BiologyISC Class 11 · Plant Growth and Development

Plant Growth Regulators: Auxins, Gibberellins, Cytokinins, Ethylene and ABA

Updated 17 October 2026 · Fact-checked

Plant growth regulators are chemical substances in small amounts that control growth. Auxins promote cell elongation, rooting and apical dominance; gibberellins promote stem elongation and bolting; cytokinins promote cell division and delay senescence; ethylene ripens fruits; abscisic acid is a stress hormone causing stomatal closure and dormancy.

Match each hormone to its main job: auxin to the tip, gibberellin to the stem, cytokinin to buds, ethylene to fruit, abscisic acid to stomata.

Illustration drawn for QuestionBuddy.

Understand Plant Growth Regulators and Photoperiodism

Auxins. Discovered from the coleoptile: Charles Darwin and Francis Darwin observed that the coleoptile tip causes bending towards light; F. W. Went isolated the substance from the tips of oat seedlings. Natural auxin: indole-3-acetic acid (IAA); synthetic: NAA, 2,4-D. Effects: cell elongation, apical dominance (the apical bud inhibits the growth of lateral buds), initiation of roots in stem cuttings, prevention of fruit and leaf drop, parthenocarpy (seedless fruit, e.g. tomato), and 2,4-D as a weedkiller for dicots.

Gibberellins. Discovered from the "bakanae" (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi (Kurosawa). GA₃ is the best known. Effects: increase the length of stems (sugarcane: increased yield), bolting in cabbage and beet (elongation of internodes before flowering), seedless fruits (grapes), improved malting in brewing, breaking of seed dormancy.

Cytokinins. Skoog and Miller discovered kinetin from degraded DNA; natural cytokinin zeatin from maize. Effects: promote cell division (cytokinesis), release of lateral buds from apical dominance, delay of leaf senescence, and morphogenesis in tissue culture (shoot formation at a high cytokinin to auxin ratio).

Ethylene. A gaseous hormone. Effects: promotes ripening of fruits (tomato, apple), breaks seed and bud dormancy, increases the number of female flowers in cucumber, promotes root hair formation, and causes leaf abscission; ethephon releases ethylene.

Abscisic acid (ABA). A stress hormone: closes stomata under water stress, induces seed dormancy, inhibits seed germination and promotes abscission and senescence; it acts as an antagonist to gibberellins.

Photoperiodism and vernalisation. Photoperiodism is the response of flowering to the duration of day and night: short-day plants (SDP: Xanthium, chrysanthemum) flower when the day is shorter than a critical length and long-day plants (LDP: wheat, spinach) when it is longer; day-neutral plants are not affected. The darkness length is critical. Vernalisation is the promotion of flowering by a period of low temperature (winter varieties of wheat, rye).

Plant Growth Regulators and Photoperiodism: key facts

Auxins: $I AA (na t u r a l) ∣ N AA, 2, 4 - D (sy n t h e t i c)$; Elongation, apical dominance, rooting, parthenocarpy.
Gibberellins: GA₃ | Bakanae disease of rice; Stem elongation, bolting, malting.
Cytokinins: $K in e t in ∣ Z e a t in$; Cell division, delay of senescence.
Ethylene and ABA: Ethylene: fruit ripening | ABA: stomatal closure, dormancy; ABA is an antagonist to GA.

Plant Growth Regulators and Photoperiodism 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.

Seedless fruits and heavy crops

Suppose a farmer wants seedless grapes with large bunches and a sugarcane crop with taller canes.

Name the hormone for each and explain.

Try it first, then show the solutionSolution

Gibberellins (GA₃) increase the length of grape stalks and fruit size, and produce seedless fruit.
Spraying sugarcane with gibberellins increases the length of the stem and so the yield of sugar.
Auxins can also cause parthenocarpy in tomato.

Answer: Gibberellins for both; auxins can give seedless tomatoes.

Memory hook: Gibberellin means stem elongation.

Tricks to remember Plant Growth Regulators and Photoperiodism

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

A-G-C-E-A
Auxin (elongation), Gibberellin (bolting), Cytokinin (division), Ethylene (ripening), Abscisic acid (stress).
Use it when: Matching hormones with function.
Dark counts
Flowering depends on the length of the night period.
Use it when: Photoperiodism questions.

Plant Growth Regulators and Photoperiodism: solved examples

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

Example 1: Fruit ripening hormone · 1 mark · MCQBoard level

The hormone that promotes the ripening of fruits is:
(a) auxin
(b) gibberellin
(c) ethylene
(d) cytokinin

Try it first, then show the solutionSolution

Ethylene, a gaseous hormone, promotes ripening.

Answer: (c) ethylene

Example 2: Auxin effects · 3 marks · Short answerBoard level

Name the natural auxin and the scientist who isolated auxin from oat coleoptile tips, and give three physiological effects of auxins.

Try it first, then show the solutionSolution

F. W. Went isolated the growth-promoting substance (auxin) from oat coleoptile tips; the natural auxin later identified chemically is IAA (indole-3-acetic acid). (1 mark)
Effects: cell elongation and apical dominance. (1 mark)
Initiates rooting in cuttings, prevents fruit and leaf drop, causes parthenocarpy. (1 mark)

Answer: Auxin was isolated by F. W. Went from oat coleoptile tips; the natural auxin is IAA. Effects: elongation, apical dominance, rooting.

Example 3: Apical dominance · 4 marks · Short answerHarder board level

What is apical dominance? Which hormone causes it and which hormone reverses it? How does a gardener use this knowledge?

Try it first, then show the solutionSolution

Apical dominance is the suppression of lateral bud growth by the apical bud. (1 mark)
Auxin from the apical bud causes it. (1 mark)
Cytokinins promote the growth of lateral buds, reversing it. (1 mark)
Pruning the tip (removing the apical bud) gives a bushy plant, as in hedges and tea. (1 mark)

Answer: Auxin inhibits lateral buds; cytokinin releases them; pruning gives a bushy plant.

Example 4: Gibberellins and ABA · 4 marks · Short answerHarder board level

Differentiate gibberellins and abscisic acid on effect on stems, on seeds and on stomata.

Try it first, then show the solutionSolution

Gibberellins promote stem elongation and bolting; ABA inhibits growth. (1.5 marks)
Seeds: gibberellins break dormancy and promote germination; ABA induces dormancy. (1.5 marks)
Stomata: ABA closes stomata under water stress; gibberellins do not. (1 mark)

Answer: Gibberellins promote growth and germination; ABA is the inhibitory stress hormone.

Example 5: Photoperiodism and vernalisation · 5 marks · Long answerHOTS / competency

Explain short-day, long-day and day-neutral plants with examples. Why are the SDP considered "long-night" plants? Define vernalisation and give one use.

Try it first, then show the solutionSolution

SDP flower when the day length is below a critical value (chrysanthemum, Xanthium); LDP when above (wheat, spinach); day-neutral plants flower regardless (tomato, cotton). (2 marks)
The critical factor is the uninterrupted night length: SDP need a long continuous dark period, and a brief flash of light in the night prevents flowering. (1.5 marks)
Vernalisation is the promotion of flowering by a period of low temperature. (0.75 mark)
It lets winter varieties of wheat and rye be sown in spring and flower normally. (0.75 mark)

Answer: SDP, LDP, day-neutral; the dark period matters; vernalisation uses cold to promote flowering.

Where students lose marks in Plant Growth Regulators and Photoperiodism

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

Mistake: Reversing the roles of auxins and cytokinins in apical dominance
Giving cytokinin as the cause.
Do this: Auxin causes apical dominance; cytokinin reverses it.
Usually costs: 2 marks.
Mistake: Naming ethylene as a liquid hormone
Wrong state.
Do this: Ethylene is a gas.
Usually costs: 1 mark.
Mistake: Confusing SDP and LDP
Wrong critical night length.
Do this: SDP need a long dark period; LDP need a short one.
Usually costs: 1 to 2 marks.

Exam tips

Make a table of the five hormones with discovery, effects and uses.
Mention the scientist or the disease for the discovery.
Give examples of plants for photoperiodic responses.
Describe vernalisation with an example.

Practice questions on Plant Growth Regulators and Photoperiodism

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
Removing the tip of a shoot makes the plant bushy because the removed tip was the source of:
(a) cytokinin
(b) ethylene
(c) auxin
(d) abscisic acid
Hint
The apical bud suppresses lateral buds.
Answer
(c) auxin
Q2 · 2 marks · Short answerBoard level
Name the hormone used to kill dicot weeds in a wheat field.
Hint
It is a synthetic auxin.
Answer
2,4-D (2,4-dichlorophenoxyacetic acid).
Q3 · 3 marks · Short answerHarder board level
State two effects of cytokinins.
Hint
Think of cell division.
Answer
They promote cell division and delay leaf senescence; they also help lateral bud growth and shoot formation in tissue culture.
Q4 · 3 marks · Short answerHarder board level
What is parthenocarpy? Name two hormones that can induce it.
Hint
Fruit without fertilisation.
Answer
Formation of fruit without fertilisation, giving seedless fruit. Auxins (tomato) and gibberellins (grapes) induce it.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Fruits ripen faster when kept with a ripe banana. Reason (R): The ripe banana releases ethylene. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Think of the gaseous hormone.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a nursery treats stem cuttings with a powder and sees early root formation.
(i) Name the hormone type used. (1 mark)
(ii) Name two examples. (1 mark)
(iii) State another use of the hormone in fruit crops. (1 mark)
(iv) State the hormone that would inhibit such growth in stress. (1 mark)
Hint
Think of auxins and ABA.
Answer
(i) Auxin. (ii) IBA and NAA (or IAA). (iii) Prevention of premature fruit drop. (iv) Abscisic acid.

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.Natural auxin?Show answer
IAA.
2.Gaseous hormone?Show answer
Ethylene.
3.Stress hormone?Show answer
Abscisic acid.
4.Hormone for cell division?Show answer
Cytokinin.

Plant Growth Regulators and Photoperiodism: frequently asked questions

What is a bolting effect?

The rapid elongation of internodes just before flowering in rosette plants such as cabbage, caused by gibberellins.

Why is ABA called an antagonist to gibberellins?

It induces dormancy and inhibits growth, whereas gibberellins break dormancy and promote growth.

What is florigen?

A hypothetical flowering hormone made in leaves in response to photoperiod and transported to the shoot apex.