NCERT Grounding
NCERT Class 11 Biology, Chapter 13 (Plant Growth and Development), Section 13.4.3.4 explicitly categorises ethylene as a "simple gaseous PGR" and states: "It is synthesised in large amounts by tissues undergoing senescence and ripening fruits." The text lists horizontal growth, swelling of axis, apical hook formation, senescence, abscission, fruit ripening, respiratory climacteric, internode elongation in deep-water rice, root growth, and root hair formation as its effects — making it the most effect-rich PGR in the chapter after auxin. NIOS Chapter 20 corroborates that ethylene increases width (not length) of cells and promotes senescence and abscission of leaves and flowers.
"Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called respiratory climactic."
NCERT Class 11 Biology, Chapter 13, §13.4.3.4
Chemistry and Biosynthesis
Ethylene is the simplest alkene: two carbon atoms joined by a double bond (CH₂=CH₂), molecular weight 28 g/mol. It is active at concentrations as low as 0.1 ppm and diffuses freely through intercellular air spaces — no vascular transport is needed. This gaseous nature sets it apart from all other PGRs, which are dissolved molecules transported in the phloem or xylem.
Threshold Concentration
Ethylene is physiologically active at as little as 0.1 parts per million — far lower than any solid or liquid PGR. It diffuses through intercellular air spaces without vascular transport.
Biosynthetically, ethylene is derived from the amino acid methionine via the Yang cycle. The key intermediate is ACC (1-aminocyclopropane-1-carboxylic acid); the enzyme ACC oxidase converts ACC to ethylene in the presence of oxygen. Tissues that produce the most ethylene include ripening fruits, senescing leaves and flowers, germinating seeds, and submerged or mechanically stressed tissues. Flooding triggers rapid ACC accumulation in roots, which is then transported to shoots where ACC oxidase converts it to ethylene — the signal that drives internode elongation in deep-water rice.
Physiological Effects — Full Catalogue
NCERT lists eight distinct physiological roles for ethylene. The table below organises them by plant organ and direction of effect, which is the most NEET-useful framework.
| Effect | Organ / Stage | Mechanism / Notes | NEET Relevance |
|---|---|---|---|
| Fruit ripening | Climacteric fruits (mango, banana, tomato, apple) | Cell-wall softening; starch → sugar conversion; respiratory climacteric (CO₂ surge) | High — concept tested multiple years |
| Triple response in seedlings | Etiolated dicot seedlings | Inhibition of elongation + radial swelling + horizontal growth | High — classic definition Q |
| Internode / petiole elongation | Deep-water rice (flooded plants) | Flooding → ACC accumulates → ethylene → rapid elongation; leaves stay above water | Very high — NEET 2023 Q.124 |
| Female flower induction | Cucumbers (monoecious) | Shifts sex ratio toward pistillate (female) flowers; increases fruit yield | Very high — NEET 2022 Q.117 |
| Root growth & root hair formation | Root system; stem cuttings | Promotes adventitious root initiation; expands absorptive surface | High — NEET 2022 Q.108 |
| Abscission | Mature leaves, mature fruits, flowers | Stimulates formation of abscission zone; cell wall dissolution by pectinases | Moderate — compare to auxin which delays abscission at early stages |
| Senescence promotion | Leaves, flowers | Accelerates chlorophyll degradation; opposes cytokinins | Moderate |
| Epinasty | Leaf petioles | Greater elongation of upper cells → petiole curves downward (twisting/drooping) | Moderate |
| Seed / bud dormancy break | Peanut seeds; potato tubers | Initiates germination in peanut; breaks sprouting dormancy in potatoes | Low–Moderate |
The Triple Response — Deep Dive
The triple response was first described by Neljubov in 1901 when pea seedlings grown near illuminating gas (which contained ethylene) showed abnormal growth. When etiolated (dark-grown) dicot seedlings are exposed to ethylene, three simultaneous and coordinated changes occur in the hypocotyl or epicotyl:
Triple Response of Ethylene in Etiolated Dicot Seedlings
-
Step 1
Inhibition of elongation
Hypocotyl / epicotyl ceases longitudinal growth. Cells do not elongate normally in the presence of ethylene.
↓ length -
Step 2
Radial swelling
The same cells expand laterally, producing a noticeably thickened, barrel-like axis — ethylene promotes width, not length (NIOS).
↑ diameter -
Step 3
Horizontal growth
Seedling grows sideways (diagravitropically) instead of upright. This is an ethylene-specific response used as a bioassay.
↔ direction
Biologically, the triple response is an obstacle-avoidance strategy. When a germinating seedling encounters a physical barrier in the soil, ethylene (produced under mechanical stress) triggers the triple response: the thickened, horizontal shoot can push through the obstacle more effectively than a thin, upright one.
Figure 1. Comparison of normal etiolated dicot seedling versus triple response under ethylene exposure: shortened axis, radially swollen hypocotyl, and horizontal (diagravitopic) growth direction.
Deep-Water Rice — Internode Elongation
The deep-water rice adaptation is one of ethylene's most ecologically striking roles and a confirmed NEET 2023 topic. When paddy fields flood, the partially submerged stem produces ethylene rapidly because oxygen scarcity (anoxia) slows ethylene breakdown, and the mechanical pressure of water stimulates ACC synthesis. The resulting high ethylene concentration acts on the intercalary meristems of internodes and petioles, driving rapid cell elongation. Growth rates of 20–25 cm per day have been recorded in some deep-water cultivars. This allows the canopy to stay above the flood surface, maintaining photosynthesis and ultimately the plant's survival.
Sex Determination in Cucumber
Cucumbers are monoecious plants — male and female flowers occur on the same plant. The sex ratio is not fixed: it is under hormonal control. Ethylene shifts the ratio toward female (pistillate) flowers at the expense of male (staminate) flowers. Commercially, applying ethephon to young cucumber plants significantly increases the number of female flowers per plant, which translates directly to higher fruit yield. This effect — confirmed in NEET 2022 — is ethylene-specific; auxins are not involved in cucumber sex determination.
Commercial Applications — Ethephon
Pure ethylene gas is impractical to apply to field crops. The solution is ethephon (2-chloroethylphosphonic acid), a compound that releases ethylene endogenously after uptake. Once inside the plant tissue (where pH is typically above 4), ethephon hydrolyses spontaneously:
ClCH₂CH₂PO(OH)₂ + H₂O → C₂H₄ + H₃PO₄ + HCl
Key principle: Ethephon does not enter as ethylene — it enters as a stable phosphonic acid and releases ethylene only after hydrolysis inside the plant. NEET sometimes phrases questions to test whether students know that ethephon releases ethylene, not that it is ethylene.
Fruit Ripening
Crops: Tomatoes, apples, mangoes, bananas
Accelerates colour development, softening, and sugar accumulation; used for off-vine ripening of tomatoes for transport.
NCERT examplePineapple Flowering
Agent: Ethephon + auxin (synergistic)
Synchronises flowering across the entire crop, ensuring uniform fruit set and single-pass harvesting.
NEET 2019 Q.77Abscission / Thinning
Crops: Cotton, cherry, walnut
Accelerates formation of the abscission zone at the base of bolls or fruit stalks, enabling mechanical harvest.
NCERT exampleMango Flowering
Agent: Ethephon spray
Induces off-season flowering in mango, allowing fruit production outside the natural season.
NCERT exampleWorked Examples
A farmer wants to increase the yield of his cucumber crop. Which plant growth regulator should he apply to the young plants, and what mechanism underlies its effect?
Answer: The farmer should apply ethylene (commercially delivered as ethephon). Ethylene shifts the male-to-female flower ratio in cucumber toward female (pistillate) flowers. Since cucumber fruits develop from fertilised female flowers, increasing the number of female flowers per plant directly increases fruit (yield) per plant. Auxin is not appropriate here — it promotes flowering in pineapple but does not alter the sex ratio in cucumber.
A batch of raw tomatoes was placed in a sealed container along with a ripe banana overnight. The next morning the tomatoes showed signs of ripening. Explain the mechanism.
Answer: Ripe bananas actively synthesise and release ethylene gas. Because the container was sealed, ethylene accumulated around the unripe tomatoes. Ethylene at concentrations above its physiological threshold (≈0.1 ppm) activates genes encoding cell-wall-degrading enzymes (cellulase, polygalacturonase) and triggers the respiratory climacteric — a sharp, transient surge in CO₂ production as starch is rapidly broken down to sugars. The combined effect is softening of the flesh, colour change (lycopene synthesis), and sweetening, i.e., ripening. This is the principle behind commercial ethephon treatment of tomatoes.
During flooding, deep-water rice plants can elongate their internodes at remarkable speed. Which hormone drives this response, and what is the adaptive advantage?
Answer: Ethylene is the hormone responsible. Flooding creates anaerobic conditions in the soil and around the submerged stem. Oxygen shortage inhibits the enzyme ACC oxidase in roots, so ACC (the ethylene precursor) accumulates and is transported to the shoot. In the shoot, where oxygen is available, ACC oxidase converts it to ethylene. The elevated ethylene activates the intercalary meristems of internodes and petioles, driving rapid elongation — the internodes can grow 20–25 cm per day in susceptible cultivars. The adaptive advantage is that the leaf canopy is pushed above the flood surface, allowing continued photosynthesis and thus survival.
Common Confusion and NEET Traps
Ethylene
Gas
Physical state at room temperature
- Promotes abscission of mature leaves and fruits
- Promotes senescence of leaves and flowers
- Also promotes fruit ripening (climacteric)
- Promotes root growth and root hair formation
- Promotes female flowers in cucumber
- Triple response in etiolated seedlings
- Internode elongation in flooded rice
Abscisic Acid
Liquid
Dissolved in sap; a terpenoid
- Also promotes abscission and senescence
- Inhibits seed germination (dormancy inducer)
- Closes stomata during drought (stress hormone)
- Antagonises gibberellins
- Does NOT promote fruit ripening
- Does NOT cause triple response
- Does NOT elongate internodes under flooding