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ZingPath: Plant Structure and Function

Phototropism in Plants

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Plant Structure and Function

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Phototropism in Plants

Biology

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You will observe a phototropism in plants.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Define phototropic responses in plants.
  • Give examples of tropic responses in plants.
  • Design an experiment to show phototropism.
  • Explain the role of auxin in phototropism.

Everything You'll Have Covered

Although plants are not mobile, they demonstrate tropisms, or responses to directional stimuli. Phototropism occurs when light (a directional stimuli) is applied. Plants demonstrate phototropism when they bend toward a source of light. Plant phototropism has been studied since the time of Charles Darwin, when he and son Francis worked with oat (Avena sativa) seedlings at the coleoptile stage. The coleoptile is a protective sheath around the young shoot. Oat seedling coleoptiles have a strong phototropic response.

In their experiment, the Darwins covered different portions of the coleoptile and determined that light perception at the tip was necessary for a phototropic response to occur.

Decades later, Peter Boysen-Jensen continued the study of phototropism in oat coleoptiles by removing their tips and placing an agar (gelatin) barrier between the tips and the coleoptiles. He also placed a mica chip between the tips and the coleoptiles of other shoots. The mica chip, a non-porous barrier, prevented the phototropic response, while the agar, a porous material, preserved it. Boysen-Jensen concluded that a chemical growth regulator, produced in the coleoptile tip in response to directional light, migrates down the coleoptile, where it causes elongation of the cells. Further experiments supported this hypothesis.

Another scientist, Frits Went, isolated the chemical signal by placing cut coleoptile tips on agar blocks, allowing compounds in the tips to diffuse into the agar. He placed the agar blocks alone on the severed coleoptiles and observed that even in the absence of light, the treated agar affected growth. Specifically, when placed to one side of the cut tip, the coleoptile bent toward the opposite side. When agar was placed directly on top of the cut tip, no bending occurred. Went concluded that a chemical signal from the tip traveled down the coleoptile and caused cells to elongate. This, and other experiments, showed that the signaling molecule accumulates on the shaded side of a coleoptile tip in response to unidirectional light, and then migrates down the same side of the coleoptile, where it softens the cell wall and causes it to take in water. The accumulated water causes the cells to enlarge and elongate, making the plant bend toward a light source. Went later determined the molecular identity of the signal, indole acetic acid (IAA), the first auxin. A class of plant hormones, auxins travel from specialized cells to other regions and affect gene expression.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should understand the general characteristics of plants, the structure of plants (root, stem, leaf), and how plants grow by photosynthesis.
Course Biology
Type of Tutorial Experiment
Key Vocabulary auxin, Boysen-Jensen, coleoptile