This MCQ module is based on: Cytokinins Ethylene Aba
TOPIC 59 OF 90
Cytokinins Ethylene Aba
🎓 Class 11
Biology
CBSE
Theory
Ch 13 – Plant Growth and Development
⏱ ~14 min
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Cytokinins, Ethylene, Abscisic Acid and the Control of Flowering
Three PGRs remain: one that drives cell division, one that is a gas, and one that stops everything. Taken with the auxins and gibberellins of Part 3, they complete a control system in which no hormone acts alone — and the last section of this part shows how the outside world reaches into that system.
13.4.3.3 Cytokinins
Cytokinins have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine) from autoclaved herring sperm DNA.
An important qualification. Kinetin does not occur naturally in plants. The search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn kernels and coconut milk. Since the discovery of zeatin, several naturally occurring cytokinins, and some synthetic compounds with cell division promoting activity, have been identified.
Natural cytokinins are synthesised in regions where rapid cell division occurs — for example root apices, developing shoot buds, young fruits.
Their physiological effects:
- They help to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
- Cytokinins help overcome the apical dominance — the direct opposite of the auxin effect you met in Part 3.
- They promote nutrient mobilisation, which helps in the delay of leaf senescence.
13.4.3.4 Ethylene
Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits.
Being a gas makes ethylene unlike every other PGR: it needs no transport tissue, and it can pass from one fruit to another through the air between them.
Effects on seedlings and on senescence
- Influences on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
- Ethylene promotes senescence and abscission of plant organs, especially of leaves and flowers.
Fruit ripening
Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits.
Respiratory climactic. This rise in the rate of respiration is called respiratory climactic.
Dormancy, elongation and roots
- Ethylene breaks seed and bud dormancy, and initiates germination in peanut seeds and sprouting of potato tubers.
- Ethylene promotes rapid internode and petiole elongation in deep water rice plants. It helps leaves and upper parts of the shoot to remain above water.
- Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
Why deep water rice is worth pausing on. A plant drowning in a flood faces a straightforward problem: its leaves must reach air or it dies. Ethylene is the ideal signal, because being a gas it accumulates in submerged tissue where it cannot diffuse away — so the depth of the water effectively sets the strength of the signal, and the plant elongates by just about as much as it needs. Notice also that ethylene is doing here the opposite of its usual job: it is largely an inhibitor of growth, yet in this one situation it drives rapid elongation.
Agricultural applications and ethephon
- Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples. It also induces flowering in mango.
- Since ethylene regulates so many physiological processes, it is one of the most widely used PGRs in agriculture.
Ethephon. The most widely used compound as a source of ethylene is ethephon. Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. It hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers, thereby increasing the yield.
The reason ethephon exists at all is practical: you cannot spray a gas onto a field. Ethephon is a liquid that the plant absorbs and then breaks down internally, releasing the gas slowly from inside the tissue where it is needed.
13.4.3.5 Abscisic acid
Abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy. But like other PGRs, it also has other wide ranging effects on plant growth and development.
What ABA does.
• It acts as a general plant growth inhibitor and an inhibitor of plant metabolism.
• ABA inhibits seed germination.
• ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore it is also called the stress hormone.
• ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
• In most situations, ABA acts as an antagonist to GAs.
• It acts as a general plant growth inhibitor and an inhibitor of plant metabolism.
• ABA inhibits seed germination.
• ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore it is also called the stress hormone.
• ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
• In most situations, ABA acts as an antagonist to GAs.
How the PGRs work together
The chapter's own summary. For any and every phase of growth, differentiation and development of plants, one or the other PGR has some role to play. Such roles could be complementary or antagonistic. These could be individualistic or synergistic.
Furthermore, there are a number of events in the life of a plant where more than one PGR interacts to affect that event — for example dormancy in seeds and buds, abscission, senescence, apical dominance.
| Event | Promoted by | Opposed / reversed by |
|---|---|---|
| Apical dominance | Auxins — the apical bud inhibits lateral buds | Cytokinins — they help overcome apical dominance |
| Seed dormancy | ABA — induces dormancy, helping seeds withstand desiccation | Ethylene — breaks seed and bud dormancy; and GAs, to which ABA is an antagonist |
| Leaf senescence | Ethylene — promotes senescence | Cytokinins (nutrient mobilisation) and gibberellins (delay senescence) |
| Abscission | Ethylene; ABA; and auxins for older mature leaves and fruits | Auxins at early stages — they prevent fruit and leaf drop |
| Seed germination | GAs; ethylene in peanut seeds | ABA — inhibits seed germination |
Where PGRs sit in the larger picture. The role of PGR is of only one kind of intrinsic control. Along with genomic control and extrinsic factors, they play an important role in plant growth and development. And crucially: many of the extrinsic factors such as temperature and light control plant growth and development via PGRs. Some such events are vernalisation, flowering, dormancy, seed germination and plant movements.
Light and temperature in the initiation of flowering
The chapter closes by pointing to the role of light and temperature — both extrinsic factors — on the initiation of flowering. Two phenomena are named, and since Exercises 7 and 9 of this chapter ask about them, they are set out here.
Photoperiodism is the response of plants to the relative lengths of day and night in the initiation of flowering. Plants are grouped by the critical duration of light they require:
- Long day plants (LDP) flower when the light period is longer than a critical duration — e.g. wheat, spinach, radish.
- Short day plants (SDP) flower when the light period is shorter than a critical duration — e.g. rice, cotton, chrysanthemum, tobacco.
- Day-neutral plants show no such dependence — e.g. tomato, cucumber, sunflower.
The detail that decides the exercises. Two points matter. First, it is the uninterrupted dark period that is actually critical, not the light period — a short day plant given a brief flash of light in the middle of the night will fail to flower, which is why some authors prefer the terms long night and short night plants. Second, the site of perception is the leaf: a hormonal substance is produced in the leaves in response to the photoperiod and migrates to the shoot apices to induce flowering. Both facts are needed for Exercise 9.
Vernalisation is the promotion of flowering by a period of low temperature. Some plants depend on a spell of cold before they will flower at all — it prevents premature reproductive development late in the growing season and lets the plant reach maturity first. Biennials such as sugar beet, cabbages and carrots require it, and so do winter varieties of wheat, barley and rye: sown in autumn, they germinate, overwinter as seedlings, and flower the following summer. Sown in spring without the cold treatment, they will not flower.
📐 Activity 13.4 — Ripen a fruit with a fruit
What to do. Take four unripe green bananas, firm and of the same size. Place banana 1 in a paper bag on its own. Place banana 2 in a paper bag along with a fully ripe apple or an over-ripe banana. Leave banana 3 on an open plate. Seal banana 4 in a bag with a ripe apple plus a small dish of moist potassium permanganate (which absorbs ethylene), if your school has it. Keep all four at room temperature and check each day for four to five days, recording colour, firmness and smell.
Predict: which banana ripens first, which last, and what must be passing between the fruits in bag 2? Also predict what bag 4 will show if the permanganate does its job.
Observations. Banana 2 ripens fastest — yellow, soft and sweet-smelling well before the others. Banana 1 ripens next, since the bag traps the gas it releases itself. Banana 3, in the open, is slowest because the gas disperses. Banana 4 lags behind banana 2 despite sharing its bag with a ripe apple, because the ethylene is being absorbed before it can act.
What it demonstrates. Exactly the observation that led to the discovery of the hormone: H. H. Cousins in 1910 confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas, later identified as ethylene. The ripe fruit is the source, because ethylene is synthesised in large amounts by tissues undergoing senescence and by ripening fruits, and it is highly effective in fruit ripening. Bag 4 is the control that proves the agent is a gas and not warmth, moisture or contact.
Two things to add to your notebook. First, ripening involves a measurable rise in respiration, the respiratory climactic — a ripening fruit briefly respires harder than at any other time in its life. Second, this is why commercial growers pick fruit green, ship it cold, and then ripen it on arrival with ethephon, which in aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. It is also why one over-ripe fruit spoils a whole basket, and why apples are best stored away from other produce.
🎯 Interactive: Which PGR, and which way?
PGR responsible: Cytokinins
Cytokinins help overcome apical dominance, directly opposing the auxin effect in which the growing apical bud inhibits the lateral buds. They also promote lateral shoot growth and adventitious shoot formation.
🎯 Competency-Based Questions
Scenario: A fruit trader stores unripe mangoes in three rooms. Room A is sealed with a crate of ripe bananas inside. Room B is sealed and empty apart from the mangoes. Room C is ventilated. Separately, a grower sprays ethephon on one tomato field and, in a cucumber field, on half the plants only. A third grower sows winter wheat in spring and finds it never flowers.
Q1. Rank the three rooms by how fast the mangoes ripen and explain. L3 Apply
A fastest, then B, then C. Ethylene is synthesised in large amounts by tissues undergoing senescence and ripening fruits, and is highly effective in fruit ripening. Room A has an external source — the ripe bananas — in a sealed space, so the concentration builds fastest. Room B accumulates only the ethylene the mangoes produce themselves, but being sealed it retains it. Room C is ventilated, so the gas disperses and ripening is slowest. Because ethylene is a gas, no vascular connection is needed for the signal to pass from fruit to fruit.
Q2. Predict what ethephon does in the tomato field and in the treated half of the cucumber field. L3 Apply
In the tomato field it hastens fruit ripening, giving a more uniform, earlier harvest. In the treated half of the cucumber field it promotes female flowers, thereby increasing the yield — since only female flowers set fruit, shifting the sex ratio raises the number of cucumbers. Ethephon works because in an aqueous solution it is readily absorbed and transported within the plant and releases ethylene slowly, which is how a gas can be applied as a spray.
Q3. Fill in the blanks: Cytokinins were discovered as ______ from autoclaved ______ DNA, but it does not occur ______ in plants. The first natural cytokinin, ______, was isolated from ______ and ______. L1 Remember
kinetin; herring sperm; naturally; zeatin; corn kernels; coconut milk.
Q4. The winter wheat sown in spring grows well but never flowers. Diagnose the problem and name the process. L4 Analyse
The crop has missed its required vernalisation — the promotion of flowering by a period of low temperature. Winter varieties of wheat, barley and rye are normally sown in autumn so that they germinate and overwinter as seedlings, experience the cold, and flower the following summer. Sown in spring, the plants never receive the low-temperature spell, so the transition to flowering is never triggered no matter how well they grow vegetatively. This is an instance of the chapter's general point that many of the extrinsic factors such as temperature and light control plant growth and development via PGRs, and vernalisation is one of the events it names. The practical fix is either autumn sowing or using a spring variety that does not require the cold treatment.
Q5. “Each PGR has its own job, so you can always name one hormone as the cause of a given effect.” Evaluate. L5 Evaluate
This is the single most common misconception about PGRs, and the chapter contradicts it directly. It states that any PGR has diverse physiological effects on plants, that diverse PGRs also manifest similar effects, and that PGRs may act synergistically or antagonistically — their roles being complementary or antagonistic, individualistic or synergistic. It further names events where more than one PGR interacts: dormancy in seeds and buds, abscission, senescence and apical dominance.
Three concrete refutations. (i) One effect, several hormones. Delaying leaf senescence is done by cytokinins through nutrient mobilisation and by gibberellins; promoting it is done by ethylene and ABA. (ii) One hormone, opposite effects. Auxins prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits — the same molecule on both sides of the same process. (iii) Direct opposition. Auxin imposes apical dominance while cytokinin helps overcome it; ABA acts as an antagonist to GAs in most situations.
The better formulation is that a plant's response is set by the balance and ratio of several PGRs acting on a particular tissue at a particular stage, not by any one hormone's identity. The practical proof is in the laboratory: it is the auxin-to-cytokinin ratio in a tissue culture medium, not either hormone alone, that decides whether a callus makes roots or shoots.
Three concrete refutations. (i) One effect, several hormones. Delaying leaf senescence is done by cytokinins through nutrient mobilisation and by gibberellins; promoting it is done by ethylene and ABA. (ii) One hormone, opposite effects. Auxins prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits — the same molecule on both sides of the same process. (iii) Direct opposition. Auxin imposes apical dominance while cytokinin helps overcome it; ABA acts as an antagonist to GAs in most situations.
The better formulation is that a plant's response is set by the balance and ratio of several PGRs acting on a particular tissue at a particular stage, not by any one hormone's identity. The practical proof is in the laboratory: it is the auxin-to-cytokinin ratio in a tissue culture medium, not either hormone alone, that decides whether a callus makes roots or shoots.
🧠 Assertion–Reason Questions
For each pair choose: (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.
Assertion (A): Abscisic acid is called the stress hormone.
Reason (R): It stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses.
Answer: A. Both are true and the reason is exactly why the name is used. Closing the stomata conserves water, which is the first line of defence against drought.
Assertion (A): Kinetin is the naturally occurring cytokinin of plants.
Reason (R): Kinetin was discovered as a modified form of adenine from autoclaved herring sperm DNA.
Answer: D. The assertion is false — kinetin does not occur naturally in plants; the first natural cytokinin found was zeatin, from corn kernels and coconut milk. The reason correctly states its origin, and in fact hints at why it is not natural: it came from animal DNA that had been autoclaved.
Assertion (A): Ethylene promotes rapid internode elongation in deep water rice plants.
Reason (R): Ethylene is largely an inhibitor of growth activities.
Answer: B. Both statements are true, but the reason does not explain the assertion — it appears to contradict it. Ethylene is indeed largely an inhibitor, yet in submerged rice it drives rapid internode and petiole elongation so that the leaves and upper shoot remain above water. This is a good illustration that a PGR's effect depends on the tissue and the situation, not on a fixed label.
Coming next. Part 5 is the exercise part: the chapter summary followed by full worked solutions to all nine NCERT exercise questions of Chapter 13, including the eight definitions, the four descriptions of growth patterns, the PGR note, and the two questions on photoperiodism.
Frequently Asked Questions - Cytokinins, Ethylene, Abscisic Acid and Flowering
What are cytokinins and where are they made?
Cytokinins have specific effects on cytokinesis and were discovered as kinetin, a modified form of adenine, from autoclaved herring sperm DNA. Natural cytokinins are synthesised in regions where rapid cell division occurs, such as root apices, developing shoot buds and young fruits.
Does kinetin occur naturally in plants?
No. Kinetin does not occur naturally in plants. The search for natural substances with cytokinin-like activity led to the isolation of zeatin from corn kernels and coconut milk, after which several naturally occurring cytokinins were identified.
What do cytokinins do?
They help produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation. They help overcome apical dominance, and they promote nutrient mobilisation which helps delay leaf senescence.
What are the main effects of ethylene?
Ethylene causes horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings; promotes senescence and abscission especially of leaves and flowers; is highly effective in fruit ripening and enhances the respiration rate during it; breaks seed and bud dormancy; promotes internode elongation in deep water rice; and promotes root growth and root hair formation.
What is respiratory climactic?
It is the rise in the rate of respiration during the ripening of fruits, which ethylene enhances.
What is ethephon and why is it used?
Ethephon is the most widely used source of ethylene. In aqueous solution it is readily absorbed and transported within the plant and releases ethylene slowly, which allows a gas to be applied as a spray. It hastens ripening in tomatoes and apples, accelerates abscission for thinning cotton, cherry and walnut, and promotes female flowers in cucumbers to increase yield.
Why is abscisic acid called the stress hormone?
Because it stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. It also acts as a general plant growth inhibitor and an inhibitor of plant metabolism, inhibits seed germination, and by inducing dormancy helps seeds withstand desiccation.
How do plant growth regulators interact with one another?
For any phase of growth, differentiation and development one or another PGR has a role, and those roles may be complementary or antagonistic, individualistic or synergistic. Several PGRs interact in events such as dormancy in seeds and buds, abscission, senescence and apical dominance. In most situations ABA acts as an antagonist to the gibberellins.
What is photoperiodism?
It is the response of plants to the relative lengths of day and night in the initiation of flowering. Long day plants flower when the light period exceeds a critical duration, short day plants when it is shorter than a critical duration, and day-neutral plants show no such dependence. The uninterrupted dark period is what actually matters, and the photoperiod is perceived by the leaves.
What is vernalisation?
It is the promotion of flowering by a period of low temperature. Biennials such as sugar beet, cabbage and carrot require it, and so do winter varieties of wheat, barley and rye, which are sown in autumn so that they overwinter as seedlings and flower the following summer.
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