11.10 Using Plant Hormones

A free GCSE Biology lesson for AQA (8461) from OBA School: (Higher tier) Commercial uses of auxins, ethene and gibberellins. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how auxins are used as weedkillers, rooting powders and in tissue culture
• Explain how ethene is used to control fruit ripening during storage and transport
• Describe how gibberellins are used to end seed dormancy, promote flowering and increase fruit size
• Evaluate the use of plant growth hormones in agriculture and horticulture

Key words

• Selective weedkiller: A weedkiller containing auxin that kills broad-leaved weeds but not narrow-leaved crops such as wheat.
• Rooting powder: A powder containing auxin that makes cuttings grow roots quickly.
• Cutting: A piece of a plant (e.g. a shoot) used to grow a new, genetically identical plant.
• Tissue culture: Growing new plants from small groups of cells in a sterile medium containing nutrients and hormones.
• Seed dormancy: A resting state in which a seed does not germinate even if conditions seem suitable.

Using auxins

This section is Higher tier content. Farmers and gardeners (horticulturists) use synthetic plant hormones to control how plants grow.

• Weedkillers; How it works Selective weedkillers contain auxin. Broad-leaved weeds absorb more and respond strongly: they grow too fast and die. Narrow-leaved crops (wheat, grass) are hardly affected, so the crop has less competition for light, water and minerals.
• Rooting powders; How it works A cut shoot is dipped in powder containing auxin before planting. It grows roots quickly, giving many identical plants (clones).
• Tissue culture; How it works Auxins (with other hormones) in the growth medium promote cell division and growth of roots and shoots from small pieces of tissue.

Worked example: reading the chart

Calculate the percentage increase in the mean number of roots between “none” and “medium”.
(11.2 − 2.4) ÷ 2.4 × 100 = 8.8 ÷ 2.4 × 100 = 366.66…
Notice the “high” concentration gives fewer roots than “medium”: too much auxin can inhibit root growth.

Answer: 367% (3 s.f.)

Using ethene

Ethene is a gas that controls the ripening of fruit. The food industry uses it to control ripening during storage and transport.

• Unripe fruit is firm, so it is less easily damaged during transport.
• Ripening can be timed so fruit arrives in shops ready to eat, reducing waste.
• Removing ethene from stores (and keeping fruit cool) slows ripening, so fruit can be stored for longer.

Using gibberellins

• End seed dormancy; Example Barley seeds are made to germinate at the same time in malting (for brewing); seeds of some crops germinate when needed.
• Promote flowering; Example Some plants can be made to flower without the usual conditions (e.g. a cold period), or earlier, for sale.
• Increase fruit size; Example Sprayed on seedless grapes to produce bigger fruit and looser bunches.

Summary: Auxin: weeds, roots, tissue culture. Ethene: ripening. Gibberellins: dormancy, flowering, fruit size.

Evaluating plant hormone use

• Higher crop yields and less competition from weeds; Drawbacks Weedkillers can harm other plants and animals and reduce biodiversity
• Many identical plants quickly and cheaply from cuttings; Drawbacks Clones have no genetic variation, so a disease could affect all of them
• Less waste of fruit; fruit available all year round; Drawbacks Cost of chemicals; some people prefer food grown without them
• Flowers and fruit produced to order; Drawbacks Workers need protective equipment when spraying

Worked example: exam tip

In “evaluate” questions, give a benefit and a drawback for each use you mention, then a conclusion, e.g. “The benefits outweigh the risks if the hormones are used carefully and in the right amounts.”

Common mistakes

• Saying weedkillers “stop weeds growing”. Auxin makes broad-leaved weeds grow too fast, so they die.
• Saying ethene ripens fruit in the field before picking. It is used after picking, in storage and transport.
• Mixing up the hormones: gibberellins (not auxins) end seed dormancy and increase fruit size.
• Saying more rooting powder is always better. Very high auxin concentrations can reduce root growth.

Revision questions

Try these, then check your answers below each one.

Question 1. Why does a selective weedkiller kill broad-leaved weeds but not wheat? (Choose from: Broad-leaved weeds respond strongly to auxin, grow too fast and die, Wheat absorbs all the weedkiller first, Weeds cannot photosynthesise, The weedkiller only works in the dark.)

Answer: Broad-leaved weeds respond strongly to auxin, grow too fast and die

Working: Broad-leaved weeds absorb more auxin and grow out of control; narrow-leaved cereals are hardly affected.

Question 2. True or false? Fruit is often picked unripe and later exposed to ethene so it ripens close to when it will be sold.

Answer: True

Working: True. This reduces damage during transport and waste.

Question 3. Why are gibberellins used in malting barley for brewing? (Choose from: To end seed dormancy so the seeds germinate, To kill weeds in the barley field, To make roots grow on cuttings, To slow down ripening.)

Answer: To end seed dormancy so the seeds germinate

Working: Gibberellins end seed dormancy, so barley seeds germinate together.

Related lessons

• Previous lesson: 11.9 Plant Hormones and Responses
• Next lesson: 11.11 Required Practicals: Hormonal Coordination
• In this chapter: 11.1 Principles of Hormonal Control
• In this chapter: 11.2 The Control of Blood Glucose Levels
• In this chapter: 11.3 Treating Diabetes

Continue in the full course

This lesson is section 11.10 (AGCBH1110) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.