5.26 Uses of plant hormones
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Describe two ways auxins are used in agriculture or horticulture.
Mark scheme
Any two from: (selective) weedkillers / rooting powders (for cuttings) / promoting growth in tissue culture (1 each)
Model answer
As selective weedkillers, and in rooting powders that make cuttings grow roots.
Explain how a selective weedkiller containing auxin can kill weeds in a wheat field without killing the wheat.
Mark scheme
- weeds are broad-leaved; wheat is narrow-leaved (1)
- broad-leaved weeds absorb more auxin (1)
- weeds grow too fast / abnormally and die (1)
Model answer
The weeds are broad-leaved, so they absorb much more of the auxin than the narrow-leaved wheat. The auxin makes the weeds grow too quickly and abnormally, so they die, while the wheat is not affected.
Bananas are picked unripe and transported long distances. Ethene is added before they are sold.
Explain the advantages of doing this.
Mark scheme
- unripe bananas are firm / less likely to be damaged in transport (1)
- ethene makes them ripen in time for sale / ripe when sold / less waste (1)
Model answer
Unripe bananas are firm, so they are less likely to be bruised during transport, and adding ethene makes them ripen just in time to be sold.
Give two uses of gibberellins.
Mark scheme
Any two from: end seed dormancy / start germination / promote flowering / increase fruit size / produce seedless fruit (1 each)
Model answer
To end seed dormancy so seeds germinate, and to increase the size of fruit.
Evaluate the use of auxin weedkillers by farmers.
Mark scheme
- advantage: kills weeds so less competition for light / water / minerals (1)
- advantage: higher crop yield / cheaper food (1)
- disadvantage: reduces biodiversity / removes food and habitats for insects and birds (1)
- disadvantage: may harm other plants / drift / pollute water (1)
- a justified conclusion, e.g. use carefully / leave field margins (1)
Max 4; must include an advantage and a disadvantage
Model answer
Weedkillers remove weeds that compete with the crop for light, water and mineral ions, so the farmer gets a bigger yield. However, removing the weeds also removes food and habitats for insects, birds and pollinators, so biodiversity on the farm falls, and spray may drift onto hedgerows or wash into rivers. They are useful, but farmers should use them carefully, for example by leaving field margins unsprayed.
A gardener wants many identical copies of a plant with attractive flowers.
Explain how rooting powder helps.
Mark scheme
- cuttings are dipped in rooting powder containing auxin (1)
- (auxin) makes the cuttings grow roots quickly, producing clones (1)
Model answer
The gardener takes cuttings and dips them in rooting powder, which contains auxin. This makes the cuttings grow roots quickly, so each grows into a new plant that is a clone of the parent.
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