OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(c) the experimental evidence for the role of auxins in the control of…
Full specification wording
Plant and animal responses: (c) the experimental evidence for the role of auxins in the control of apical dominance
Apical dominance is the suppression of lateral bud growth by the shoot apex.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 3 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Apical dominance is the suppression of lateral bud growth by the shoot apex. The evidence rests on experiments that remove or apply auxin and observe the effect on side shoots. Decapitation (removing the shoot tip) releases lateral buds, which then grow; applying auxin (IAA) in a paste to the cut stump restores suppression, so the buds stay dormant. This shows the apex produces a substance that inhibits bud outgrowth, and that the substance is auxin. Further support comes from auxin-transport experiments: auxin moves down the stem in a polar direction, and blocking that transport reduces the dominance. The evidence is correlational plus manipulative, so a good answer distinguishes removing the source from replacing the chemical and explains why the replacement step is the key control.
What earns the marks
- Decapitation of the shoot tip removes the source of auxin and lateral buds grow out, showing the apex suppresses bud growth.
- Applying auxin (IAA) paste to the decapitated stump restores apical dominance, so lateral buds remain dormant.
- The replacement experiment is the key evidence: it shows auxin itself, not merely wounding, causes the suppression.
- Auxin is transported polarly down the stem from the apex, and interfering with this transport reduces dominance.
- Control treatments (lanolin paste without auxin, or intact plants) are needed to show the effect is due to auxin.
- Conclusions are limited because the experiments are on particular species and show correlation plus manipulation, not the full molecular mechanism.
Common mistakes and how to put them right
Thinking decapitation alone proves auxin causes dominance.
Put it right: Decapitation only removes the source; the auxin-replacement step is what links auxin to the effect.
Confusing apical dominance with phototropism.
Put it right: Apical dominance is suppression of lateral buds by the apex, a separate response.
Assuming any paste restores dominance.
Put it right: A control paste without auxin (e.g. plain lanolin) must fail to restore it, showing auxin is the active agent.
Exam tips
- State the manipulation and the observation for each step: remove apex → buds grow; replace auxin → buds stay dormant.
- Name the control treatment explicitly so the causal claim is supported.
- Use the term polar transport when explaining how auxin from the apex reaches the buds.
Try the lesson
The start of the Plant and animal responses: (c) the experimental evidence for the role of auxins in the control of apical dominance lesson, made from checked teaching notes for this specification point.
Have a go first
In a classic apical dominance experiment, what is observed when the shoot tip of a young plant is removed?
Listen
Apical dominance is the suppression of lateral bud growth by the shoot apex. The evidence rests on experiments that remove or apply auxin and observe the effect on side shoots. Decapitation (removing the shoot tip) releases lateral buds, which then grow; applying auxin (IAA) in a paste to the cut stump restores suppression, so the buds stay dormant. This shows the apex produces a substance that inhibits bud outgrowth, and that the substance is auxin. Further support comes from auxin-transport experiments: auxin moves down the stem in a polar direction, and blocking that transport reduces the dominance. The evidence is correlational plus manipulative, so a good answer distinguishes removing the source from replacing the chemical and explains why the replacement step is the key control. A strong answer earns marks for these points. Decapitation of the shoot tip removes the source of auxin and lateral buds grow out, showing the apex suppresses bud growth. Applying auxin (IAA) paste to the decapitated stump restores apical dominance, so lateral buds remain dormant. The replacement experiment is the key evidence: it shows auxin itself, not merely wounding, causes the suppression. Auxin is transported polarly down the stem from the apex, and interfering with this transport reduces dominance. A common slip: Thinking decapitation alone proves auxin causes dominance. Instead, decapitation only removes the source; the auxin-replacement step is what links auxin to the effect. Exam tip: State the manipulation and the observation for each step: remove apex → buds grow; replace auxin → buds stay dormant.
See it
Key words
Say what each word means before you turn the card.
Apical dominance
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Decapitation
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Auxin (IAA)
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Polar auxin transport
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See every definition
- Apical dominance
- The suppression of lateral bud growth by the shoot apex.
- Decapitation
- Removing the shoot tip, which releases lateral buds so that they then grow.
- Auxin (IAA)
- A substance produced by the apex that inhibits bud outgrowth; applied in a paste to the cut stump it restores suppression so the buds stay dormant.
- Polar auxin transport
- The movement of auxin down the stem in a polar direction, where blocking that transport reduces the dominance.
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