Plant Hormones and Responses

Plants do not have nerves or muscles, yet they still respond to their environment in coordinated ways — bending towards the light, sending roots downwards, and ripening their fruit at the right time. They do this using plant hormones, chemical messengers that control growth. This lesson explains how plants respond to light and gravity through growth responses called tropisms, how the hormone auxin brings these responses about, a required practical investigating them, and the many commercial uses of plant hormones in farming and horticulture. This is part of Topic B3 of OCR Gateway Science A.

Separate science note: The commercial uses of plant hormones (auxins as weedkillers and in rooting powder, gibberellins, and ethene in fruit ripening) are content for the separate (triple) Biology course. That section is flagged accordingly. The mechanism of auxin in roots is Higher tier.

By the end of this lesson you should be able to define phototropism and gravitropism, explain how auxin controls them, describe a required practical on plant growth responses, and (separate science) describe the commercial uses of plant hormones.

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Tropisms: Growth Responses in Plants

A tropism is a growth response of a plant towards or away from a stimulus. Because plants respond by growing, tropisms are usually slow compared with animal responses. The two you must know are:

• Phototropism — a growth response to light. Plant shoots grow towards light, so they are positively phototropic. This helps the leaves capture more light for photosynthesis.
• Gravitropism (also called geotropism) — a growth response to gravity. Plant roots grow towards gravity (downwards), so they are positively gravitropic; shoots grow away from gravity (upwards), so they are negatively gravitropic.

Tropism	Stimulus	Shoots	Roots
Phototropism	Light	Grow towards light (positive)	Grow away from light (negative)
Gravitropism / geotropism	Gravity	Grow away from gravity, i.e. upwards (negative)	Grow towards gravity, i.e. downwards (positive)

These responses make biological sense: shoots grow up and towards the light to reach sunlight for photosynthesis, while roots grow down into the soil to anchor the plant and reach water and minerals.

Exam Tip: Watch the words positive and negative. Positive means growth towards the stimulus; negative means growth away from it. A shoot is positively phototropic (towards light) but negatively gravitropic (away from gravity, i.e. upwards).

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Auxin: the Growth Hormone

The plant hormone responsible for tropisms is auxin. The key facts are:

• Auxin is made at the tips of shoots and roots.
• It moves back from the tip and controls cell elongation — how much the cells just behind the tip grow in length.
• Auxin becomes unevenly distributed when a shoot or root is lit or oriented unevenly, and this unevenness is what makes the plant bend.

How auxin makes a shoot bend towards light

When light shines on a shoot from one side:

1. Auxin made at the tip moves to the shaded side of the shoot (it moves away from the light).
2. The higher concentration of auxin on the shaded side makes those cells elongate more (auxin promotes cell elongation in shoots).
3. Because the shaded side grows faster (its cells get longer) than the lit side, the shoot bends towards the light.

How auxin affects roots (Higher tier only)

Higher tier only: Roots respond to gravity in the opposite way to shoots, and the reason is that auxin has the opposite effect in roots:

1. In a root lying on its side, auxin gathers on the lower side (gravity pulls it down).
2. But in roots, a high concentration of auxin inhibits (slows down) cell elongation — the opposite of its effect in shoots.
3. So the lower side of the root, with more auxin, grows less, while the upper side grows more.
4. Because the upper side grows faster, the root bends downwards, towards gravity — exactly what a root needs to grow into the soil.

The crucial Higher-tier idea is that auxin promotes growth (elongation) in shoots but inhibits growth in roots. The same uneven distribution of auxin therefore bends a shoot up towards the light and a root down towards gravity.

Exam Tip (Higher): The fact that catches people out: more auxin makes a shoot cell grow more but makes a root cell grow less. Hold these opposite effects in mind and the directions of bending all follow logically.

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Required Practical: Investigating Plant Growth Responses

A core practical of Topic B3 is to investigate the effect of light (or gravity) on the growth of newly germinated seedlings (for example cress or mustard seedlings).

flowchart TD
  A["Set up dishes of germinating seedlings"] --> B["Give each dish a different light condition:<br/>all-round light, light from one side, or darkness"]
  B --> C["Keep all other conditions the same<br/>(temperature, water, type and number of seeds)"]
  C --> D["Leave for several days"]
  D --> E["Measure the direction and amount<br/>of shoot growth"]
  E --> F["Record results and compare conditions"]

Key points the exam rewards:

• The independent variable is the direction of the light (or the orientation relative to gravity); the dependent variable is the direction/amount the seedlings grow.
• Keep control variables the same — temperature, water, and the number and type of seeds — so it is a fair test.
• Seedlings given light from one side bend towards it (positive phototropism); those in all-round light grow straight up; those in the dark grow tall, pale and spindly (they keep growing upwards searching for light).

Example readings

The table below shows example readings (illustrative, not real data) for cress seedlings after four days.

Light condition	Direction of shoot growth	Mean shoot height (mm)
All-round light	Straight up	28
Light from one side	Bent towards the light	30
Darkness	Straight up, pale and spindly	42

The seedlings in the dark grew the tallest ($42 \text{ mm}$42 mm) but were pale and weak, because without light they cannot photosynthesise and keep elongating in a search for light. The seedlings lit from one side clearly bent towards it — direct evidence of positive phototropism in shoots.

Exam Tip: A common "explain the results" question asks why dark-grown seedlings are tall but pale. The answer: they keep growing (elongating) to try to reach light, but without light they cannot make chlorophyll/photosynthesise, so they are pale and spindly.

Worked Example — Interpreting tropism results

In an experiment, seedlings lit from the left bent to the left, and seedlings lit from the right bent to the right. What does this show, and how does auxin explain it?

Step 1 — identify the response. The shoots grew towards the light in both cases, so this is positive phototropism.

Step 2 — explain with auxin. Auxin made at the tip moves to the shaded side; that side's cells elongate more, so the shoot bends towards the light.

Answer: the results show positive phototropism (shoots grow towards light). Auxin collects on the shaded side, where it makes cells elongate more, so the shoot bends towards the light. Because the bending direction always followed the light direction, the cause must be the light.

Common error: saying the plant "wants" or "tries" to reach the light. Examiners want the mechanism — uneven auxin causing uneven growth — not intention.

Worked Example — Explaining a gravitropism result (Higher)

A germinating seed is placed on its side in the dark. After two days the root has bent downwards and the shoot has bent upwards. Explain these observations.