Phototropism

Phototropism is the growth of an organism in response to a light stimulus. It is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth.

History

The concept of phototropism was first studied by Charles Darwin and his son Francis Darwin. They studied the phototropic response of canary grass (Phalaris canariensis) and several other plant species. The Darwins discovered that light was perceived by the tip of the plant (the coleoptile), and that the response (bending) took place in a different part of the plant. They also found that the bending towards the light only occurred when the tip was exposed to light.

Mechanism

Phototropism is driven by the plant hormone auxin. Auxin is synthesized in the tip of the shoot and is transported down the shoot, where it promotes elongation of the cells it passes. When a shoot is exposed to light, auxin is transported towards the darker side of the shoot. This causes the cells on the darker side to elongate more than the cells on the brighter side, causing the shoot to bend towards the light.

See also

• Heliotropism
• Gravitropism
• Thigmotropism
• Hydrotropism

References

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Phototropism

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• Model for auxin transport
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• Phototropism
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