BiologyICSE Class 10 · Chemical Coordination in Plants

Auxins, Gibberellins and Cytokinins: Plant Growth Promoters (ICSE Class 10 Biology)

Updated 16 October 2026 · Fact-checked

Auxins, gibberellins and cytokinins are plant growth promoters. Auxins, made at shoot tips, lengthen cells, cause apical dominance and tropic bending, and root cuttings. Gibberellins lengthen internodes, break seed dormancy and enlarge fruits such as grapes. Cytokinins, made in root tips, promote cell division and delay the ageing of leaves.

Match each hormone to where it acts: auxin at the shoot tip, gibberellin on the internodes, cytokinin at the root tip for cell division.

Illustration drawn for QuestionBuddy.

Understand Auxins, Gibberellins and Cytokinins: Growth Promoters

Plant growth regulators. Plants coordinate growth with chemicals called plant growth regulators (phytohormones). They are made in small amounts in one part, often travel to another, and act on growth and development. They fall into two groups: growth promoters (auxins, gibberellins, cytokinins) and the growth inhibitor abscisic acid; ethylene mainly controls ripening and ageing. This page covers the three promoters; the syllabus asks for 'a brief idea of the physiological effects'.

Auxins. Made mainly at shoot tips and young leaves; they move downwards.

Cell elongation in the shoot just behind the tip.
Apical dominance: auxin from the apical (terminal) bud suppresses the lateral buds. Remove the tip and the side buds grow, making the plant bushy; this is why hedges are trimmed.
Tropisms: unequal distribution causes phototropic and geotropic bending.
Rooting of stem cuttings: dipping the cut end in auxin promotes adventitious roots.
Prevents premature fall of leaves and fruits.
Parthenocarpy in some plants: fruit develops without fertilisation, so it is seedless.
Weedicides: synthetic auxins such as 2,4-D in high doses kill broad-leaved weeds while cereals such as wheat are spared.

Gibberellins.

Stem elongation by lengthening the internodes; genetically dwarf plants grow to normal height when sprayed.
Bolting: sudden elongation of the flowering stem in rosette plants such as cabbage.
Break seed dormancy and promote germination.
Increase fruit size and cause parthenocarpy: sprayed on grapes, they make bunches longer and the berries larger and elongated.
Promote flowering in some plants.

Cytokinins. Made mainly in root tips and in developing seeds and fruits.

Promote cell division (together with auxin).
Delay senescence (ageing): leaves stay green and fresh longer, so they are used on cut flowers and leafy vegetables.
Promote growth of lateral buds, acting against apical dominance.
Help break dormancy of seeds and buds.

Comparison table.

Effect	Auxin	Gibberellin	Cytokinin
Main site of production	shoot tip	young leaves, seeds	root tip, seeds
Key growth effect	cell elongation	internode elongation	cell division
Buds	apical dominance	—	lateral bud growth
Use	rooting cuttings, weedicide	bigger grapes, tall dwarf plants	fresher leaves and flowers

Interaction (an idea). Hormones rarely act alone. Auxin (from the tip) holds side buds back while cytokinin (from the root) pushes them to grow; the balance decides the shape of the plant.

Flow chart: apical dominance. Apical bud makes auxin → auxin moves down the stem → lateral buds suppressed → tip removed → auxin supply falls → lateral buds grow (helped by cytokinin) → bushy plant.

Draw-and-label checklist: apical dominance. Two plants side by side: A with intact apical bud, tall with dormant lateral buds; B with tip cut, with several side branches. Labels: apical bud, lateral bud (dormant), cut tip, lateral branches, arrow 'auxin moves down'.

Auxins, Gibberellins and Cytokinins: Growth Promoters: key facts

Auxin: Cell elongation
Apical dominance
Tropic bending
Roots on cuttings
Stops premature leaf and fruit fall
Synthetic auxin as weedicide; Made at the shoot tip.
Gibberellin: Internode elongation, dwarf plants grow tall
Bolting in cabbage
Breaks seed dormancy
Larger, elongated grapes
Parthenocarpy; Fruit size is gibberellin, not ethylene.
Cytokinin: Cell division
Delays ageing of leaves
Lateral bud growth
Breaks dormancy; Made in root tips.

How to answer Auxins, Gibberellins and Cytokinins: Growth Promoters questions

ICSE gives a farming or gardening case and asks you to name the hormone, give its effect, explain a term such as apical dominance or parthenocarpy, or pick the odd one out.

1Find the effect in the case: lengthening, more branches, bigger fruit, fresh leaves, roots on a cutting.
2Match: elongation and cuttings = auxin; bigger fruit and tall dwarfs = gibberellin; division and freshness = cytokinin.
3Name the hormone, then give the effect in one line.
4Define any technical term asked (parthenocarpy: fruit without fertilisation, seedless).
5Check you have not confused size of fruit (gibberellin) with ripening (ethylene).

Auxins, Gibberellins and Cytokinins: Growth Promoters in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Taller sugarcane

Suppose sugarcane farmers spray a gibberellin solution on young cane, and at harvest the canes are noticeably longer and the yield of sugar per hectare rises.

Explain how the spray increases the yield.

Try it first, then show the solutionSolution

Gibberellins promote stem elongation by lengthening the internodes.
The sprayed canes grow taller with longer internodes.
Sugar is stored in the stem, so longer stems hold more sugar.
More cane per plant means more sugar per hectare.

Answer: Gibberellin lengthens the internodes, giving longer canes that store more sugar.

Memory hook: Gibberellin makes Giants, even in sugarcane.

Bushy tulsi

Suppose your grandmother pinches off the top of her tulsi plant every few weeks, and it becomes thick and bushy instead of tall and thin.

Explain why.

Try it first, then show the solutionSolution

The top (apical) bud makes auxin that moves down and keeps the side buds dormant: apical dominance.
Pinching the top removes this supply of auxin.
The side buds, helped by cytokinin from the roots, start to grow.
Many side branches make the plant bushy.

Answer: Removing the apical bud ends apical dominance, so lateral buds grow into branches.

Memory hook: Cut the king, the princes grow.

Tricks to remember Auxins, Gibberellins and Cytokinins: Growth Promoters

Short memory aids. Say each one aloud once, then use it on a question the same day.

A-G-C = Apex, Giant, Cell-cycle
Auxin from the Apex; Gibberellin makes Giants (tall, big grapes); Cytokinin drives Cytokinesis (cell division).
Use it when: One-line effect of each.
Cytokinin keeps it young
C for Cytokinin, C for 'Crisp' leaves that stay green.
Use it when: Delay of senescence.
Grapes go Gibberellin
Both start with G: bigger, longer grapes.
Use it when: Avoiding the ethylene error.

Auxins, Gibberellins and Cytokinins: Growth Promoters: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Hormone and effect · 1 mark · MCQBoard level

Which pair of hormone and effect is correctly matched?
(a) cytokinin: apical dominance (b) gibberellin: delays ageing of leaves (c) auxin: rooting of stem cuttings (d) cytokinin: bolting in cabbage

Try it first, then show the solutionSolution

Apical dominance is due to auxin, delayed ageing to cytokinin and bolting to gibberellin; auxin does promote roots on stem cuttings.

Answer: (c) auxin: rooting of stem cuttings.

Example 2: Cabbages that shoot up · 3 marks · Short answerBoard level

In warm weather some cabbage plants suddenly send up a tall flowering stem, spoiling the heads for sale. (a) Name this response. (b) Name the hormone responsible. (c) Which part of the stem lengthens?

Try it first, then show the solutionSolution

(a) Bolting (1 mark).
(b) Gibberellin (1 mark).
(c) The internodes (1 mark).

Answer: (a) Bolting. (b) Gibberellin. (c) Internodes.

Example 3: Auxin paste on a cut tip · 2 marks · Short answerHarder board level

A gardener cuts off the apical bud of a plant and immediately puts a dab of auxin paste on the cut stump. Predict whether the lateral buds will grow into branches, and explain what this shows.

Try it first, then show the solutionSolution

The lateral buds stay dormant, because the auxin paste replaces the auxin that the apical bud used to send down the stem (1 mark).
This shows that apical dominance is caused by auxin from the apical bud, not by the bud itself (1 mark).

Answer: Lateral buds stay dormant; the paste supplies auxin, showing that auxin from the tip causes apical dominance.

Example 4: Opposite actions on buds · 2 marks · Short answerHarder board level

Compare the effects of auxin and cytokinin on lateral buds and state where each is mainly produced.

Try it first, then show the solutionSolution

Auxin suppresses lateral buds (apical dominance); it is made at the shoot tip (1 mark).
Cytokinin promotes growth of lateral buds; it is made mainly in root tips (1 mark).

Answer: Auxin (shoot tip) suppresses; cytokinin (root tip) promotes lateral buds.

Example 5: Dwarf pea trial · 3 marks · Case-basedHOTS / competency

Suppose a dwarf pea variety reaches 12 cm. Plants sprayed with gibberellin reach 42 cm. (a) By how many times did the height increase? (b) Which part of the stem lengthened? (c) Would the seeds of the sprayed plants give tall plants? Explain.

Try it first, then show the solutionSolution

(a) 42 ÷ 12 = 3.5 times (1 mark).
(b) The internodes (1 mark).
(c) No; the hormone changes growth, not the genes, so the offspring are still dwarf (1 mark).

Answer: (a) 3.5 times. (b) Internodes. (c) No, the dwarf gene is unchanged; the effect is not inherited.

Example 6: Selective weedkiller · 3 marks · Case-basedHOTS / competency

Suppose a wheat farmer sprays a synthetic auxin at high concentration and the broad-leaved weeds die while the wheat survives. (a) Name such a chemical. (b) Why is it called a selective weedicide? (c) Why is it unwise to spray it near a mustard field?

Try it first, then show the solutionSolution

(a) 2,4-D (1 mark).
(b) At high concentration it kills broad-leaved plants but not narrow-leaved cereals such as wheat (1 mark).
(c) Mustard is broad-leaved, so drift of the spray could damage the crop (1 mark).

Answer: (a) 2,4-D. (b) It kills broad-leaved weeds only. (c) Mustard is broad-leaved and would be harmed.

Where students lose marks in Auxins, Gibberellins and Cytokinins: Growth Promoters

The slips examiners see most often, and what to write instead.

Mistake: Ethylene for elongated grapes
CISCE examiners' comments (2025): for a hormone sprayed to elongate grapes, candidates wrote ethylene, linking it to fruit ripening, and could not recall parthenocarpy, confusing it with pollination or fertilisation.
Do this: Size and shape of fruit = gibberellin; seedless fruit without fertilisation = parthenocarpy.
Usually costs: 1-2 marks.
Mistake: Cytokinin and auxin sites swapped
Students remember both 'tips' but not which; auxin is from the shoot tip, cytokinin from the root tip.
Do this: Auxin at the Apex (top); cytokinin at the root.
Usually costs: 1 mark.
Mistake: Apical dominance without the hormone
Describing the bushy plant without naming auxin and the suppressed lateral buds leaves out the explanation.
Do this: Write: auxin from the apical bud suppresses lateral buds.
Usually costs: 1 mark.

Exam tips

Promoters: auxin, gibberellin, cytokinin.
Auxin: shoot tip; cytokinin: root tip.
Bigger grapes and tall dwarfs: gibberellin.
Parthenocarpy = seedless fruit without fertilisation.

Practice questions on Auxins, Gibberellins and Cytokinins: Growth Promoters

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The hormone mainly produced in root tips and promoting cell division is:
(a) auxin (b) cytokinin (c) gibberellin (d) ethylene
Hint
Cytokinesis.
Answer
(b) cytokinin.
Q2 · 1 mark · Very short answerBoard level
Name the region of the plant where auxin is mainly produced.
Hint
Top of the shoot.
Answer
The shoot tip (apical bud) and young leaves.
Q3 · 2 marks · Short answerHarder board level
Give two effects of auxins on plants.
Hint
Shoot tip hormone.
Answer
Cell elongation; apical dominance (also tropic bending, rooting of cuttings, preventing premature fruit fall).
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A plant whose root tips are damaged may form fewer side branches.
Reason (R): Cytokinins, made mainly in root tips, promote the growth of lateral buds.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
Where is cytokinin made, and what does it do to buds?
Answer
(c).
Q5 · 3 marks · Short answerHarder board level
Name the hormone for each: (a) breaks seed dormancy and enlarges grapes; (b) helps stem cuttings root; (c) delays yellowing of leaves.
Hint
G, A, C.
Answer
(a) Gibberellin (b) Auxin (c) Cytokinin.
Q6 · 2 marks · Short answerHarder board level
Why do tea planters regularly pluck the top buds of tea bushes?
Hint
Apical dominance.
Answer
Removing the apical buds removes the auxin that suppresses lateral buds, so many side shoots grow and the bush yields more young leaves.
Q7 · 2 marks · Short answerHarder board level
Distinguish between auxin and gibberellin with respect to their effect on stem growth.
Hint
Cell versus internode.
Answer
Auxin causes elongation of cells just behind the shoot tip and bends the stem in tropisms; gibberellin lengthens the internodes of the whole stem and can make dwarf plants tall.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose 20 stem cuttings dipped in auxin solution and 20 dipped in plain water are planted. After two weeks, 18 of the auxin-treated and 6 of the water-treated cuttings have roots.
(a) Calculate the rooting percentage for each group. (1)
(b) What is the purpose of the water group? (1)
(c) State the conclusion. (1)
Hint
Out of 20.
Answer
(a) Auxin: 18/20 × 100 = 90%; water: 6/20 × 100 = 30%. (b) It is the control, showing rooting without the hormone. (c) Auxin promotes root formation on stem cuttings.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which hormone causes apical dominance?Show answer
Auxin.
2.What is parthenocarpy?Show answer
Development of fruit without fertilisation, giving seedless fruit.
3.Which hormone delays ageing of leaves?Show answer
Cytokinin.
4.Name a synthetic auxin used as a weedicide.Show answer
2,4-D.
5.Which hormone breaks seed dormancy?Show answer
Gibberellin (cytokinin also helps).

Auxins, Gibberellins and Cytokinins: Growth Promoters: frequently asked questions

Are plant growth regulators the same as plant hormones?

For ICSE, yes. Natural ones are phytohormones; synthetic ones like 2,4-D are also called growth regulators.

Do I need chemical names like indole acetic acid?

No. The syllabus asks only for a brief idea of their physiological effects.

Can both auxin and gibberellin cause parthenocarpy?

Yes, in different plants. In the grape example, gibberellin is the expected answer.

Why are cytokinins called anti-ageing hormones?

They delay senescence, so leaves stay green and fresh for longer.