Describe physiological effects and applications of ethylene.

Answer: Ethylene (C₂H₄) is a gaseous plant hormone.

Physiological effects:

1. Promotes ripening of fruits (increases rate of respiration – respiratory climacteric).
2. Promotes senescence and abscission of leaves, flowers and fruits.
3. Breaks dormancy of seeds and buds; promotes sprouting of potato tubers.
4. Causes horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings (triple response).
5. Promotes root growth and root hair formation; induces flowering in pineapple and mango and increases female flowers in cucumber.
6. Promotes rapid internode/petiole elongation in deep-water rice.

Applications: Ethephon, which releases ethylene, is used for artificial ripening of fruits (banana, mango, tomato), to hasten fruit ripening and thinning (cotton, cherry, walnut), to induce flowering and synchronise fruit set in pineapple, to increase female flowers and yield in cucumber, and to promote abscission.