Q.A farmer grows cucumber plants in his field. He wants to increase the number of female flowers in them. Which plant growth regulator can be applied to achieve this?

Ethylene: The Ripening Hormone

Imagine you've bought a bunch of raw bananas. You put them in a paper bag with a ripe apple. The next day, the bananas are noticeably yellower. What happened? The apple released a gas that told the bananas, "It's time to ripen." That gas is ethylene.

Ethylene is the only gaseous plant growth regulator (PGR). It's a simple hydrocarbon — just two carbons and four hydrogens ($C_2H_4$) — but its effects are dramatic. Unlike auxins or gibberellins that promote growth, ethylene is largely about finishing things: ripening, aging, and shedding.

The Precise Statement

Ethylene is a gaseous plant hormone that regulates fruit ripening, senescence (aging), abscission (dropping of leaves/fruits), and epinasty (downward curling of leaves). It is produced naturally by most plant tissues, especially in response to stress, wounding, or during ripening.

Key Effects Explained

Fruit Ripening — This is ethylene's most famous job. Unripe fruits produce a little ethylene, which triggers more ethylene production in nearby fruits (a positive feedback loop). That's why one rotten apple spoils the barrel — the rotten one releases ethylene that ripens (and then rots) the others. Commercially, fruits like mangoes and tomatoes are picked green, transported, then exposed to ethylene gas in ripening chambers.

Senescence and Abscission — Ethylene accelerates aging. It breaks down chlorophyll (yellowing leaves), weakens cell walls at the abscission zone (where a leaf or fruit attaches to the stem), and promotes dropping. This is why ethylene is used to defoliate cotton plants before mechanical harvesting.

Epinasty — When a plant is flooded or stressed, ethylene causes the upper side of leaf petioles to grow faster than the lower side, making leaves curl downward. This is a visible sign of stress.

Practical Applications …