BiologyCBSE Class 11 · Plant Growth and Development

Plant Growth Regulators: Auxin, Gibberellin, Cytokinin, Ethylene and ABA

Updated 17 October 2026 · Fact-checked

Plant growth regulators are small, simple organic molecules that regulate growth and development. Auxins promote rooting, apical dominance and parthenocarpy; gibberellins promote stem elongation and bolting; cytokinins promote cell division and delay senescence; ethylene promotes fruit ripening; abscisic acid causes stomatal closure and dormancy. Promoters are auxins, gibberellins and cytokinins; inhibitors are ABA and ethylene.

Understand Plant Growth Regulators: Types and Effects

PGRs. Small, simple molecules of diverse chemical composition (indole compounds, adenine derivatives, terpenes, carotenoid derivatives or gases). They are plant growth promoters (auxins, gibberellins, cytokinins: cell division, enlargement, flowering, fruiting, seed formation) and inhibitors (abscisic acid, and ethylene, which can be a promoter too; they work in stress and dormancy).

Auxins. First isolated from human urine; IAA (indole-3-acetic acid) and IBA are natural; NAA and 2,4-D are synthetic. Discovery: Charles and Francis Darwin (coleoptile bending towards light); F. W. Went isolated auxin from tips of oat (Avena) coleoptiles.

Initiate rooting in stem cuttings; promote flowering (pineapple); prevent early fruit and leaf drop but promote abscission of older mature leaves and fruits.
Apical dominance: the apical bud suppresses growth of lateral buds; removal of the shoot tip (as in tea plantations, hedge-making) helps lateral buds grow.
Produce seedless fruits (parthenocarpy), e.g. tomato.
2,4-D is a weedicide for dicots; controls weeds in lawns; helps in cell division in culture; xylem differentiation.

Gibberellins. Over 100 known (GA₃ the best studied). Discovery: Kurosawa studied the 'bakanae' (foolish seedling) disease of rice caused by the fungus *Gibberella fujikuroi*; GA isolated later.

Cause elongation of stems; in grapes increase length of stalks; sugarcane stems elongate, raising yield.
Delay senescence of fruits, extending marketing duration.
Bolting in cabbage and beet, the sudden stem elongation before flowering; speed up malting in brewing; break dormancy of seeds and buds.

Cytokinins. Promote cell division (cytokinesis). Discovery: Skoog and Miller with kinetin (from herring sperm DNA); zeatin from maize kernels and coconut milk.

Help in producing new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation; overcome apical dominance; promote nutrient mobilisation; delay leaf senescence.

Ethylene. A gaseous PGR; discovered by Cousins, who found ripened oranges hastened ripening of unripe bananas. Produced in large amounts in ripening fruits and senescing tissues.

Promotes fruit ripening (tomato, apple), senescence and abscission of leaves and flowers; breaks seed and bud dormancy; initiates germination in peanut, sprouting of potato tubers; promotes root growth and root hair formation; promotes female flowers in cucumber; promotes flowering in pineapple and mango. Ethephon (releases ethylene) is used in agriculture.

Abscisic acid (ABA). A stress hormone.

Causes stomatal closure during water stress; inhibits seed germination; promotes seed dormancy and helps seeds withstand desiccation; increases tolerance to stress; stimulates abscission; acts as an antagonist to gibberellins.

Table: promoters versus inhibitors. Auxin, GA, cytokinin: promote growth. ABA and ethylene: inhibit.

Draw and label: Went's Avena coleoptile experiment (agar block with auxin placed off-centre causing curvature).

Plant Growth Regulators: Types and Effects: key facts

Auxins: $I AA, I B A (na t u r a l) ∣ N AA, 2, 4 - D (sy n t h e t i c)$; Rooting, apical dominance, parthenocarpy
Gibberellin: GA3
stem elongation
bolting
malting; Bakanae of rice
Cytokinin: Zeatin, kinetin
cell division
delays senescence; Overcomes apical dominance
Stress PGRs: Ethylene: ripening | ABA: stomatal closure, dormancy; Inhibitors

How to answer Plant Growth Regulators: Types and Effects questions

Questions give an effect and ask which hormone, or ask for uses.

1Match the effect with the hormone: elongation = GA; division = cytokinin; ripening = ethylene; stress = ABA; rooting and dominance = auxin.
2Give the example crop or plant.
3Mention synthetic versus natural where asked.
4State antagonists: auxin and cytokinin on apical dominance; GA and ABA on dormancy.
5Link discoveries to scientists.

Plant Growth Regulators: Types and Effects 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.

Ripening in a paper bag

Suppose a hard green mango is kept in a closed paper bag with a ripe banana and ripens in two days, while one kept alone takes a week.

Explain using a PGR.

Try it first, then show the solutionSolution

Ripe fruits release the gaseous hormone ethylene.
Ethylene in the closed bag speeds up ripening of the mango.
Gas-tight space keeps its concentration high.

Answer: Ethylene accelerates ripening.

Memory hook: Gas hormone spreads and ripens.

Tricks to remember Plant Growth Regulators: Types and Effects

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

AGCEA
Auxin, Gibberellin, Cytokinin, Ethylene, ABA.
Use it when: The five PGRs
Elongate = GA
Gibberellin elongates stems.
Use it when: Effects
Ethylene is the gas
Only gaseous PGR.
Use it when: Ethylene

Plant Growth Regulators: Types and Effects: solved examples

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

Example 1: Apical dominance · 3 marks · Short answerBoard level

What is apical dominance? Which hormone causes it, and how does cytokinin affect it?

Try it first, then show the solutionSolution

The apical bud suppresses growth of lateral buds (1 mark).
Auxin from the apical bud is responsible (1 mark).
Cytokinin promotes lateral bud growth, overcoming apical dominance (1 mark).

Answer: Auxin causes it; cytokinin opposes it.

Example 2: Uses of auxin · 2 marks · Very short answerBoard level

Give four agricultural uses of auxins.

Try it first, then show the solutionSolution

Rooting of stem cuttings; prevents early fruit and leaf drop (1 mark).
Seedless fruits by parthenocarpy; 2,4-D as a weedicide for dicots (1 mark).

Answer: Rooting, abscission control, parthenocarpy, weedicide.

Example 3: Match effects · 4 marks · Short answerHarder board level

Name the PGR for each: (a) bolting in cabbage (b) closing stomata in drought (c) delaying leaf senescence (d) ripening of tomato.

Try it first, then show the solutionSolution

(a) Gibberellin (1 mark).
(b) Abscisic acid (1 mark).
(c) Cytokinin (1 mark).
(d) Ethylene (1 mark).

Answer: GA, ABA, cytokinin, ethylene.

Example 4: Discoveries · 3 marks · Short answerHarder board level

Name the scientist and discovery for bakanae disease, Went's experiment and kinetin.

Try it first, then show the solutionSolution

Bakanae: Kurosawa found the rice disease caused by Gibberella; later gibberellin isolated (1 mark).
Went isolated auxin from oat coleoptile tips (1 mark).
Skoog and Miller found kinetin as a cell division factor (1 mark).

Answer: Kurosawa; Went; Skoog and Miller.

Example 5: Tea plantation · 5 marks · Long answerHOTS / competency

Suppose a tea grower pinches off shoot tips to get a bushier plant with more leaves. Explain the physiology, name the hormones involved and predict what happens if auxin is applied to the cut tip.

Try it first, then show the solutionSolution

Apical bud makes auxin that suppresses lateral buds (1 mark).
Removing the tip removes the source of auxin (1 mark).
Lateral buds grow, giving a bushy plant (1 mark).
Cytokinins promote lateral bud growth (1 mark).
Applying auxin to the cut tip would restore suppression of lateral buds (1 mark).

Answer: Auxin suppresses laterals; pinching releases them.

Where students lose marks in Plant Growth Regulators: Types and Effects

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

Mistake: Ethylene as liquid
State confused.
Wrong working
Ethylene is a liquid hormone.
Right working
Ethylene is a gas.
Do this: Gas.
Usually costs: 1 mark.
Mistake: ABA promotes growth
Role reversed.
Wrong working
ABA promotes cell division.
Right working
ABA is an inhibitor.
Do this: Stress.
Usually costs: 1 mark.
Mistake: Cytokinin for rooting
Roles mixed.
Wrong working
Cytokinin initiates rooting of cuttings.
Right working
Auxin initiates rooting; cytokinin promotes division.
Do this: Auxin roots.
Usually costs: 1 mark.

Exam tips

Learn one effect and one example for each PGR.
Link discoveries to named scientists.
Mention antagonists.
Give 2,4-D as a weedicide for dicots.

Practice questions on Plant Growth Regulators: Types and Effects

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 weedicide 2,4-D is a synthetic
(a) gibberellin (b) cytokinin (c) auxin (d) ethylene
Hint
Dicot weed control.
Answer
(c).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): ABA is called a stress hormone.
Reason (R): It closes stomata and promotes tolerance to stress.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Water stress.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
What is parthenocarpy? Which PGR can induce it?
Hint
Seedless fruit.
Answer
Formation of fruit without fertilisation; auxin can induce it, as in tomato.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose P stretches grape stalks; Q is a gas that ripens fruit; R closes stomata in drought.
(a) Name P, Q, R. (1)
(b) Which is an inhibitor of seed germination? (1)
(c) Name one use of P in sugarcane. (2)
Hint
GA, ethylene, ABA.
Answer
(a) P gibberellin, Q ethylene, R ABA. (b) ABA. (c) Gibberellin spray elongates stems and so increases sugar yield.
Q5 · 3 marks · Short answerHOTS / competency
Suppose seeds of a plant fail to germinate in winter until a cold period passes. Which two PGRs are likely to be in balance, and how would a gibberellin spray change the situation?
Hint
Dormancy balance.
Answer
ABA promotes dormancy and inhibits germination while gibberellin breaks dormancy and promotes germination. A gibberellin spray would shift the balance towards germination even without the cold period.

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.Gaseous PGR?Show answer
Ethylene.
2.Natural auxins?Show answer
IAA and IBA.
3.Stress hormone?Show answer
ABA.
4.Kinetin source?Show answer
Herring sperm DNA.
5.Bakanae fungus?Show answer
Gibberella fujikuroi.
6.What does ethephon do?Show answer
Releases ethylene.

Plant Growth Regulators: Types and Effects: frequently asked questions

What are PGRs?

Small organic molecules regulating plant growth.

What is bolting?

Rapid stem elongation before flowering.

What does auxin do in cuttings?

Promotes rooting.

Which PGR delays senescence?

Cytokinin (also gibberellin in fruits).

What is parthenocarpy?

Seedless fruit formation.