Tropisms: directional growth responses in plants
What a tropism is
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A tropism is a growth response in a plant where the direction of growth is determined by the direction of the stimulus.
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Two features define it, and both must appear in your answer:
- The response is caused by growth, not by movement of existing tissue — so it is slow and usually permanent.
- The direction of the response is set by the direction of the stimulus.
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A tropism is positive if growth is towards the stimulus, and negative if growth is away from it.
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Tropisms are named after the stimulus:
| Tropism | Stimulus | Typical response |
|---|---|---|
| Phototropism | Light | Shoots grow positively, roots negatively |
| Gravitropism | Gravity | Roots grow positively, shoots negatively |
| Thigmotropism | Touch or contact | Tendrils coil positively around a support |
| Hydrotropism | Water gradient | Roots grow positively toward moisture |
| Chemotropism | Chemical gradient | Pollen tubes grow positively toward the ovule |
How a tropism works: the role of auxin
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Plants have no nervous system, so the signal must be chemical. The hormone responsible is auxin (indole-3-acetic acid, IAA).
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Auxin's key property is that it causes cell elongation — it loosens cell walls so cells take in water and lengthen.
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The mechanism has four steps. Learn this sequence, because it is the backbone of every Merit answer on tropisms:
- Detection. Receptors detect the stimulus. For light, phototropins in the shoot tip absorb blue light. For gravity, statoliths (dense starch grains) settle to the lower side of root cap cells.
- Redistribution. Auxin is transported laterally across the shoot or root, accumulating on the side away from light, or on the lower side in response to gravity.
- Differential elongation. The cells with more auxin respond by elongating more or less, depending on the tissue.
- Bending. Unequal elongation on the two sides curves the organ.
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The critical subtlety, and the one most often got wrong: shoots and roots respond to auxin in opposite ways.
- Shoot cells elongate more with high auxin.
- Root cells elongate less with high auxin — high concentrations inhibit elongation in roots.
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This single fact explains why one hormone produces opposite bends:
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Phototropism in a shoot. Light from one side → auxin moves to the shaded side → shaded cells elongate more → shoot bends towards the light. Positive phototropism.
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Gravitropism in a root. Statoliths settle → auxin accumulates on the lower side → lower cells elongate less → upper side outgrows the lower → root bends downwards. Positive gravitropism.
Selective advantage of tropisms
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Each tropism must be linked to survival and reproduction in the plant's niche.
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Positive phototropism in shoots.
- Leaves are positioned where light intensity is highest, so the rate of photosynthesis rises.
- More glucose is fixed, so more resources go to growth, flowering and seed production — more surviving offspring.
- This matters most in a dense canopy, such as regenerating kānuka scrub, where light is the limiting factor.
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Positive gravitropism in roots.
- Roots grow downwards into the soil, reaching water and mineral ions and anchoring the plant.
- Anchorage prevents the plant being uprooted by wind, so it survives to reproduce — significant in exposed NZ conditions.
- It also works in the dark, before a seed reaches the surface, which light-based responses could not do.
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Positive thigmotropism in tendrils.
- A climber such as kiwifruit or native clematis (puawhānanga) reaches the canopy without investing in a thick woody stem.
- The energy saved on structural tissue is available for reproduction, and the plant reaches light it could never reach unaided.
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Positive hydrotropism in roots.
- Roots grow towards higher soil moisture, maintaining water uptake during dry periods and keeping stomata open for gas exchange.
Worked Example
Worked Example
A seed germinates on its side, buried 4 cm deep in dark soil.
- After 3 days, the root has curved downwards and the shoot has curved upwards.
- No light has reached either organ.
- A second seed is germinated on a slowly rotating turntable (a clinostat), which averages the direction of gravity to zero. Its root and shoot both grow straight out sideways.
Explain how the responses in the first seed occur, and why they provide a selective advantage.
Answer:
To explain how, we work through detection, redistribution and differential elongation for each organ in turn. The clinostat result tells us the stimulus must be gravity, since removing a consistent gravity direction removes the response — light can be ruled out because the seed is in darkness.
In the root. Dense statoliths in the root cap cells settle to the lower side under gravity. This triggers lateral transport of auxin to the lower side of the root. Because high auxin concentrations inhibit elongation in root cells:
- lower-side cells elongate less
- upper-side cells elongate more
- the root curves downwards — positive gravitropism.
In the shoot. Auxin again accumulates on the lower side. But shoot cells respond in the opposite way — high auxin promotes elongation:
- lower-side cells elongate more
- the shoot curves upwards — negative gravitropism.
So one hormone, redistributed in the same direction in both organs, produces opposite bends purely because the two tissues have opposite sensitivities.
Why this is a selective advantage.
The seed has a fixed, finite food reserve in its endosperm and no way to photosynthesise until its leaves reach light. Every day spent growing in the wrong direction burns reserve it cannot replace.
- The root growing down reaches water and mineral ions and anchors the seedling before the shoot emerges into the wind.
- The shoot growing up reaches the surface by the shortest possible path, so the seedling becomes photosynthetic — self-sustaining — before its reserves are exhausted.
A seedling that germinated on its side and grew straight out sideways, as on the clinostat, would exhaust its reserve while still underground and die before reproducing. Individuals with the gravitropic response therefore survive to reproduce far more often, and the alleles for it increase in frequency.