Explain shoot gravitropism through auxin distribution
Connect the lower side of a horizontal shoot with upward growth.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Identify the root and shoot of a seedling; recall growth, cell structure and the variables in a controlled investigation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which side has more auxin in the horizontal shoot-gravity model?
What you will learn
- Identify gravity-induced unequal auxin distribution
- Explain why a horizontal shoot bends upwards
Explain shoot gravitropism through auxin distribution
| Feature | Meaning |
|---|---|
| Source and movement | Auxin made in the shoot tip; diffusion from the tip |
| Distribution | More on lower side of horizontal shoot |
| Growth | Greater lower-side cell elongation |
| Result | Upward shoot growth; negative gravitropism |
Identify gravity-induced unequal auxin distribution
For a shoot placed horizontally, gravity provides a directional stimulus even in darkness. Auxin is made in the shoot tip and diffuses through the plant from that tip. The gravity stimulus leads to unequal auxin distribution, with more auxin on the lower side of the horizontal growing region. The source and movement remain the same general shoot account as in phototropism.
Auxin stimulates shoot cell elongation, so the lower side elongates more than the upper side in the taught model. The lower side becomes the longer outside of a curve and the shoot bends towards the less-elongating upper side. New growth therefore curves upwards, away from gravity. This is negative gravitropism, although the side with more auxin is the lower side.
Explain why a horizontal shoot bends upwards
Keep the stimulus, distribution and response directions separate. Gravity acts downwards, more auxin is on the lower side, but the shoot response is upward growth. Saying that the chemical falls and drags the tip down gives the wrong mechanism and bend. The chemical distribution controls cell growth; it does not provide a mechanical weight that pulls the whole shoot.
A dark horizontal-start experiment can support a gravity response without invoking a light source above the plant. The visible curve alone does not directly measure auxin concentration, so distinguish an observed response from the chemical explanation supplied by the model. Root gravitropism is described separately at Core level; roots do not use the same simple stimulatory shoot response to higher auxin.
Pause and explain
Why does the shoot bend up when its lower side elongates more?
Worked example
Explain an upward shoot curve after a horizontal start in darkness.
Show the worked solution
- More auxin on the lower side stimulates greater lower-side elongation.
- The shoot bends towards its less-elongating upper side, away from gravity.
Answer Auxin from the shoot tip becomes greater on the lower side. Lower-side cells elongate more, so the shoot curves up away from gravity: negative gravitropism.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the complete auxin chain for a horizontal shoot kept in darkness.
Give me a hint
Include the tip, movement, lower side and upward bend.
Compare my reasoning
- Auxin is made in the tip and diffuses from it; gravity leads to more on the lower side.
- Lower-side cells elongate more, causing upward growth away from gravity.
Auxin is made in the shoot tip and diffuses through the plant from it. Gravity leads to more auxin on the lower side, where shoot cells elongate more. The shoot curves up away from gravity: negative gravitropism.
Look for these in your work
- Include source, movement and distribution
- Link unequal growth to the correct response
Compare the faster-elongating side and final bend in a horizontal shoot.
Give me a hint
The more-elongating side and bend are opposite.
Compare my reasoning
- The lower side elongates more.
- The bend is towards the upper side, away from gravity.
Lower side elongates more; the shoot bends up towards the less-elongating upper side, away from gravity.
Look for these in your work
- Identify lower-side growth
- Give the opposite bend direction
Correct: the root and shoot must both bend up because higher auxin always stimulates both parts identically.
Give me a hint
Limit the required growth rule to shoots.
Compare my reasoning
- The taught stimulation model concerns shoot cells.
- Roots usually grow towards gravity and respond differently to auxin.
The stated stimulatory elongation rule concerns shoots; it cannot be transferred unchanged to roots. Shoots usually show negative gravitropism, while roots usually show positive gravitropism.
Look for these in your work
- Retain the shoot scope
- Distinguish root and shoot response signs
Common mistakes
- Predicting downward shoot bending because auxin is lower.
- Attributing a dark response to directional light.
What can you explain now?
Why does the shoot bend up when its lower side elongates more?
Compare with the explanation
The lower side becomes longer and forms the outside of the curve
The less-elongating upper side is the inside.
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Make it stick
Explain the dark horizontal-shoot response without predicting that lower-side growth bends the tip down.
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