By the end of this lesson
- Describe auxin production and redistribution.
- Explain unequal shoot elongation.
- Interpret classic phototropism experiments.
BIG QUESTION
How can light on one side make a shoot bend toward it?
Directional light changes auxin distribution; greater auxin action on the shaded side promotes elongation and curvature.
The shoot tip detects directional light
Light-sensitive processes near the shoot tip detect a difference between illuminated and shaded sides. auxinA plant hormone made mainly in young shoot tips that controls growth by cell elongation and causes tropisms. is produced mainly in young shoot tissues and transported away from the tip.
Modern evidence supports lateral redistribution and transport; avoid the oversimplified claim that light merely destroys auxin.
Auxin action becomes greater on the shaded side
Under one-sided light, more auxin is transported to or retained on the shaded side of a shoot. At shoot concentrations, auxin promotes cell elongation.
Cells on the shaded side lengthen more than illuminated-side cells, so the shoot curves toward light.
Experiments separate detection and growth
Removing the tip or covering it with an opaque cap prevents normal phototropic curvature because light detection/auxin signalling is disrupted. A transparent cap usually permits the response.
Covering only the lower shoot does not prevent tip detection, showing that stimulus detection and elongation occur in different regions.
Classic phototropism treatments
Predictions should identify what each manipulation blocks.
| Treatment | Expected curvature | Inference |
|---|---|---|
| Intact tip + one-sided light | Toward light. | Normal detection and signalling. |
| Tip removed | Little/no curvature. | Tip is needed. |
| Opaque cap on tip | Little/no curvature. | Tip must detect light. |
| Transparent cap | Curvature occurs. | Covering alone is not the cause. |
| Opaque sleeve below tip | Curvature can occur. | Light detection is mainly at tip. |
Language that earns precision
- auxin
- Plant growth regulator that influences cell elongation and other processes.
- differential growth
- Different growth rates on opposite sides of an organ.
- phototropism
- Directional growth response to light.
- opaque
- Not allowing light to pass through.
Common misconceptions
Claim: Auxin makes the lit side grow faster in a shoot.
Correction: Greater auxin action on the shaded side promotes more elongation there.
Claim: Auxin moves toward light.
Correction: It is redistributed toward the shaded side under directional light.
Claim: Light simply destroys all auxin on one side.
Correction: Phototropism involves regulated transport and redistribution, not only destruction.
Match the answer to the command word
Light detected at tip → auxin redistributed → shaded side elongates more → shoot bends toward light.
State what the opaque cap and the transparent cap (a control experiment) each test.
Recall, apply, then justify
Explain positive phototropism in a shoot using auxin.
Hint
Detection, distribution, elongation, bend.
Suggested marking points
- Directional light is detected near the shoot tip.
- Auxin is redistributed toward the shaded side.
- Auxin promotes shoot-cell elongation.
- Shaded-side cells elongate more than lit-side cells.
- Unequal growth bends the shoot toward light.
Explain why an opaque cap prevents curvature but a transparent cap does not.
Hint
Use the transparent cap as a control experiment for covering, separate from light detection.
Suggested marking points
- An opaque cap prevents directional light reaching the tip.
- Light detection/auxin redistribution is disrupted.
- A transparent cap allows light through, so detection and curvature can continue.
Design an investigation distinguishing whether light is detected at the tip or elongation zone.
Planning hint
Use matched cap and sleeve treatments.
What a strong design includes
- Use seedlings of the same species, age and initial length.
- Expose all groups to the same one-sided light, temperature and water conditions.
- Include an intact control group.
- Place opaque caps on tips in one group.
- Place transparent caps on tips as a covering control experiment.
- Place opaque sleeves below tips on another group.
- Measure curvature angle after a fixed time.
- Use replicates and compare mean curvature between groups.