Plant tropisms and hormones: WACE Year 12 Biology
“Explain how tropisms and plant hormones such as auxin allow plants to respond to their environment”
Plants respond to directional stimuli through tropisms, growth responses toward or away from a stimulus. Phototropism is growth toward light, and gravitropism is growth in response to gravity. These responses are controlled by hormones, especially auxin, which collects on one side of a shoot or root and causes uneven growth that bends the plant in a survival-promoting direction.
What this dot point is asking
SCSA wants you to define tropisms, name the main types, and explain how the hormone auxin produces the bending response through uneven growth. A strong answer links the direction of the response to the survival benefit for the plant.
Tropisms: directional growth responses
A tropism is a growth response of a plant toward or away from a directional stimulus. It is positive if growth is toward the stimulus and negative if growth is away from it. Because plants cannot move, responding by growing is how they adjust to their environment. The main tropisms are:
- Phototropism: response to light (shoots grow toward light, positive phototropism).
- Gravitropism (geotropism): response to gravity (roots grow down, shoots grow up).
- Thigmotropism: response to touch (climbing plants coil around a support).
- Hydrotropism: response to water (roots grow toward moisture).
A tropism is a permanent change in the direction of growth, not a rapid movement. The plant bends because cells on one side grow more than cells on the other, so the response is slower than an animal's but equally directional.
The role of auxin
Auxin is a plant hormone that controls growth. It is produced at the tips of shoots and moves down the plant. Its key feature is that it can collect unevenly, and where it is more concentrated it changes the rate of cell elongation. The crucial point is that auxin has opposite effects in shoots and roots:
- In shoots, high auxin concentration increases cell elongation.
- In roots, high auxin concentration decreases cell elongation.
How auxin produces phototropism
When light shines on a shoot from one side, auxin moves to the shaded side. Because auxin promotes elongation in shoots, the cells on the shaded side grow longer than those on the lit side. The uneven growth bends the shoot toward the light. This positive phototropism brings leaves into the light, maximising photosynthesis.
Explain the auxin distribution
A seed germinates on its side. In the root, gravity causes auxin to collect on the lower side. Because high auxin slows elongation in roots, the cells on the lower side grow less than those on the upper side. The root therefore bends downward, showing positive gravitropism. Growing down anchors the plant and reaches water and nutrients in the soil, aiding survival.
Why the responses aid survival
Each tropism has a clear survival value. Growing toward light maximises the light captured for photosynthesis. Roots growing downward and toward water anchor the plant and secure water and minerals. Climbing plants coiling around supports gain access to light without building thick stems. The responses match the plant to its environment, just as homeostatic responses match an animal's internal conditions.
Tropism: a directional growth response to a stimulus. Phototropism: growth in response to light. Gravitropism: growth in response to gravity. Auxin: a plant hormone controlling cell elongation, distributed unevenly to cause bending.
Why this matters for survival
Plant responses show that surviving in a changing environment is not unique to animals. Without nerves or muscles, plants still detect light, gravity, touch and water and respond in ways that improve their survival. Auxin and tropisms are the plant equivalent of coordinated responses, allowing plants to optimise growth as conditions change around them.
- Saying auxin always promotes growth
- Auxin promotes elongation in shoots but inhibits it in roots, which is why they bend in opposite directions.
- Calling a tropism a movement
- A tropism is differential growth, not a rapid movement.
- Saying auxin collects on the lit side in phototropism
- Auxin moves to the shaded side, so that side grows more and the shoot bends toward the light.
- Forgetting the survival value
- Marks reward linking the direction of the response to a benefit such as maximising photosynthesis.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksA shoot is illuminated from one side and bends toward the light. Explain how the hormone auxin produces this phototropic response, and explain why the response is beneficial to the plant.Show worked answer →
A 6 mark answer needs the auxin mechanism plus the survival benefit.
- Auxin production and redistribution
- Auxin is produced at the shoot tip. When light comes from one side, auxin moves to the shaded side of the shoot.
- Uneven growth
- In shoots, a higher auxin concentration increases cell elongation. So the cells on the shaded side (with more auxin) elongate more than those on the lit side.
- Bending
- Because one side grows longer than the other, the shoot bends toward the light. This is positive phototropism.
- Benefit
- Bending toward light brings the leaves into the strongest light, maximising the light captured for photosynthesis and so the plant's energy supply.
Markers reward auxin moving to the shaded side, greater elongation there, the resulting bend toward light and the photosynthesis benefit.
WACE 2023-style5 marksAuxin has opposite effects on cell elongation in shoots and roots. Use this to explain why, in a germinating seed lying on its side, the shoot shows negative gravitropism while the root shows positive gravitropism.Show worked answer →
A 5 mark answer needs the opposite auxin effects applied to both organs.
- Auxin redistribution
- Gravity causes auxin to accumulate on the lower side of both the shoot and the root.
- In the shoot (negative gravitropism)
- In shoots, high auxin increases elongation, so the lower side (more auxin) grows more than the upper side. The shoot therefore bends upward, away from gravity, which is negative gravitropism. Growing up brings the shoot toward light for photosynthesis.
- In the root (positive gravitropism)
- In roots, high auxin decreases elongation, so the lower side (more auxin) grows less than the upper side. The root therefore bends downward, with gravity, which is positive gravitropism. Growing down anchors the plant and reaches water and nutrients.
Markers reward auxin on the lower side of both organs, the opposite effect on elongation in shoot versus root, and the resulting upward shoot and downward root.