Plant Hormones: Auxins and Tropisms

Plants cannot move from place to place, but they can respond to their environment by growing towards or away from stimuli. These growth responses are called tropisms and are controlled by plant hormones, particularly auxins. This lesson covers auxin action, phototropism and gravitropism, as required by Edexcel GCSE Biology (1BI0) Topic 6.

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What are Plant Hormones?

Plant hormones (also called plant growth regulators) are chemical substances produced in small quantities in one part of the plant that have an effect on another part of the plant. They coordinate and control growth, development and responses to the environment.

Unlike animal hormones, plant hormones are not produced in specific glands — they are made in various tissues (e.g., shoot tips, root tips, developing seeds).

Key Plant Hormones (for Edexcel GCSE)

Hormone	Where produced	Key effects
Auxin (IAA)	Shoot tips, root tips	Promotes cell elongation in shoots; controls tropisms
Gibberellins	Seeds, young leaves	Promote seed germination, stem elongation, flowering
Ethene	Ripening fruits, ageing tissues	Promotes fruit ripening

This lesson focuses on auxin and tropisms. Gibberellins and ethene are covered in the next lesson on commercial uses.

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Auxin

Auxin (specifically indole-3-acetic acid, or IAA) is the most important plant hormone for GCSE Biology.

Key Facts About Auxin

• Produced in the tips of shoots and roots (the meristems — regions of actively dividing cells)
• Moves (diffuses) away from the tip to the region just behind it (the zone of elongation)
• Causes cell elongation — cells in the zone of elongation absorb water and expand, making the shoot or root grow longer
• Auxin's effect depends on its concentration AND on which organ it is in (shoot or root):
  • In shoots: auxin promotes cell elongation (higher concentration → more growth)
  • In roots: auxin inhibits cell elongation at high concentrations (roots are much more sensitive to auxin than shoots)

Exam Tip: The key difference between auxin's effect on shoots and roots is crucial. In shoots, more auxin = more growth. In roots, more auxin = less growth. This is what allows roots and shoots to respond differently to the same stimulus.

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Tropisms

A tropism is a growth response of a plant towards or away from a directional stimulus.

• Positive tropism: growth towards the stimulus
• Negative tropism: growth away from the stimulus

Types of Tropism

Tropism	Stimulus	Positive response	Negative response
Phototropism	Light	Growth towards light	Growth away from light
Gravitropism (geotropism)	Gravity	Growth towards gravity (downward)	Growth away from gravity (upward)

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Phototropism

Phototropism is the growth response of a plant to the direction of light.

Phototropism in Shoots

Shoots show positive phototropism — they grow towards the light. This is beneficial because it maximises the amount of light reaching the leaves for photosynthesis.

How Auxin Controls Phototropism

When light shines on a shoot from one side:

1. Auxin is produced in the shoot tip
2. Auxin migrates to the shaded side of the shoot (the side away from the light)
3. The shaded side now has a higher concentration of auxin than the illuminated side
4. Since auxin promotes cell elongation in shoots, cells on the shaded side elongate more than cells on the light side
5. The unequal growth causes the shoot to bend towards the light

graph LR
    A["Light from one side"] --> B["Shoot tip detects light"]
    B --> C["Auxin moves to shaded side"]
    C --> D["Shaded side: HIGH auxin"]
    C --> E["Lit side: LOW auxin"]
    D --> F["More cell elongation"]
    E --> G["Less cell elongation"]
    F --> H["Shoot bends towards light"]
    G --> H

    style A fill:#f1c40f,color:#333
    style D fill:#27ae60,color:#fff
    style E fill:#95a5a6,color:#fff
    style H fill:#2980b9,color:#fff

Diagram Explanation

Imagine a shoot with light coming from the right:

• Left side (shaded): high auxin → cells elongate more → this side grows faster
• Right side (lit): low auxin → cells elongate less → this side grows slower
• Result: the shoot curves to the right (towards the light)

Exam Tip: When explaining phototropism, you MUST mention: (1) auxin moves to the shaded side, (2) this causes more cell elongation on the shaded side, (3) the unequal growth bends the shoot towards the light. Missing any of these steps will lose marks.

Phototropism in Roots

Roots show negative phototropism — they grow away from light. This helps roots grow downward into the soil, away from the surface.

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Gravitropism (Geotropism)

Gravitropism is the growth response of a plant to gravity. It is also called geotropism (from "geo" meaning earth).

Gravitropism in Shoots

Shoots show negative gravitropism — they grow upward, away from gravity.

Mechanism:

1. If a shoot is placed horizontally, gravity causes auxin to accumulate on the lower side
2. The lower side has more auxin → more cell elongation → the lower side grows faster
3. The shoot bends upward (away from gravity)

Gravitropism in Roots

Roots show positive gravitropism — they grow downward, towards gravity.

Mechanism:

1. If a root is placed horizontally, gravity causes auxin to accumulate on the lower side
2. In roots, a high concentration of auxin inhibits cell elongation (roots are much more sensitive to auxin than shoots)
3. The lower side (high auxin) grows less than the upper side (low auxin)
4. The upper side elongates more, causing the root to bend downward (towards gravity)

Summary of Auxin Effects

Organ	High auxin concentration	Low auxin concentration	Tropism result
Shoot	More cell elongation	Less cell elongation	Bends towards light (phototropism); bends upward against gravity (gravitropism)
Root	Less cell elongation (inhibition)	More cell elongation	Bends away from light; bends downward with gravity

Exam Tip: The crucial difference is that auxin has OPPOSITE effects on shoots and roots. In shoots, high auxin → more growth. In roots, high auxin → less growth. This is the key to understanding why roots and shoots respond differently to gravity.

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Evidence for Auxin and Tropisms

Classic Experiments (Darwin, Boysen-Jensen, Went)

Although detailed knowledge of historical experiments is not required at GCSE, understanding the principles helps:

1. Removing the tip: If the shoot tip is removed, the plant no longer responds to light. This shows the tip is where auxin is produced and where the stimulus is detected.
2. Covering the tip: If the tip is covered with an opaque cap, the shoot does not bend towards light. If the tip is covered with a transparent cap, it does bend. This shows the tip detects light direction.
3. Agar block experiments (Went): If the tip is removed and placed on an agar block, auxin diffuses into the agar. Placing this agar block on one side of a decapitated shoot causes it to bend away from the block side (due to unequal auxin distribution).

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The Clinostat Experiment