Photomorphogenesis (page 2)

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Author: Janice VanCleave

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    Photomorphogenesis: Plant Responses to Light

    1. How much light is needed to prevent etiolation? Repeat the experiment cutting a 2-inch (5-cm) diameter hole in the side of the box. Cover the hole with a piece of opaque paper (see Figure 14.2). At the same time each day during the test period, lift the paper cover from the hole and allow light to enter the box for five minutes. Close the cover and secure it with masking tape to prevent any extra light from entering. Note: Further testing of the quantity of light needed to inhibit etiolation can be done by using larger or smaller holes in the box and by increasing or decreasing the amount of time the light enters the box.
    2. Does the type of light affect the results? Repeat the experiment using different light sources, such as a fluorescent light, incandescent bulbs, and/or plant grow-lights.
    1. How does solar window film affect plant growth? Repeat the preceding experiment covering the hole in the box with a piece of solar window film. (For a list of companies who sell solar window film, look under "Glass Coating and Tinting" in your telephone directory.)
    2. Does the color of the light affect etiolation? Repeat the experiment using different colors of cellophane to cover the hole in the box. Science Fair Hint: Use photographs to represent the procedure and results of the experiment.

Design Your Own Experiment

  1. Can etiolation occur in the light? Test the effect of ethylene—a gas released by ripening fruit that affects plant growth—on plant cell elongation by placing two bean seedlings, one each, inside two large glass jars. Add a piece of ripened fruit, such as a banana, to one of the jars. Secure the lids on the jars and place them near a window where they will receive equal amounts of light. Observe the growth of the plants for one week. Find out more about the effect of ethylene on plant growth and include these facts along with the results of the experiment in your project summary.
    1. Another way to demonstrate etiolation is to grow bean seeds inside a closed aluminum foil package. Prepare a package by placing six or eight pinto or lima beans on a paper towel. Roll the towel around the beans and place the roll in the center of a piece of aluminum foil. Moisten the paper towel with water. Fold the foil around the paper roll and close up each end of the foil. After ten days, open the package and observe the stems of the seedlings.
    2. Design ways of using the foil packages containing beans to test the effect of the quantity of light on etiolation. One way might be to punch holes in the aluminum foil to allow a specific amount of light to enter each package.

Get the Facts

  1. Many types of seed remain more or less dormant if kept completely in the dark, but light inhibits the germination of other seeds. Find out more about the effect of light on seed germination. Which wavelengths of light are most effective in promoting germination? Which wavelength of light inhibits seed germination? What role does phytochrome (a light-absorbing pigment) play in the effect that different light waves have on seed germination?
  2. The response of any organism to light is mediated by light-sensitive pigments (molecules that change when they absorb light). Many plants respond to changes in the photoperiod (period of daylight) by making physiological changes such as the formation of flowers. How do pigments affect the ability of plants to respond to photoperiods? Learn more about photoperiodic control of plants. Discover the biochemistry behind plants' seeming ability to tell time. What is the critical photoperiod? Give examples and differences between short-day, long-day, and day-neutral plants.
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