A farmer wants to use a plant hormone to control weeds growing among their crop plants. Explain how auxins can be used as selective weedkillers to kill the weeds without harming the crop.

Edexcel GCSE Biology (1BI0) — 6.3 Plant hormones · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Broad-leaved weeds, such as dandelions, often compete with narrow-leaved cereal crops for resources. Farmers can spray fields with synthetic auxins to remove these weeds.

Model answer (4 marks)

Auxins are plant hormones that stimulate cell elongation in shoots.
When sprayed in high concentrations they cause uncontrolled, excessive growth in broad‑leaved weeds.
This over‑growth leads to wilting, collapse and death of the weeds.
Narrow‑leaved crop plants absorb less of the synthetic auxin and therefore are not affected, so they survive.

Examiner tips

  • Use the word ‘stimulate’ for auxin action and ‘uncontrolled growth’ for the effect. Mention that the crop is ‘unaffected’ because it absorbs less of the hormone.

Common mistakes

  • Confusing auxin with other hormones such as gibberellins. Saying the crop is ‘killed’ instead of ‘unaffected’. Omitting the reason why the crop is unharmed (lower auxin uptake).

Mark scheme (4 marks)

  1. Auxins are plant hormones that stimulate growth in shoots
  2. When sprayed in high concentrations, auxins cause uncontrolled/excessive growth in broad-leaved weeds
  3. This excessive growth kills the broad-leaved weeds
  4. Narrow-leaved crop plants are not affected / absorb less auxin, so they are unharmed

Key terms in this question

auxins · selective weedkillers

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