By Vijayalaxmi Kinhal; Science Writer, CID Bio-Science; Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Ethylene (C2H4) is a small hydrocarbon, which is colorless and odorless. This gaseous phytohormone that is produced by plants has various valuable applications in the agriculture industry. It can be both beneficial and harmful because it promotes and inhibits plant growth and development at various stages in a plant’s life. It is, however, best known for its ripening effect on fruit.
How ethylene acts
Ethylene has three pathways which produce a variety of effects on plants depending on their sensitivity to it and their life stage. The three modes of actions are:
1) Ripening: Ethylene is released in high concentrations by fruit that is ripening. It acts by:
- Dissolving pectin and softening the fruit,
- Changing the color of the fruit, and
- Converting stored starches and acids into sugars, making the fruit sweet.
Once ripening is underway, it, in turn, triggers the production of more ethylene to continue the process of ripening.
2) Senescence: Ethylene brings about ageing, or senescence, by triggering programmed cell or organ death. This is not as terrible as it seems, as the organism discards old cells or unnecessary parts to make way for new cells or the next stage of growth. An example of this is abscission. Ethylene is also produced for the purpose of starting senescence when any part of the plant is damaged.
3) Response to stress: Ethylene production is increased as a response to stress. When this happens, it can cause inhibition of stem elongation, stem thickening, and epinasty, the bending or curving of stems or roots to avoid obstacles.
Effects of ethylene
Ethylene can affect all parts of a plant as a result of its three pathways.
- Germination: It breaks seed dormancy and promotes germination, for example in groundnuts.
- Sprouting: Ethylene can trigger the sprouting of buds in tubers when its levels increase, as seen in potatoes.
- Root development: It encourages the growth of adventitious roots on the stems of some vegetables such as tomatoes.
- Stem: It promotes elongation of petioles and internodes, which helps paddy seedlings to emerge quickly out of the water.
- Leaves: When leaves grow old, ethylene removes chlorophyll from leaves, causing them to change color. This is followed by abscission, or the falling of leaves.
- Flowers: Ethylene causes wilting in flowers and their abscission. Conversely, it also promotes flowering in fruits and vegetables.
- Fruit: It is best known commercially for its action on fruit, both in ripening them and in causing abscission.
Application in food production
Ethylene levels are an important consideration in agriculture due to ethylene’s positive applications and the problems it creates for the food industry.
1) Beneficial effects
Ethylene can increase profits by extending the time from harvest to the shop, and by improving the quality and quantity of food. The gas can be delivered through ethephon (a liquid), a cylinder, or a catalytic generator.
- Ripening fruit: Thanks to ethylene, mature fruit is harvested before ripening. When it is ready to be sold, fruit is ripened artificially with ethylene. Besides ripening, ethylene brings about color changes in citrus fruit through the removal of chlorophyll.
- Increasing flowering: Ethylene is used to promote the formation of female flowers in cucumbers to increase overall yield.
2) Problems caused by ethylene
Being a gas, ethylene spreads out of a piece of fruit and into the environment when it is produced and quickens the ripening of other fruit nearby. Moreover, “one rotten apple can spoil the whole basket,” if the process continues unchecked.
Not all fruit and vegetables produce or need ethylene. For example, cherries and blueberries do not. However, many temperate and tropical fruits do start to ripen when exposed to heightened ethylene levels.
- Producers: Fruits producing ethylene in their cells are apples, bananas, tomatoes, avocados, melons, pears, squash, and stone fruits such as mangos.
- Sensitive fruit: Plants sensitive to ethylene that do not produce it themselves are broccoli, cabbage, cauliflower, leafy greens, and lettuce.
In both cases, where ethylene needs to be added or removed to maintain fruit quality, it is necessary to measure levels of ethylene to avoid wasting the gas or to prevent the spoilage of produce, respectively. Different methods can be used to measure ethylene in the air.
- Micro gas chromatography systems are highly efficient but expensive.
- Gas detector tubes, which can measure ethylene in the air, comprise a simple handheld system.
- Electrochemical sensors are available as small but precise handheld instruments. Some examples are:
- The F-950 Handheld Ethylene Analyzer, which can detect ethylene levels as low as 0.5 ppm, and can measure between 0 - 200 ppm of the gas.
- The F- 940, which is used in storage facilities, has a resolution of 0.1ppm, with a range of 0 - 10 ppm.
- The F-960, used during ripening, has a resolution of 1 ppm, with a range of 0 - 500 ppm
When recorded levels during storage and transport are high, there are many techniques that can be used to control the level of ethylene, such as scrubbing, ventilation, or the use of UV radiation.
- Scrubbing: Dry chemical scrubbers are the traditional method of removing ethylene. Air is sucked from the bottom of the room where ethylene sinks, as it is heavier than the air, and is then passed through filters with potassium permanganate (KMnO4), which oxidizes and neutralizes ethylene. The cleaned air is released free of the gas. Some systems use ozone instead of potassium permanganate.
- Absorption Filters: There are simple ethylene absorption filters that can be hung in storage rooms and vehicles to absorb the gas given out by fruits and vegetables.
- Ventilation: Air in refrigerated containers and rooms is circulated regularly with fans to help the produce and fruits to respire, and remove accumulated ethylene and carbon dioxide. Ventilation increases cooling costs, as the introduced air needs to be cooled to maintain storage temperatures. The rate of ventilation varies depending on the fruit or vegetable
- UV radiation: UV radiation is used to remove ethylene, as it reacts with oxygen in the air to produce ozone, which in turn oxidizes the gas.
Control to avoid waste
Besides removing ethylene, many measures can be taken to prevent the spread and accumulation of ethylene by separating fruits producing the gas from ones sensitive to it. Other methods include keeping temperatures low, reducing oxygen content, and increasing carbon dioxide levels to inhibit ethylene production in fruits. In every case, precise and regular measurement of the levels of ethylene is essential to monitoring its level and effects.