Plant Hormones » Using Plant Hormones

Plant hormones at work

Farmers and gardeners do not just wait for plants to do what they want. Plant growth hormones are used in agriculture (farming) and horticulture (growing plants in gardens, nurseries and greenhouses) to control how plants grow. The hormones are either the natural ones or man-made chemicals that act in the same way.

This lesson is about what people use the hormones for. If you need to know how the hormones work inside the plant, look back at the lesson on Plant Hormones and Tropisms.

Key terms:

• Weed: a wild plant growing where it is not wanted, such as in a crop or a lawn.
• Weed killer: a chemical that kills weeds.
• Cutting: a piece cut from a parent plant, such as a short stem with leaves, which is used to grow a new plant.
• Rooting powder: a powder containing auxin that helps a cutting grow roots.
• Dormancy: a resting state in which a seed does not grow, even if it is alive.

Uses of auxins

Auxins are used in three main ways.

Auxins as weed killers

Most weeds in lawns and cereal crops have broad leaves. Hormone weed killers contain auxins in a large amount. When sprayed on the field, the weeds take in so much auxin that they grow in an uncontrolled way and die. The crop plants, such as wheat, are not killed. Because the weed killer kills only some plant species, it is described as selective.

The benefit is that the crop has less competition from weeds for light, water and mineral ions, so the farmer gets a bigger yield.

Auxins as rooting powders

A gardener can make new plants by taking cuttings from a parent plant. A cutting has no roots, so it cannot take in water at first. If the cut end is dipped in rooting powder, the auxin makes the cutting grow roots much faster. The cutting then becomes a new plant that is a copy of its parent.

This is useful for nurseries, which can produce many identical plants of a good variety quickly.

Auxins in tissue culture

In tissue culture, tiny pieces of plant tissue are grown in a clean, sterile dish on a jelly that contains nutrients. Auxin is added to the jelly to promote the growth of the tissue. The details of tissue culture are covered in the lesson on Cloning.

Ethene and fruit ripening

Ethene is a plant hormone that makes fruit ripen. It is used in the food industry to control the ripening of fruit during storage and transport.

• Fruit that is ripe is soft and easily bruised. If it ripens on the plant, it may be spoiled before it reaches the shops.
• So some fruit, such as bananas, is picked while it is still unripe and firm. It is stored and transported unripe.
• When the fruit arrives, ethene is added so that it all ripens at the same time, ready for sale.
• Ripening can also be slowed by keeping the fruit away from ethene, so that it lasts longer in storage.

The result is less wasted fruit, and fruit that arrives in the shop in good condition.

Uses of gibberellins

Gibberellins are used in three ways.

Controlling when seeds germinate and when plants flower lets a grower produce a crop at the right time, and have many plants ready together. Bigger fruit is easier to sell. For example, gibberellins are sprayed on seedless grapes to make them larger.

Hormone weed killers and biodiversity

Biodiversity is the variety of living things in an area. The everyday use of hormone weed killers can reduce it.

• The weed killer kills many species of wild plants, so there is a smaller variety of plants in the field.
• Insects and other animals that feed on those plants, or use them for shelter, have less food and fewer places to live. Birds that feed on those insects can also be affected.
• If the spray drifts or is washed off, it can harm wild plants outside the field, such as in hedgerows.

So there is a trade-off. Farmers get a bigger yield of their crop, but there are fewer wild plants and animals in their fields. Each is a point you can use in an exam answer.

Putting it together

• Auxins: weed killers, rooting powders, promoting growth in tissue culture.
• Ethene: controls ripening of fruit in storage and transport.
• Gibberellins: end seed dormancy, promote flowering, increase fruit size.
• Hormone weed killers can reduce biodiversity.

Exam-style question

A farmer sprays his wheat field with a hormone weed killer. A fruit company ships bananas that are picked while green.

(a) Name the type of plant hormone that is used in the weed killer. [1 mark]

(b) Suggest one way the weed killer may reduce biodiversity in the field. [2 marks]

(c) Name the hormone used to ripen the bananas when they arrive, and give one advantage of shipping them green. [2 marks]