Chemical Coordination in Plants ICSE Class 10: Growth Regulators and Tropisms

What Chemical Coordination in Plants is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Chemical Coordination in Plants is in the 2. Plant Physiology (iv) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

Auxins

Auxins are produced mainly at shoot tips and young leaves and move downwards.

Effects:

• Promote cell elongation in shoots, so stems grow taller.
• Cause apical dominance: the main (apical) bud's auxin suppresses the growth of side (lateral) buds.
• Control phototropism and geotropism by unequal distribution.
• Promote root formation in stem cuttings (used by gardeners).
• Prevent premature dropping of leaves and fruits.
• Can cause parthenocarpy (seedless fruits without fertilisation) in some plants.
• Synthetic auxins (such as 2,4-D) in high doses are used as weedicides (kill broad-leaved weeds).

Worked example

A gardener pinches off the tip of a plant and it becomes bushy. Explain.

Try it first, then show the solutionSolution

1. The shoot tip produces auxin.
2. Auxin from the tip suppresses side buds (apical dominance).
3. Removing the tip removes this suppression, so side buds grow into branches.

Answer: Removing the tip ends apical dominance by auxin.

Gibberellins

Effects:

• Cause stem elongation by lengthening the internodes; they can make genetically dwarf plants grow tall, and cause bolting (sudden elongation of the flowering stem) in plants like cabbage.
• Break seed dormancy and promote germination.
• Promote parthenocarpy and increase the size of fruits: sprayed on grapes, they make the bunches longer and the berries larger and elongated.
• Help flowering in some plants.

Worked example

A grape farmer sprays a hormone at fruiting to make the grapes elongated and seedless. Name the hormone and the term for fruit formation without fertilisation.

Try it first, then show the solutionSolution

1. Gibberellins increase fruit size and elongate grape bunches.
2. Fruit forming without fertilisation is parthenocarpy.

Answer: Gibberellin; parthenocarpy.

Cytokinins

Cytokinins are produced mainly in root tips and developing seeds and fruits.

Effects:

• Promote cell division (cytokinesis), together with auxins.
• Delay ageing (senescence) of leaves, keeping them green and fresh longer (used to keep cut flowers and leafy vegetables fresh).
• Promote the growth of lateral buds, working against apical dominance.
• Help break dormancy of seeds and buds.

Worked example

Which hormone would you use to keep harvested spinach green for longer? Why?

Try it first, then show the solutionSolution

1. Leaves turn yellow as they age (senescence).
2. Cytokinins delay senescence.

Answer: Cytokinin.

Abscisic acid and ethylene

Abscisic acid (ABA), a growth inhibitor:

• Induces dormancy of seeds and buds (e.g. before winter or dry season).
• Causes closing of stomata during water shortage, so it is called the stress hormone.
• Promotes abscission (falling) of leaves, flowers and fruits.
• Opposes the effects of growth promoters like gibberellins.

Ethylene, a gaseous hormone:

• Promotes ripening of fruits (e.g. mango, banana, tomato).
• Promotes abscission and ageing of leaves and flowers.

Ripe fruits release ethylene, which is why one ripe banana in a bag speeds up the ripening of unripe ones.

Worked example

Why is abscisic acid called the stress hormone?

Try it first, then show the solutionSolution

1. In drought, ABA levels rise.
2. ABA causes stomata to close, reducing water loss.
3. It also induces dormancy to survive bad conditions.

Answer: It helps the plant survive stress such as drought, mainly by closing stomata.

Tropic movements

Tropism	Stimulus	Example
Phototropism	light	shoot bends towards light (positive); roots grow away from light (negative)
Geotropism	gravity	roots grow down (positive); shoots grow up (negative)
Hydrotropism	water	roots grow towards water (positive)
Thigmotropism	touch	tendrils of pea, cucumber and grapevine coil around a support
Chemotropism	chemicals	pollen tube grows towards the ovule, attracted by chemicals

A tropism is a directional growth movement of a plant part in response to a stimulus.

Worked example

A pea plant's thin coiling structures wind around a stick. Name the structures and the tropism.

Try it first, then show the solutionSolution

1. The coiling structures are tendrils (modified leaflets in pea).
2. They respond to the touch of the support by unequal growth.
3. Response to touch: thigmotropism.

Answer: Tendrils; thigmotropism.

How auxin causes phototropism

When light falls on a shoot from one side:
1. Auxin moves to the shaded side of the shoot tip.
2. Cells on the shaded side elongate more than those on the lit side.
3. The shoot therefore bends towards the light.

If the tip is removed or covered with an opaque cap, the shoot does not bend, because the tip senses light and makes the auxin. In roots, high auxin concentrations inhibit elongation, which is part of why roots respond differently from shoots.

Worked example

A seedling on a window sill bends towards the window. On which side of the stem are the cells longer, and why?

Try it first, then show the solutionSolution

1. Auxin collects on the side away from the light (shaded side).
2. Auxin makes those cells elongate more.
3. Longer cells on the shaded side push the tip towards the light.

Answer: The shaded side; it has more auxin.

Chemical Coordination in Plants in real life

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

Topics in this chapter

• Tropic Movements in Plants
• Auxins, Gibberellins and Cytokinins: Growth Promoters
• Abscisic Acid and Ethylene in Plants

Chemical Coordination in Plants: key facts

Phototropism chain

$\text{one-sided light}\to \text{auxin to shaded side}\to \text{more elongation there}\to \text{bends to light}$

Shoot tip senses light.

Promoters and inhibitor

$\text{promoters: auxin, gibberellin, cytokinin};\text{inhibitor: abscisic acid}$

Ethylene: ripening and abscission.

Sign of a tropism

$\text{towards stimulus}=+;\text{away}=-$

Root: +geotropic, +hydrotropic; shoot: +phototropic, -geotropic.

Chemical Coordination in Plants: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Chemical Coordination in Plants

The question patterns to expect, and the method that works for each.

Tricks to remember Chemical Coordination in Plants

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

• A-G-C grow, ABA stops, E ripens

  Auxin, Gibberellin, Cytokinin promote growth; Abscisic acid stops it; Ethylene ripens.

  Use it when: Classifying hormones.

• Gibberellin = Giant

  Gibberellins make dwarf plants giant and grapes large.

  Use it when: Gibberellin effects.

• Cytokinin = Cell division + Can't age

  Cytokinins drive cell division and delay ageing.

  Use it when: Cytokinin effects.

• Thigmo = touch (think 'thumb')

  Thigmotropism: tendrils feel the support like a thumb.

  Use it when: Naming tropisms.

Chemical Coordination in Plants: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Chemical Coordination in Plants

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

• ✗Mistake: Phototropism for a tendril

  CISCE examiners' comments, 2024: for pea plants using a support, candidates wrote phototropism, missed 'touch stimulus' and 'growth movement' in the definition, and named the structure as stem or creeper instead of tendril.

  Wrong working

  Pea plant climbing a stick: phototropism; structure: stem.

  Right working

  Thigmotropism (growth movement in response to touch); structure: tendril.

  ✓Do this: Coiling around a support = touch = thigmotropism; name the tendril.

  Usually costs: 1-2 marks.

• ✗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.

  Wrong working

  Ethylene elongates grapes.

  Right working

  Gibberellin elongates and enlarges grapes; seedless fruit without fertilisation is parthenocarpy.

  ✓Do this: Size of fruit = gibberellin; ripening = ethylene.

  Usually costs: 1-2 marks.

• ✗Mistake: Calling a tropism a quick movement

  A slip worth checking in your own answer (not taken from a CISCE report): tropisms are slow growth movements, so 'growth' must appear in the definition.

  Wrong working

  Phototropism is the movement of a plant towards light.

  Right working

  Phototropism is the directional growth movement of a plant part in response to light.

  ✓Do this: Include 'growth' and 'direction of the stimulus'.

  Usually costs: 1 mark.

• ✗Mistake: Signs of geotropism reversed

  A slip worth checking in your own answer (not taken from a CISCE report): shoots grow against gravity, so they are negatively geotropic.

  Wrong working

  Shoots are positively geotropic.

  Right working

  Shoots: negatively geotropic; roots: positively geotropic.

  ✓Do this: Towards = positive; roots go towards gravity.

  Usually costs: 1 mark.

Practice questions on Chemical Coordination in Plants

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

1. Q1 · 1 mark · MCQBoard level

   The hormone that promotes cell division is:
   (a) cytokinin
   (b) abscisic acid
   (c) ethylene
   (d) auxin

   Hint

   Cyto-kinesis.

   Answer

   (a)

2. Q2 · 1 mark · MCQBoard level

   Roots growing towards a leaking water pipe show:
   (a) geotropism
   (b) hydrotropism
   (c) chemotropism
   (d) phototropism

   Hint

   Water.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerHarder board level

   Odd one out, with the category of the rest: auxin, abscisic acid, gibberellin, cytokinin.

   Hint

   Promoters vs inhibitor.

   Answer

   Abscisic acid; the rest are growth promoters.

4. Q4 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Removing the tip of a shoot makes a plant bushy.
   Reason (R): Cytokinins from the tip suppress the lateral buds.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Which hormone gives apical dominance?

   Answer

   (c): auxin from the tip suppresses lateral buds.

5. Q5 · 2 marks · Very short answerBoard level

   Name the hormone: (a) used as a weedicide in synthetic form; (b) that induces dormancy; (c) that is a gas; (d) that makes dwarf plants tall.

   Hint

   Half mark each.

   Answer

   (a) Auxin (2,4-D) (b) Abscisic acid (c) Ethylene (d) Gibberellin

6. Q6 · 2 marks · Short answerHarder board level

   Explain positive geotropism of roots and negative geotropism of shoots with examples.

   Hint

   Gravity.

   Answer

   Roots grow downwards, in the direction of gravity (positive geotropism), anchoring the plant; shoots grow upwards, against gravity (negative geotropism), towards light.

7. Q7 · 2 marks · Short answerHarder board level

   Why do florists use cytokinin sprays on cut flowers and leafy greens?

   Hint

   Senescence.

   Answer

   Cytokinins delay senescence (ageing), so leaves and petals stay fresh and green for longer.

8. Q8 · 3 marks · Short answerHOTS / competency

   A potted plant is laid on its side in the dark for a few days. Predict the direction of growth of the root and shoot, and name the responses.

   Hint

   No light, only gravity.

   Answer

   The root tip bends downwards (positive geotropism); the shoot tip bends upwards (negative geotropism). Light plays no part since it is dark.

9. Q9 · 3 marks · Short answerHOTS / competency

   Unripe tomatoes picked green ripen during transport in closed trucks. Explain, and suggest how a trader could slow down ripening.

   Hint

   Gas build-up.

   Answer

   Tomatoes release ethylene, which builds up in the closed truck and promotes ripening. Ventilating the truck (removing ethylene) and keeping it cool would slow ripening.

10. Q10 · 3 marks · Short answerHarder board level

    Name the tropism and the plant part involved in each: (a) pollen tube; (b) cucumber tendril; (c) sunflower stem in a field.

    Hint

    Chemicals, touch, light.

    Answer

    (a) Chemotropism (b) Thigmotropism (c) Phototropism

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. 1.Hormone responsible for apical dominance?Show answer

   Auxin.

2. 2.Hormone that breaks seed dormancy and causes bolting?Show answer

   Gibberellin.

3. 3.Hormone that delays leaf ageing?Show answer

   Cytokinin.

4. 4.Gaseous plant hormone?Show answer

   Ethylene.

5. 5.Growth inhibitor hormone?Show answer

   Abscisic acid.

6. 6.Growth of a pollen tube towards the ovule?Show answer

   Chemotropism.

7. 7.Stimulus in hydrotropism?Show answer

   Water.

8. 8.Roots: geotropism sign?Show answer

   Positive.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give two effects each of auxin, gibberellin, cytokinin, ABA and ethylene?
• ☐Can you explain apical dominance and parthenocarpy?
• ☐Can you name the tropism, stimulus and sign for roots, shoots, tendrils and pollen tubes?
• ☐Can you explain phototropism using auxin?

Chemical Coordination in Plants: frequently asked questions