Use of plant hormones (HT only) — AQA GCSE Biology
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Use of plant hormones (HT only) explained
Students must understand the commercial applications of auxins, ethene, and gibberellins.
Read the full explanation
Auxins promote cell elongation and are used as selective weed killers (disrupting growth in broad-leaved weeds but not narrow-leaved crops), in rooting powders to stimulate root growth in cuttings, and in tissue culture to promote growth. Ethene is a gas that controls cell division and the ripening of fruits; it is widely used in the food industry to control ripening during storage and transport. Gibberellins initiate seed germination and are used commercially to end seed dormancy, promote flowering throughout the year, and increase the size of fruits, such as seedless grapes.
Plant growth hormones are used in agriculture and horticulture.
Agriculture and horticulture both rely on plant growth hormones to manage crops and ornamental plants. In agriculture, auxin-based selective weed killers reduce competition from broad-leaved weeds in cereal fields, improving yield. In horticulture, rooting powders containing auxins help cuttings such as pelargonium or rosemary develop roots quickly and uniformly, producing many identical plants. Tissue culture uses auxins and other hormones to multiply rare or valuable plants from small pieces of tissue under sterile conditions. Growers may also use hormones to control fruit ripening, delay leaf fall or regulate growth so plants are a convenient size for transport and sale. These uses show how understanding hormone action allows people to manipulate plant growth deliberately.
Auxins are used: • as weed killers • as rooting powders • for promoting growth in tissue culture.
Auxins have three important commercial uses. As weed killers, synthetic auxins are sprayed onto fields; broad-leaved weeds absorb them and grow abnormally, while narrow-leaved cereals are less affected, so the treatment is selective. As rooting powders, auxin is applied to the cut end of a stem cutting; it stimulates root formation so the cutting establishes quickly and many identical plants can be produced. In tissue culture, small pieces of plant tissue are grown on a sterile nutrient medium containing auxins and other hormones; the auxins promote cell division and shoot or root development, producing many plantlets from one parent. Each use depends on auxin promoting or disrupting normal growth and development.
Ethene is used in the food industry to control ripening of fruit during storage and transport.
Ethene is a gaseous plant hormone that promotes fruit ripening. In the food industry, it is applied to unripe fruit during storage and transport so that ripening occurs at a controlled time, often just before sale. For example, bananas are harvested unripe, transported in controlled conditions, then exposed to ethene to ripen uniformly. Students should understand that ethene controls the rate and timing of ripening, reducing spoilage and waste. They should also recognise that ethene is a plant hormone, not a nutrient or pesticide, and that its use allows fruit to be stored and transported without over-ripening. The skill involves explaining how ethene is used and why this benefits the food industry, using correct terminology such as hormone, ripening, storage and transport.
Your focus
- Describe the commercial uses of auxins as weed killers, rooting powders, and in tissue culture.
- Explain how ethene is used to control the ripening of fruit during transport and storage.
- Describe the use of gibberellins in ending seed dormancy, promoting flowering, and increasing fruit size.
Show all 12 objectives
- Identify agricultural and horticultural contexts in which plant growth hormones are used.
- Describe at least one agricultural use and one horticultural use of plant growth hormones.
- Link each commercial use to the effect of the hormone on plant growth.
- Describe how auxins are used as weed killers, rooting powders and in tissue culture.
- Explain the growth effect of auxins in each of these three commercial uses.
- Relate each use of auxin to the control of plant growth in agriculture and horticulture.
- State that ethene is a plant hormone used to control fruit ripening.
- Describe how ethene is used during storage and transport of fruit.
- Explain the benefits of using ethene in the food industry.
Use of plant hormones (HT only) exam tips
Marking Points
- Auxins are used as selective weed killers, targeting broad-leaved weeds without harming narrow-leaved crops.
- Auxins are used in rooting powders to stimulate root formation in plant cuttings, and in tissue culture to promote growth.
- Ethene is used in the food industry to control the ripening of fruit during storage and transport.
- Gibberellins are used to end seed dormancy, encouraging seeds to germinate at all times of the year.
- Gibberellins are also used to promote flowering and to increase the size of fruits.
- States that plant growth hormones are used commercially in agriculture and horticulture to control plant growth.
- Gives an agricultural example, such as auxin selective weed killers reducing competition from broad-leaved weeds in cereal crops.
- Gives a horticultural example, such as auxin rooting powder encouraging cuttings to form roots.
- Explains that tissue culture uses plant hormones to produce many genetically identical plants from small explants.
- Links each use to the underlying hormone effect, such as cell elongation, root initiation or controlled development.
- States that auxins can be used as selective weed killers that disrupt the growth of broad-leaved weeds.
- Explains that auxin rooting powders stimulate root formation in stem cuttings.
- Describes tissue culture as growing small explants on a sterile medium containing auxins to promote growth and development.
- Links each use to auxin action, such as cell elongation, root initiation or controlled cell division and differentiation.
- Recognises that these uses allow growers to control plant growth for agricultural or horticultural purposes.
- Ethene is a plant hormone that promotes the ripening of fruit.
- It is used to control the timing of ripening so fruit can be stored and transported before it ripens.
- Fruit is often harvested unripe and exposed to ethene when ripening is required, for example before sale.
- This reduces waste and allows fruit to reach consumers in good condition.
- Ethene is applied as a gas in controlled conditions during storage or transport.
Examiner Tips
- 💡Memorise at least one specific commercial use for each of the three hormones: auxins, ethene, and gibberellins.
- 💡When discussing ethene, mention its use in transport, allowing fruit to be picked unripe and ripened just before sale.
- 💡Give one clear agricultural use and one clear horticultural use, each with the hormone and its effect, to cover the statement fully.
- 💡Use precise terms such as selective weed killer, rooting powder and tissue culture rather than vague phrases like chemicals for plants.
- 💡If asked to compare uses, comment on whether the hormone promotes or disrupts growth and which plants are affected.
- 💡For each of the three uses, write one sentence naming the auxin use and one sentence explaining the growth effect it causes.
- 💡Use the term selective when writing about auxin weed killers, and contrast broad-leaved weeds with narrow-leaved crops.
- 💡In tissue culture answers, mention sterile conditions and the role of auxins in promoting growth from a small explant.
- 💡Link ethene to its role as a plant hormone and its effect on ripening.
- 💡Use examples such as bananas to explain how ethene is used in storage and transport.
- 💡Explain the benefit to the food industry, such as reducing waste or ensuring fruit ripens at the right time.
Common Mistakes
- Confusing the roles of different hormones; the correction is to remember auxins are for roots and weeds, ethene is for ripening, and gibberellins are for seeds, flowers, and fruit size.
- Stating that weed killers kill all plants; the correction is that selective weed killers specifically target broad-leaved plants and leave narrow-leaved crops unaffected.
- Forgetting that ethene is a gas; the correction is to explicitly state that ethene is a gaseous hormone used to control fruit ripening.
- Describing agriculture and horticulture as identical; the correction is that agriculture mainly involves crop production while horticulture involves garden and ornamental plant production, though both use plant hormones.
- Naming a use without naming the hormone or its effect; the correction is to state the hormone involved and the growth response it produces.
- Assuming all plant hormone uses increase growth; the correction is that some uses, such as weed killers, disrupt or inhibit normal growth.
- Saying auxin weed killers kill only weeds and never affect crops; the correction is that they are selective because they affect broad-leaved plants much more than narrow-leaved cereals, but selectivity is not absolute.
- Confusing rooting powder with fertiliser; the correction is that rooting powder contains auxin, a hormone that stimulates root growth, not nutrients.
- Describing tissue culture as simply growing plants in soil; the correction is that tissue culture uses small explants on a sterile nutrient medium with hormones under controlled conditions.
- Confusing ethene with ethylene glycol or other chemicals; correction: ethene is the correct name for the plant hormone.
- Thinking ethene is used to prevent ripening; correction: ethene promotes or controls ripening, not prevents it.
- Believing ethene is a nutrient for fruit; correction: it is a hormone that regulates ripening.