How Does Ethylene Influence Plant Growth and Development?
What is Ethylene?
Ethylene is considered as one of the important plant metabolites and growth regulators. It is the only gaseous plant hormone that causes the ripening of fruit. It is commonly seen that unripe fruits kept with ripe banana ripen overnight. It is because of ethylene released from the ripe banana, which promotes the ripening of the other fruits.
Ethylene is a colorless, inflammable gas with a sweet musky odor. It is the smallest member of the alkene family.
Ethylene is widely used in agricultural industries for the artificial ripening of fruits and vegetables. It is also used in the chemical and plastic industry for the production of several commonly used compounds like polyethylene.
Ethylene Formula
The IUPAC name for ethylene is ethene. It consists of two carbon atoms and four hydrogen atoms. The formula for ethene is C2H4 or H2C=CH2.
Ethylene Structure
Belonging to the alkene family, ethylene is considered to be an unsaturated hydrocarbon with a double bond between the two carbon atoms. The two carbon atoms and the four hydrogen atoms are coplanar. Two hydrogen atom forms bond with each carbon atom in an H-C-H manner at an angle of 117.4°, which is close to 120° required for sp2 hybridization. The π-bond between the two carbon atoms makes the molecule rigid; breaking the C-C bond requires high energy. The reactivity of ethylene is conferred by this π-bond. The double bond between the two carbon atoms has high electron density and is the main region for electrophilic reactions.
Ethylene As A Plant Hormone
Chinese used to burn incense to ripen fruits in ancient times. Similarly, Egyptians used gash figs for the same purpose. As said previously, ethylene hormone is the only gaseous hormone that plays an important role in:
Hypocotyl elongation and root initiation.
Promotes growth of root to increase the absorbing capacity for water and minerals.
Promotion of Apo-geotropism during root development.
Possible role in adventitious root development.
Affect gravitropism.
Stimulates stem growth.
Plays an important role in bud outgrowth.
Plays an essential role in stem branching.
Stimulates bending of stem and branches in the direction of light.
Promotes root and shoot differentiation in a triple response.
Interferes with the transport of other hormones like auxin.
Prevents stem trichome formation in Arabidopsis.
Promotes leaf growth.
Promotes leaf area expansion.
Involved in leaf development and leaf senescence.
Dismantling of leaf cells and nutrient recycling from senescent leaves to other plant parts.
Regulatory role in the transition from vegetative to reproductive phase of some plants.
Sex determination of flower and production of female flower in male plants
Promotes flowering in pineapples.
Induces flowering n Bromeliad.
Induces the opening of flowers.
Stimulates flower senescence.
Breaks seed dormancy and promote seed germination.
Induces epinasty.
Rise in the respiration of some climacteric fruits.
Fruit ripening.
Change in color of the fruit during the ripening process.
Promotes fruit senescence.
It helps plants to respond to environmental conditions.
Function Of Ethylene
Ethylene has several defined functions at different stages of plant development. The roles of ethylene are studied at the cellular and tissue level in detail. Some of the functions of ethylene are defined below:
Ethylene in cell division
Ethylene has conflicting roles in the cell division process, depending on the type of tissues. It promotes cell division in the sub-epidermal layers leading to apical hook development, where it works in conjunction with auxin. However, it inhibits cell division in the root apical meristem. It also controls cell division rate and, subsequently, the differentiation of vascular tissue. It inhibits the mitotic cell cycle in the abaxial surface of the leaf petiole, and in a way, contributes to the hyponastic response.
Ethylene in cell elongation
The role of ethylene in the cell elongation process is also conflicting, depending on the cell type. It inhibits cell elongation in light-grown and dark-grown seedlings. However, it stimulates cell elongation in the growing hypocotyl of light-grown seedlings.
Ethylene and cell death
Ethylene has also been found to play a critical role in cell death. During xylogenesis (formation of xylem cells), ethylene production is high. Xylogenesis is a part of programmed cell death. Ethylene production is also enhanced during bending of branches due to gravitation, due to modifications of the xylem morphology.
Aerenchyma formation, which promotes gas exchange in hypoxic conditions, is also a form of PCD. Ethylene production is found to be enhanced during aerenchyma formation.
Response to environmental condition and competition
Any plant has to face several environmental challenges. They have to avoid shade regions and compete with other plants for sunlight. Upward leaf movement (hyponasty) is an adaptive response in this process, and ethylene plays an important role in it. Additionally, root hair formation is enhanced in poor nutrient conditions. The external application of ethylene promotes root hair formation.
Thigmomorphogenesis is the plant response to mechanical stress like touch and wind. These responses include reduced root area, shoot elongation, and dry weight accumulation. Ethylene plays a crucial role in thigomomorphogenetic responses.
Ethylene in plant growth
Ethylene promotes horizontal growth of plants and swelling of the plant axis. It inhibits longitudinal growth.
Ethylene in response to gravity
Ethylene desensitizes the plant towards the gravitational pull. It makes the stems positively geotropic, and the flowers and leaves droop due to ethylene production.
Senescence and abscission
Ethylene stimulates senescence and abscission in flowers, leaves, and fruits. It also promotes hydrolase formation.
Apical Dominance
Apical dominance is enhanced, and lateral bud dormancy is prolonged during ethylene treatment.
Flowering and fruit ripening
Ethylene promotes flowering and fruit ripening, mainly in climacteric fruits. It also induces flower fading in some plants.
Use Of Ethylene
Ethylene is used as an artificial fruit ripener.
Ethylene is used to develop female flowers in male plants.
Ethylene is used to promote sprouting in rhizomes.
Ethylene is used to thin tough fruits like walnut and prevent excess flowering.
Used in the manufacturing of polyethylene bags.
FAQs on Ethylene: Formula, Roles, and Applications in Biology
1. What is ethylene, and what is its chemical formula as per the Biology syllabus?
Ethylene is a simple gaseous plant growth regulator (PGR) or phytohormone. Its chemical formula is C₂H₄, and its IUPAC name is Ethene. It is widely known for its role in the ripening of fruits and is synthesised in large amounts by plant tissues undergoing senescence (ageing) and ripening.
2. How would you describe the molecular structure of ethylene?
The ethylene molecule (C₂H₄) has a planar structure. It is composed of two carbon atoms connected by a double bond, which consists of one sigma (σ) bond and one pi (π) bond. Each carbon atom is also bonded to two hydrogen atoms. This arrangement results from the sp² hybridization of the carbon atoms, giving the molecule a flat geometry.
3. What is the primary role of ethylene in the fruit ripening process?
Ethylene plays a crucial role in initiating and accelerating the fruit ripening process, especially in climacteric fruits. Its main functions include:
- Triggering a sharp increase in the rate of respiration, known as the respiratory climacteric.
- Promoting the breakdown of chlorophyll and the synthesis of other pigments (like carotenoids and anthocyanins), which causes the fruit to change colour.
- Facilitating the enzymatic breakdown of complex carbohydrates into simple sugars, making the fruit sweeter.
- Causing the softening of the fruit by degrading pectin in the cell walls.
4. What are some key examples of ethylene's applications in agriculture?
In agriculture, Ethephon, a compound that releases ethylene, is widely used. Key applications include:
- Inducing uniform flowering and synchronising fruit set in pineapples.
- Promoting the development of female flowers in cucumbers, which significantly increases the crop yield.
- Accelerating the abscission (shedding) of older flowers and fruits to help in thinning crops like cotton, cherry, and walnut.
- Breaking seed and bud dormancy in various plants, such as promoting sprouting in potato tubers.
5. Why is ethylene considered a unique plant hormone compared to others like auxins or gibberellins?
Ethylene is unique among plant hormones primarily because it is a volatile gas. This physical state allows it to diffuse through the air and intercellular spaces, enabling it to affect not only the tissue that produces it but also adjacent plant parts or even neighbouring plants. Furthermore, it exhibits a distinct dual functionality, acting as both a growth promoter (e.g., in fruit ripening) and a powerful growth inhibitor (e.g., in promoting senescence and abscission).
6. How does ethylene induce the 'triple response' in seedlings, and what is its biological importance?
The 'triple response' is a critical survival strategy for seedlings, particularly dicots, when they encounter a physical obstacle while growing in the dark. Ethylene triggers this response, which involves three specific actions:
- Inhibition of stem elongation: Prevents the delicate shoot from damaging itself by growing against the barrier.
- Promotion of lateral expansion: The stem swells and thickens, making it stronger.
- Horizontal growth: The seedling grows horizontally instead of vertically, allowing it to navigate around the obstacle.
7. How does ethylene's role in senescence and abscission differ from its role in promoting growth?
Ethylene demonstrates a stark contrast in its functions. As a growth promoter, it initiates germination, promotes root and root hair formation for better absorption, and induces flowering in certain plants. In its role related to senescence and abscission, it acts as an inhibitor by accelerating the ageing process. This includes promoting the yellowing of leaves, the fading of flowers, and the formation of an abscission layer that leads to the shedding of leaves, flowers, and mature fruits from the parent plant.
8. What is the difference between climacteric and non-climacteric fruits in relation to ethylene?
The primary difference lies in their ripening behaviour and response to ethylene.
- Climacteric fruits (e.g., bananas, mangoes, tomatoes) exhibit a dramatic increase in ethylene production and respiration rate as they ripen. They can continue to ripen even after being harvested. Ethylene is essential to trigger this ripening cascade.
- Non-climacteric fruits (e.g., grapes, citrus fruits, strawberries) do not show a spike in ethylene production or respiration during ripening. They must ripen on the plant to achieve full flavour and quality, as ethylene treatment has little to no effect on their post-harvest ripening.
