) in rosette plants; ethylene triggers the respiratory climacteric (a burst of respiration accompanying fruit ripening); cytokinins, in contrast, promote cell division and lateral bud growth, and are known to overcome apical dominance rather than cause it.
Step-by-Step Solution
- (A) Auxin induces parthenocarpy — correct, well documented (e.g., in tomato, using auxin sprays to get seedless fruit).
- (B) Gibberellin induces bolting — correct, classic effect in cabbage/rosette plants.
- (C) Ethylene → respiratory climacteric — correct, ethylene triggers the ripening-associated rise in respiration in climacteric fruits. …
- AP EAPCET 2021Set ap-2021-09-06-FN1 markMCQQ.Match the following? Column I:(a) Francis Darwin(b) F. W. Went(c) E. Kurosava(d) F. Skoog Column II:(i) Kinetin(ii) Gibberellin(iii) Phototropism(iv) Auxin (A) (a - iv), (b - iii), (c - ii), (d - i) (B) (a - iii), (b - iv), (c - ii), (d - i) (C) (a - i), (b - ii), (c - iii), (d - iv) (D) (a - ii), (b - i), (c - iv), (d - iii)
›Reveal solutionSolution
Francis Darwin is linked to phototropism, F.W. Went to auxin, E. Kurosawa to gibberellin, and F. Skoog to kinetin — matching to (B).
Concept and Intuition
The history of plant hormone discovery is a classic NCERT topic: Charles Darwin and his son Francis Darwin observed canary grass coleoptiles bending toward light and attributed it to a transmissible influence from the tip — the phenomenon later named phototropism. Frits W. Went extracted this diffusible substance from Avena coleoptile tips onto agar blocks and named it auxin, formally identifying the hormone responsible for the bending. E. Kurosawa, studying the "bakanae" (foolish seedling) disease of rice caused by the fungus Gibberella fujikuroi, isolated the causal chemical, later named gibberellin. Folke Skoog, together with Carlos O. Miller, isolated kinetin from autoclaved herring sperm DNA while working on cell division factors, thereby discovering the cytokinin class.
Step-by-Step Solution
- Francis Darwin — associated with the coleoptile bending observations that defined phototropism (iii). …
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