Compare shoot and root responses to directional light
Describe observed signs while retaining the limits of root-light claims.
Section 14.5 · Tropic responses
Core and Extended; 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 light response is common in young shoots?
What you will learn
- Describe a typical shoot and a stated root response
- Avoid universal claims from one species or light condition
Compare shoot and root responses to directional light
| Feature | Meaning |
|---|---|
| Shoot | Commonly positive phototropism |
| Root model | Negative phototropism under the stated blue-light conditions |
| Gravity | Continues to act during the light investigation |
| Limit | Root sign is not universal across species and light conditions |
Describe a typical shoot and a stated root response
A young shoot commonly shows positive phototropism: its growing region curves towards a one-sided light source. This changes the direction in which new shoot growth develops. When a source is on the left, a shoot bending left is growing towards the source even though gravity continues to act downwards. Identify the part, source and growth direction in that order.
Roots can show negative phototropism under suitable conditions, including the unilateral blue-light conditions used in published Arabidopsis experiments. A root growing downwards can also curve laterally away from a left-hand source. Its downward gravitropic growth and lateral light response are different descriptions relative to different stimuli; one does not automatically cancel the other.
Avoid universal claims from one species or light condition
Root responses depend on species, light quality and experimental conditions. Original Arabidopsis studies have reported negative responses to blue light and positive responses to red light. Root-light curvature can be weaker than the gravity response. Therefore the useful description is the response observed under the stated conditions, not a rule that every root always grows away from every source.
If a supplied root record shows no clear lateral curvature, describe that observation without inventing a negative or positive response. A strong downward gravity response, unsuitable exposure or measurement uncertainty may obscure the light comparison. Repeated, controlled measurements and a suitable control are needed. The auxin explanations in this syllabus section concern shoots; do not apply the shoot elongation rule to roots without a different supporting account.
Pause and explain
What is a defensible root-light statement?
Worked example
In a stated blue-light model, a source is left, the shoot curves left and the root curves right while growing down. Describe both light responses.
Show the worked solution
- The shoot grows towards the source.
- The root grows laterally away from the source.
Answer Shoot: positive phototropism. Root in this stated model: negative phototropism; it can still grow down in response to gravity.
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Test this topic →From guided practice to a new situation
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.
Describe a shoot curving towards a one-sided source.
Give me a hint
Name the part and source-relative sign.
Compare my reasoning
- The shoot grows towards light.
- Towards the source is positive.
The shoot shows positive phototropism.
Look for these in your work
- Name shoot growth
- Give the correct sign
A vertical root curves laterally away from a left-hand blue source while continuing downwards. Describe its light and gravity responses separately.
Give me a hint
Different stimuli provide different references.
Compare my reasoning
- Away from the source is negative phototropism.
- Down towards gravity is positive gravitropism.
In this stated record the root shows negative phototropism and positive gravitropism.
Look for these in your work
- Use the light-source reference
- Use the gravity reference separately
Correct: a root with no measurable sideways curve proves that all roots are unable to respond to light.
Give me a hint
Keep the conclusion limited to the observation.
Compare my reasoning
- The record has no clear lateral response under its conditions.
- Other species or light conditions can show a response.
This record shows no clear lateral curvature under the stated conditions. It does not prove that all roots cannot respond to light.
Look for these in your work
- Report the actual observation
- Reject the unsupported universal claim
Common mistakes
- Claiming all roots always grow away from all light.
- Using the root’s downward direction alone to judge phototropism.
What can you explain now?
What is a defensible root-light statement?
Compare with the explanation
Report the observed sign under the stated species and light conditions
Root responses are condition-dependent.
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Make it stick
Describe both light responses in a stated record and explain why roots cannot be assigned one universal light-response rule.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.