Triple Science Only - Other Plant Hormones And Their Uses (Higher Tier)
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of auxins.
What is a tropism?
A directional growth response of a plant to a stimulus.
Which plant hormone controls phototropism and gravitropism?
Auxin.
In shoots, what do high concentrations of auxin lead to?
Faster growth, for example on the shaded side of a shoot in one directional light.
Topic Explainer Video
Check out this @lauradoesbiology video that explains other plant hormones and their uses, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Effects and Uses of Plant Hormones
Auxins- quick re-cap
What are they?
- Auxins are plant hormones that control growth at the tips of shoots and roots.
Tropisms controlled by auxins:
- Phototropism: Growth towards light.
- Gravitropism (also known as geotropism): Growth in response to gravity.
How auxins work in tropisms:
- In shoots: More auxin = more growth.
- In roots: More auxin = less growth.
This leads to shoots bending towards light and roots growing downwards with gravity.
See the previous 2 study notes for more on auxin and tropisms.
Plant growth hormones are also used in agriculture and horticulture- such as auxins, gibberellins and ethene.
Uses of Auxins
|
Use |
Explanation |
|---|---|
|
Weed killers |
Some weedkillers contain high concentrations of auxins that cause uncontrolled growth in weeds, killing them without harming other plants. |
|
Rooting powder |
Contains auxins to encourage root growth in plant cuttings. This is a type of plant cloning. |
|
Tissue culture |
Auxins are added to nutrient media to stimulate growth of plant cells into full plants. Tissue culture is another type of plant cloning. |
Weedkillers can affect biodiversity if not used carefully - they may kill wild plants that support other organisms. This can disrupt food chains.
Uses of Gibberellins
Role: Promote germination, flowering, and fruit size.
|
Use |
Effect |
|---|---|
|
End seed dormancy |
Seeds germinate earlier, even when temperatures are cold, so their growing season is longer. |
|
Promote flowering |
Speeds up flowering or triggers it without changing light conditions. |
|
Increase fruit size |
Larger, more marketable fruit. |
Uses of Ethene
Role: Controls cell division and ripening of fruit.
|
Use |
Explanation |
|---|---|
|
Fruit ripening control |
Ethene gas is used in transport to ripen fruit just before sale – prevents spoilage during shipping. |
Common in supermarkets: Bananas are picked and transported green to prevent damage and ripened with ethene on arrival.
Key Terms & Definitions
|
Key Term |
Definition |
|---|---|
|
Tropism |
Directional growth of a plant in response to a stimulus. |
|
Auxin |
Plant hormone that controls growth in shoots and roots. |
|
Phototropism |
Plant growth towards light. |
|
Gravitropism |
Plant growth in response to gravity. |
|
Gibberellins |
Plant hormones used to promote seed germination, flowering, and fruit growth. |
|
Ethene |
A gas that controls cell division and fruit ripening. |
|
Weedkiller |
A chemical that targets and kills weeds without damaging crops or other plants. Weedkillers are also known as herbicides. |
No answer provided.
Exam Tips
When answering questions on plant hormones:
- State which hormone is involved (auxin, gibberellin, ethene).
- Include what it controls and how it is used.
No answer provided.
Practice Questions
Q1: Farmers can use plant hormones to control the growth of plants. Give three commercial uses of auxin. (3 marks)
Q2: A farmer sprayed an apple tree with gibberellin. Suggest two reasons why the farmer sprayed the apple tree with gibberellin. (2 marks)
Model Answers:
Q1:
- weed killers
- rooting powders / to grow plants from cuttings
- promoting growth in tissue culture
Q2: Any two from the following:
- promote / start flowering / to grow more flowers
- to grow more fruit
- to grow bigger fruits
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!