Ethylene
Ethylene is a gas produced by various parts of plants, as well as fungi and bacteria. It serves as a fruit ripening hormone. The gas speeds up respiration in fresh fruits and vegetables, promoting maturity, senescence, and tissue softening. Ethylene accumulation can lead to the yellowing of green vegetables. Major sources of ethylene include shoot apex (apical tissues) and senescing tissues. Ethylene is triggered by physiological stresses such as wounding, anaerobic conditions, flooding, chilling, disease, and drought.
1.0Discovery of Ethylene
In 1886, Dimitry Neljubow, a Russian graduate student, first observed the impact of ethylene on plants when studying abnormal growth in dark-grown pea seedlings exposed to illuminating gas. However, interest in ethylene's role in plant growth only gained momentum when its commercial implications were recognized.
People involved in fruit shipping and storage have long known that ripe or rotting fruit can hasten the ripening of nearby fruit. For instance, bananas shipped from Cuba often arrived in New York overripe and unsellable. The link between these effects and a volatile substance emitted by plant tissue was highlighted in a 1910 report by H. H. Cousins from the Jamaican Department of Agriculture. He found that ripe oranges emitted a volatile product that sped up the ripening of nearby bananas. Similar reports in the early 1930s showed that emanations from apples caused growth abnormalities in tomato seedlings and respiratory changes linked to ripening. In 1934, R. Gane provided conclusive evidence identifying the volatile substance as ethylene.
2.0Biosynthesis of Ethylene
methionine is the precursor of ethylene in plants, formation of ethylene is a cyclic process which is methionine cycle, also known as the Yang cycle, ensures the recycling of sulfur back to methionine to sustain ethylene production.
3.0Physiological Effects
fruit ripening: Ethylene stimulates fruit ripening through processes like chlorophyll breakdown, leading to a change in fruit color (e.g., apples turning from green to red), fruit softening via cell wall breakdown by enzymes, and conversion of starch and acids to sugars.
Promotes abscission: The abscission zone in leaves experiences an upsurge in ethylene production, initiating the breakdown of the middle lamella. This process ultimately leads to abscission, or the shedding of leaves.
Plumular Hook Formation : In etiolated dicot seedlings, the plumular tip (shoot apex) exhibits a hooked shape, providing an advantage for soil penetration and protecting the delicate growing point from damage. This hook formation is attributed to ethylene production, causing uneven growth on the two sides of the plumular tip. Ethylene induces faster elongation on the outer side of the plumular tip.
Triple Response : Ethylene causes triple response of etiolated seedling such as in pea which consists of inhibition of stem elongation, stimulation of radial swelling of stems and horizontal growth of stems with respect to gravity.
Leaf Epinasty : Epinasty occurs when the upper side (adaxial) of a leaf's petiole grows faster than the lower side (abaxial), resulting in a downward curvature. This phenomenon is induced by ethylene in dicot plants like tomato, potato, pea, and sunflower. Young leaves are more responsive to ethylene compared to older ones. It's worth noting that monocots do not display this specific response.
Flowering : While ethylene generally inhibits flowering in plants, it has the opposite effect in certain species like pineapple and mango, where it actually induces flowering. Commercially, ethylene is utilized to synchronize flowering and fruit set in pineapple.
Sex expression : In monoecious species, where distinct male and female flowers exist, ethylene plays a significant role in stimulating the development of female flowers. Consequently, it suppresses the formation of male flowers to a considerable extent.