Ethylene and abscisic acid

A bad apple spoils the box

Everyone knows the saying. One rotten fruit in a basket can make the others go bad. A more cheerful version: a ripe banana in the fruit bowl makes green fruit ripen sooner. The cause is a gas.

In 1910 Cousins noticed that oranges stored with bananas made the bananas ripen faster. The ripe oranges gave off a gas. Later work showed that the gas is ethylene, and that plants make it themselves.

Ethylene

Ethylene is a simple gaseous PGR, with the formula $\text{C}_2\text{H}_4$. It is made in large amounts by tissues that are ripening or senescing.

NCERT gives these effects.

• Seedling growth. It changes how a seedling grows. It affects the horizontal growth of seedlings, the swelling of the axis and the apical hook of dicot seedlings. The apical hook is the bend at the top of a seedling that keeps the soft tip safe as the seedling pushes through the soil.
• Senescence and abscission. It promotes the senescence and abscission of plant parts, mostly leaves and flowers.
• Ripening. It is highly effective in fruit ripening. It also enhances the respiration rate of the fruit during ripening, and this sharp rise is called the respiratory climacteric.
• Breaking dormancy. It breaks the dormancy of seeds and buds. It starts germination in peanut seeds and sprouting in potato tubers.
• Deep-water rice. It promotes the quick elongation of the internodes and petioles in deep-water rice, so that the leaves and the upper parts stay above the water.
• Roots. It promotes root growth and root hair formation, so the plant absorbs more.
• Flowering. It starts flowering and helps all the fruits of a plant to set at one time in pineapple. It also induces flowering in mango.

In the picture the fruit makes ethylene first. Then the rate of respiration shoots up to a peak, which is the climacteric. After the peak the fruit is ripe and begins to age.

Some fruits show a climacteric, such as banana, apple, tomato and mango. Others do not, such as citrus fruits and grapes. Beyond NCERT, this is a favourite NEET detail.

You should find that more ethylene moves the peak to an earlier day. The shape of the curve stays the same.

Ethephon

A gas is hard to use in a field. The most widely used compound that gives ethylene is ethephon, a liquid. Ethephon is taken in by the plant and then breaks down slowly to release ethylene.

• It hastens ripening of tomatoes and apples.
• It speeds up the abscission of flowers and fruits, so it is used for thinning, for example in cotton, cherry and walnut.
• It promotes female flowers in cucumber, which raises the yield, since only female flowers make fruit.

A triple response, beyond the text

Neljubow, in 1901, found that pea seedlings grown in the laboratory air of a city, where gas lamps burned, grew in a strange way. They were short and thick and they grew sideways. This is the triple response to ethylene: a shorter axis, a thicker axis and a horizontal growth. The gas from the lamps was ethylene. This goes beyond the NCERT text.

Abscisic acid

Abscisic acid, or ABA, is a derivative of carotenoids. It was found as a chemical that caused the abscission of cotton fruits (Addicott) and the dormancy of buds of sycamore (Wareing, who named it dormin). They later turned out to be the same compound. Its name fits only part of its work.

ABA is a general growth inhibitor, and it also slows the metabolism of the plant. NCERT lists these effects.

• It inhibits seed germination.
• It stimulates the closure of stomata in the epidermis.
• It raises the tolerance of the plant to stress of many kinds. For this reason it is called the stress hormone.
• It has an important role in seed development, maturation and dormancy. It helps seeds to survive desiccation, which is drying out, and other conditions that are bad for growth.
• In most cases, ABA is an antagonist of gibberellins.

Think of a dry season. The plant makes ABA, the guard cells close the stomata, and less water escapes. When the rain returns, the ABA level falls.

Which regulator does which job?

Three traps catch students again and again.

• Delay of senescence is done by gibberellins and cytokinins. Ethylene and ABA promote senescence and abscission.
• Breaking the dormancy of seeds and buds is done by gibberellins and ethylene. Starting and keeping dormancy is done by ABA.
• Auxins prevent the early drop of leaves and fruits. They promote the abscission of old ones. Ethylene promotes abscission and ABA also promotes it.

Try it in Python

The program shows how the days to ripening of green bananas shorten as the ethylene in the room increases.

Practise with new numbers

Common mistakes

• Calling ethylene a liquid. It is a gas. Ethephon is the liquid that releases it.
• Saying that ABA only causes abscission. Its main work is stress response, stomatal closure and dormancy.
• Giving the climacteric to every fruit. Banana, apple and tomato have it. Citrus and grapes do not.
• Saying that GA and ABA work together. In most cases they are antagonists.
• Forgetting that auxin prevents early drop. Auxin prevents the drop of young fruits and leaves, and promotes the drop of old ones.

Remember these

• Ethylene, $\text{C}_2\text{H}_4$, is a gas made by ripening fruit and senescing tissue. It ripens fruit, causes the respiratory climacteric, and promotes senescence and abscission.
• Ethylene breaks seed and bud dormancy (peanut, potato), lengthens the internodes of deep-water rice, and makes root hairs.
• Ethephon gives ethylene. It is used to ripen tomatoes and apples, thin cotton, cherry and walnut, and give female flowers in cucumber.
• ABA is a carotenoid derivative. It inhibits germination, closes stomata, helps tolerance of stress, and keeps seeds dormant. It is the antagonist of GA.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the respiratory climacteric?

2. Name three uses of ethephon.

3. Why is abscisic acid called the stress hormone?

4. Which two regulators delay senescence, and which two promote it?

5. How does ABA act against gibberellins in a seed?