What is Phototropism?
Phototropism is a plant's growth response toward or away from a light source, one of the clearest examples of how hormones shape plant behavior. Shoots typically grow toward light (positive phototropism), maximizing light capture for photosynthesis. The mechanism relies on the hormone auxin being redistributed unevenly across the stem.
Phototropism is the directional growth of a plant in response to light, caused by auxin accumulating on the shaded side of a shoot, making those cells elongate more and bending the stem toward the light source.
- 1↓Light detectionPhototropin, a blue-light photoreceptor in the shoot tip, detects the direction of light.
- 2↓Auxin redistributionAuxin (IAA) is transported laterally, accumulating more on the shaded side of the stem.
- 3↓Unequal cell elongationHigher auxin concentration causes shaded-side cells to elongate more than lit-side cells.
- 4Stem bendingUneven elongation curves the stem, bending it toward the light source.
Step-by-step worked examples
A seedling on a windowsill is rotated 180° so its bent stem now points away from the window. What happens over the next few days?
Phototropin at the shoot tip detects the new direction of light Auxin redistributes to the new shaded side of the stem Cells on the new shaded side elongate more The stem gradually re-bends until it points toward the window again
A stem is illuminated evenly from all sides in a growth chamber. Will it show phototropism?
Phototropism requires an uneven (directional) light source to create an auxin gradient With equal light on all sides, auxin distributes evenly around the stem No side elongates more than another The stem grows straight up rather than bending in any direction
Why do plant roots often grow away from light (negative phototropism) while shoots grow toward it (positive phototropism)?
In shoots, auxin accumulation on the shaded side promotes elongation, bending the stem toward light In roots, the same auxin concentration is inhibitory rather than stimulatory to cell elongation So the shaded side of a root grows less, and the lit side grows more This causes the root to curve away from light, into the soil
Flashcards
Quick quiz
Q1.Which hormone is primarily responsible for phototropism?
Q2.What causes the stem to bend toward light?
Q3.What photoreceptor detects the light direction for phototropism?
Q4.Root phototropism is usually…
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Common mistakes
Thinking light itself directly stretches the stem cells. — Correct: Light triggers a hormonal (auxin) response — the bending is caused by unequal cell elongation, not direct light stretching.
Assuming auxin always promotes elongation everywhere. — Correct: Auxin's effect depends on the tissue: it promotes elongation in shoots but can inhibit it in roots at the same concentration.
Confusing phototropism with photosynthesis. — Correct: Phototropism is a growth movement toward light; photosynthesis is the chemical process of making food from light.
Believing phototropism happens instantly. — Correct: It's a gradual growth response taking hours to days, not an instant movement like a Venus flytrap closing.
FAQ
What is phototropism?
Phototropism is a plant's directional growth response to light, where shoots typically bend toward the light source and roots bend away from it.
What is the mechanism behind phototropism?
Blue light is detected by phototropin, which triggers auxin to accumulate on the shaded side of the stem, causing those cells to elongate more and bend the stem toward light.
What are examples of phototropism?
A windowsill plant leaning toward the glass, a seedling re-orienting after being rotated, and stems in a forest growing toward gaps in the canopy are all examples.
How is phototropism different in shoots and roots?
Shoots show positive phototropism (grow toward light) because auxin promotes elongation there, while roots show negative phototropism because the same auxin level inhibits elongation.