Ethylene: Discovery and Physiological Effects, Practice Problems and FAQs

We all get a variety of fruits and vegetables from the market. Sometimes you might have accompanied your parents to the market. When we buy fruits from the market, what are the factors commonly we consider that tell us the fruit is ready to eat?

Yes, first we see the colour of the fruit, once ripened it changes the colour. Secondly we smell the fruit. If it gives a sugary fragrance, it is a ripened fruit and ready to eat. Lastly, we check the fruit by smoothly pressing it with our hands. If the fruit is ripened, it is palpable. If the fruit is unripened, it does not give a sugary fragrance and is non palpable.

                                          Fig: Ripened fruits

For example, the ripened mangoes give a sweet fragrance and the colour changes from green to yellow. On the other hand, the unripened mangoes are green in colour and they are not sweet in taste.

So, have you ever wondered which chemical is responsible for ripening of fruits? Yes, it is ethylene, you are correct. It is a gaseous hormone which stimulates the ripening of fruits. Due to the presence of this hormone, we can eat sweet and juicy fruits. Let’s understand more about ethylene hormones in this article.

Table of contents:

• Discovery of ethylene
• Physiological effects of ethylene
• Practice Problems
• FAQs

Discovery of ethylene

The fruit merchants used kerosene lamps and hay to fasten the ripening process in fruits. They were aware that the ripened and rotten fruits are able to accelerate the ripening in other fruits. H. H. Cousins confirmed that ripened oranges hastened the ripening of stored unripened bananas, when they were kept near. The spoiled oranges released a volatile substance, which was later identified as ethylene. R. Gane provided evidence for the presence of these volatile substances. 

                                                  Fig: Experiment by H.H Cousins

Chemical formula of ethylene

The chemical formula of ethylene is C2H4 and its structure is given below:

                                      Fig: Ethylene

Chemical formula of ethephon 

Ethephon is the most widely used commercial source of ethylene, because ethylene is a gas and is difficult to use. Ethephon is 2-chloroethylphosphonic acid which is a solid and can easily be used in a solution. Plants can easily absorb and transport ethephon in solution.

                                       Fig: Ethephon

Physiological effects of ethylene

Ethylene is a volatile hormone. It has a promoting effect on all the stages of plant life. Ethylene show its effects on the following plant parts:

• Seeds and seedlings
• Shoots
• Roots
• Flowers
• Fruits
• Various organs

Effect of ethylene on seeds and seedlings

Ethylene breaks seed dormancy and stimulates seed germination along with gibberellin. Ethylene promotes the horizontal growth of seedling. 

                      Fig: Horizontal growth of seedlings

It increases the girth of seedling. It promotes apical hook formation in dicot seeds. It prevents damage to the growing tips.

                       Fig: Apical hook formation

Effect of ethylene on shoots

It breaks bud dormancy. It inhibits the growth of lateral buds. It causes apical dominance. It is identified to promote the sprouting in potato tubers.

                              GIF: Sprouting of potato tubers

It promotes internode or petiole elongation in deep-water rice plants. The rice plants are evolved to tolerate huge quantities of water. The leaves and upper parts of the rice plant remain above water and will not suffocate because of this. It increases the harvest yield.

                        Fig: Internode or petiole elongation

Effect of ethylene on root

Ethylene promotes root growth. It also promotes the formation of root hair to increase the area for absorption.

                                         Fig: Formation of root hair

Effect of ethylene on flowers

Promotes flowering

Ethylene promotes flowering in pineapples and mangoes. It increases the yield. Ethylene synchronises fruit set in pineapple. The fruit set is described as the transition from flower to fruit. It makes the pineapple plant to flower and produce fruit at the same time. This is because natural flowering is variable in pineapple.

                            Fig: Pineapple

Sex expression

Ethylene favors female flowers over male flowers in cucumber. It increases yield.

                     Fig: Setting of female flowers in cucumber

Effect of ethylene on fruit

Ethylene is used to ripen the fruit. Ethephon is the wide source of ethylene. Ethephon is absorbed by the plants and converts into ethylene. Ethylene leads to respiratory climacteric. It is the final stage when fruit changes its color from green to yellow, red or so on. At this stage, cellular respiration increases. In some fruits, sugars are released during this time.

                                    GIF: Respiratory climacteric

Effect of ethylene on various organs

Ethylene induces senescence. Senescence is considered as the process of aging of a plant and its parts. It happens in all the plant parts like flowers, leaves, fruits etc.

Ethylene promotes abscission, i.e. it normally causes the leaves to fall off from the plants. 

Thinning is reducing the number of fruits or other plant parts so that the remaining ones can grow better. It is used for thinning in agriculture. Ethephon is commonly used for thinning.

                             Fig: Thinning in agriculture

Practice Problems

Q 1. Let’s put some unripened fruits with a ripened fruit which is going to spoil. What will you observe? The unripened fruits start ripening because the spoiled fruit releases ethylene. This condition explains which property of ethylene?

a. Ethylene is a volatile
b. Ethylene has slightly sweet smell
c. Ethylene is a non polar molecule
d. Ethylene is reactive

Answer: Ethylene is a volatile hormone. It has a chemical formula of C2H4. It has a promoting effect on all the stages of life. Therefore, ethylene releases from spoiled fruit slowly and steadily ripe the other unripened fruits. Hence, the correct option is a.

Q 2. From the given options, identify which is not a function of ethylene.

a. Breaks seed dormancy
b. Induces senescence
c. Bending of shoot towards light
d. Formation of root hair

Answer: Phototropism is characterised as the bending of shoot towards light. The shoot of a plant shows positive phototropism. This is accelerated by auxin hormone. Ethylene is a stimulating hormone that performs various functions in different parts of the plant. Ethylene breaks seed dormancy and stimulates seed germination along with gibberellin. Ethylene induces senescence. Senescence is considered as the process of aging of plants and its parts. Ethylene promotes root growth. It also promotes the formation of root hair to increase the area for absorption. Hence, the correct option is c.

Q 3. Take an unripened banana and wrap it in newspaper. After 5 - 6 days, unwrap it and see the changes. You see the unripe banana is converted into ripened one. It also gives a fruity fragrance. This is because of which hormone?

a. Gibberellin
b. Ethylene
c. Abscisic acid
d. Auxin

Answer: Ethylene is a volatile hormone. It has a chemical formula of C2H4. It has a promoting effect on all the stages of life. Ethylene is used to ripen the fruit. Ethephon is the wide source of ethylene. Ethephon is absorbed by the plants and converts into ethylene. Ethylene leads to respiratory climacteric. It is the final stage when fruit changes its color from green to yellow, red or so on. At this stage, cellular respiration increases. In some fruits, sugars are released during this time. Hence option b is correct.

Q 4. What is the effect of ethylene on rice plants?
Answer: Ethylene promotes internode or petiole elongation in deep-water rice plants. The rice plants are evolved to tolerate huge quantities of water. The leaves and upper parts of the rice plant remain above water and don’t suffocate due to this. It increases harvest yield in this way.

FAQs

Q 1. What role does ethylene play in fruit development?
Answer: Ethylene is used to ripen the fruit. Ethephon is the wide source of ethylene. Ethephon is absorbed by the plants and converts into ethylene. Ethylene leads to respiratory climacteric. It is the final stage when fruit changes its color from green to yellow, red or so on.

Q 2. What are the common effects of ethylene in plants?
Answer: Ethylene shows many physiological effects in different plant parts as follows:

• Ethylene breaks seed dormancy and stimulates seed germination along with gibberellin.
• It breaks bud dormancy. 
• It promotes the sprouting of potato tubers.
• Ethylene promotes root growth.
•  It also promotes the formation of root hair to increase the area for absorption.
• Ethylene promotes flowering in pineapples and mangoes.
• Ethylene is used to ripen the fruit. 
• Ethylene induces senescence and abscission.

Q 3. What is the source of ethylene in plants?
Answer: Ethylene is a natural hormone produced by different plant parts, such as leaves, stems, roots, flowers, tubers, seeds and fruits. 

Q 4. What is the effect of ethylene on plant leaves?
Answer: Ethylene induces senescence. Senescence is the process of aging of plants. Ethylene promotes abscission, i.e. it causes the leaves to fall off. 

Q 5. How is ethylene hormone normally transported in plants?
Answer: Ethylene hormone is produced in leaves and transported through the phloem part to the shoot apical meristem of growing tips and buds.

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