Plant Hormones [H]

This lesson covers plant hormones and their roles in controlling plant growth and development. This is Higher Tier content on the AQA GCSE Biology specification. It includes the Required Practical on investigating the effect of light on seedling growth, which demonstrates phototropism.

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Plant Growth Responses

Unlike animals, plants cannot move to respond to changes in their environment. Instead, they grow towards or away from stimuli. These directional growth responses are called tropisms.

Tropism	Stimulus	Positive Response (towards)	Negative Response (away from)
Phototropism	Light	Shoots grow towards light	Roots grow away from light
Gravitropism (geotropism)	Gravity	Roots grow towards gravity (downwards)	Shoots grow away from gravity (upwards)
Hydrotropism	Water/moisture	Roots grow towards water	—
Thigmotropism	Touch	Tendrils of climbing plants wrap around supports	—

Why Tropisms Are Important

• Phototropism ensures that shoots grow towards light for maximum photosynthesis.
• Positive gravitropism in roots ensures they grow downwards to anchor the plant and absorb water and minerals.
• Negative gravitropism in shoots ensures they grow upwards towards the light.
• Hydrotropism helps roots find water in the soil.

Exam Tip: Make sure you use the correct terminology: a positive tropism means growth towards the stimulus; a negative tropism means growth away from the stimulus. Roots show positive gravitropism but negative phototropism. Shoots show positive phototropism but negative gravitropism.

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Auxins — The Key Plant Hormone

Auxin (specifically indole-3-acetic acid, or IAA) is the main plant hormone involved in tropisms. It is produced in the tips of shoots and roots and controls cell elongation.

How Auxin Works in Shoots

• Auxin is produced in the shoot tip.
• Auxin promotes cell elongation in shoots — cells on the side with more auxin grow longer.
• In a shoot exposed to light from one side, auxin moves to the shaded side.
• The cells on the shaded side elongate more than those on the lit side.
• This causes the shoot to bend towards the light (positive phototropism).

How Auxin Works in Roots

• Auxin is also present in root tips, but it has the opposite effect — auxin inhibits cell elongation in roots.
• If a root is growing horizontally, gravity causes auxin to accumulate on the lower side.
• The lower side (with more auxin) grows more slowly; the upper side grows faster.
• This causes the root to bend downwards (positive gravitropism).

Feature	Effect on Shoots	Effect on Roots
High auxin concentration	Promotes cell elongation (faster growth)	Inhibits cell elongation (slower growth)
Low auxin concentration	Slower growth	Faster growth
Phototropism	Shoot bends towards light	Root bends away from light
Gravitropism	Shoot bends away from gravity (upwards)	Root bends towards gravity (downwards)

flowchart TD
    subgraph Phototropism_in_Shoots
        A1[Light from one side] --> A2[Auxin moves to shaded side]
        A2 --> A3[Shaded side: high auxin = more elongation]
        A2 --> A4[Lit side: low auxin = less elongation]
        A3 --> A5[Shoot bends towards light]
    end
    subgraph Gravitropism_in_Roots
        B1[Root growing horizontally] --> B2[Gravity pulls auxin to lower side]
        B2 --> B3[Lower side: high auxin = inhibits elongation]
        B2 --> B4[Upper side: low auxin = promotes elongation]
        B3 --> B5[Root bends downwards]
    end

Exam Tip: The key difference is that auxin promotes elongation in shoots but inhibits elongation in roots. This is the reason shoots and roots respond differently to the same stimulus. Many students lose marks by forgetting this distinction.

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Required Practical: Investigating the Effect of Light on Seedling Growth

This required practical investigates phototropism by observing how seedlings grow in response to directional light.

Method

1. Germinate cress or grass seeds in several identical pots with damp cotton wool.
2. Set up the following conditions:

Setup	Description	Expected Result
A — Light from one side	Place seedlings in a box with a window cut in one side, allowing light to enter from only one direction	Seedlings bend towards the light (phototropism)
B — Light from all sides (control)	Place seedlings in an open area with uniform light	Seedlings grow straight upwards
C — Complete darkness	Place seedlings in a completely sealed dark box	Seedlings grow straight upwards (no directional stimulus)
D — Tip covered	Cover the tips of some seedlings with aluminium foil caps, then expose to one-sided light	Seedlings grow straight upwards (auxin production blocked at tip)
E — Tip removed	Cut the tips off some seedlings, then expose to one-sided light	Seedlings grow straight upwards (no auxin produced)

3. Leave all setups for 5-7 days in the same temperature conditions.
4. Observe and measure the direction and angle of growth.

Variables

Variable	Details
Independent variable	Direction/presence of light
Dependent variable	Direction and degree of bending of the seedlings
Control variables	Type of seed, volume of water, temperature, number of seeds, pot size, duration of experiment

Expected Results and Explanations

Setup	Result	Explanation
A	Seedlings bend towards the light	Auxin moves to the shaded side, causing more elongation on that side
B	Seedlings grow straight up	Light is even on all sides, so auxin is evenly distributed
C	Seedlings grow straight up (etiolated — pale and thin)	No directional light stimulus; auxin is evenly distributed
D	Seedlings grow straight up	The tip is covered, so auxin cannot be redistributed by light
E	Seedlings grow straight up (or do not grow much)	The tip is removed, so no auxin is produced

Key Conclusions

• Phototropism is controlled by auxin produced in the shoot tip.
• Light causes auxin to redistribute to the shaded side of the shoot.
• The tip must be present and uncovered for the phototropic response to occur.
• This experiment provides evidence that the tip is the site of auxin production and light perception.

Exam Tip: In the exam, you may be asked to design or evaluate this experiment. Always explain why control setups are needed (to prove that bending is caused by light direction, not other factors). Mention specific control variables and explain how they are kept constant.

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Other Plant Hormones

While auxin is the most important plant hormone for GCSE, you should also know about: