Ethylene
எத்திலீன்
Ethylene is the only gaseous plant growth regulator. Do not confuse its role in senescence with Abscisic Acid (ABA). While ABA promotes abscission, ethylene accelerates the entire process of fruit ripening and organ shedding.
The idea
Ethylene is a simple gaseous PGR synthesized from methionine. It promotes horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings. Crucially, it induces fruit ripening and increases the rate of respiration, a phenomenon termed respiratory climacteric. It also breaks seed and bud dormancy and initiates germination in peanut seeds and sprouting of potato tubers.
Why it matters
It exists to coordinate the ripening of fruits and the shedding of old organs, allowing the plant to manage resources efficiently and disperse seeds.
NEET lens
NEET focuses on the 'respiratory climacteric' and the specific physiological effects like horizontal growth. Expect matching questions linking PGRs to their functions. Never mark ethylene as a growth inhibitor; it is a broad-spectrum PGR with both promoting and inhibiting roles. Memorize the list of effects: ripening, senescence, and dormancy breaking.
Clinical significance
In human physiology, ethylene gas exposure is toxic, but its derivative, ethylene oxide, is a critical medical sterilant. It is used to sterilize heat-sensitive surgical instruments that cannot withstand autoclaving. Failure to properly aerate instruments after sterilization leads to severe tissue necrosis in patients.
Introduction
Ethylene (C₂H₄) is a gaseous plant hormone produced during fruit ripening, senescence, and stress. It promotes fruit ripening, abscission (leaf/fruit drop), and epinasty.
Core concept
Chemical: simple alkene (C₂H₄), gas at room temperature, the only gaseous PGR. Synthesis: from methionine → SAM → ACC → ethylene (key enzyme: ACC synthase). Synthesis induced by: wounding, flooding, pathogen attack, other hormones (especially IAA at high concentrations). Effects: fruit ripening (climacteric fruits — banana, mango, apple — show ethylene burst at ripening). Senescence and abscission. Epinasty (leaves bend downward). Triple response in seedlings: swelling + shortening + horizontal growth. Inhibits longitudinal growth, promotes lateral growth.
Go deeper
Climacteric vs non-climacteric fruits: climacteric (banana, apple, mango, tomato, avocado) — show sudden rise in respiration + ethylene at ripening → ripen after harvest. Non-climacteric (grape, citrus, strawberry) — no ethylene burst → must ripen on plant. This determines handling and storage in agriculture.
From Class 10
When one apple in a bag ripens and gets damaged, the others ripen faster too. The signal is a gas — ethylene — which triggers ripening.