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Phototropism: Plant Movement in Different Light Intensity

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Author: Janice VanCleave

Plants move in response to light. Phototropism, or plant movement due to light, will be either positive or negative. Positive phototropism is a movement toward light, and negative phototropism is a movement away from light. In this project, you will study the response of plants to light. You will also determine the effect of light intensity and different colors of light on plant movement as well as the rate at which plants move in response to light.

Getting Started

Purpose: To determine the response of oat seedlings to light.

Materials

  • Flower pot
  • Potting soil
  • 1 tablespoon (15 ml) of oat seeds
  • Water

Procedure

  1. Fill the flower pot with potting soil to within 2 inches (5 cm) from the top.
  2. Sprinkle the oat seeds over the surface of the soil.
  3. Cover the seeds with about 1 inch (2.5 cm) of soil.
  4. Moisten the soil with water and keep it moist, but not dripping wet.
  5. Place the pot on a table near a window.
  6. Allow the pot to remain undisturbed for 14 days.
  7. Make daily observations and record the growth of the oat seeds above the surface of the soil (see Figure 13.1).

Results

The first signs of growth appear in four to six days. Straight, closed tube-like structures break through the soil and grow toward the light. After several more days, leaves break through the ends of these tubes. The tubes and leaves bend toward the light.

Phototropism: Plant Movement Due to Light

Why?

As the seeds germinate, each primary leaf is protected by a hollow, cylindrical structure, the coleoptile, which surrounds it. After the coleoptile has grown above the surface of the soil, it stops growing and the primary leaf breaks through. The coleoptile shoot and the leaf bend toward the light as a result of the buildup of the growth hormone auxin. The auxin migration theory for this phototropism (growth toward the light) states that light-sensitive auxin moves from the light side to the dark side of unevenly lighted growing tips. The cells on the shaded side contain a higher concentration of auxin and grow more rapidly than the cells on the lighted side. As a result, the plant bends toward the light.

Try New Approaches

Phototropism: Plant Movement Due to Light

  1.  
    1. At what rate do the leaves move when seeking light? Place a strip of masking tape around the outside rim of the flowerpot containing the oat seedlings. Use a marking pen to mark each quarter point on the circular strip of tape. Start with 0° on the side the seedlings are leaning toward and mark each 90° interval. Place a piece of tape on the table next to the side of the pot facing the window. Use the tape as a reference marker. Turn the pot so that the side marked 180° points to the tape reference marker (see Figure 13.2). The oat leaves should be facing away from the light coming from the window. Record the position of the leaves at four-hour intervals during the daylight periods until the leaves face the window. Record the light conditions for each day. Is it a bright, sunny day? Overcast? Partly cloudy?
    2. Is the movement of the leaves toward the light from side to side or do they flip over? Repeat the previous experiment two times, first placing the pot with the leaves facing at an angle or 90° (perpendicular) to the light source, and then having the leaves face 180° (in the opposite direction) to the light source. Observe the position of the leaves at two-hour intervals during the day until the leaves turn toward the light. Make note of the position of the leaves at each observation. To record the changes in the leaves' position, take photographs every two hours. Science Fair Hint: Display these time-lapse photographs to indicate the direction of motion of the leaves.
  2. Do plants grow toward an artificial light source? Repeat the original experiment placing the oat seedlings in front of a desk lamp. Make sure that the only light source in the room is the incandescent light from the lamp.
  3. Charles Darwin and his son Francis discovered that the tip of the coleoptile is necessary for phototropism in oat seedlings. Test this fact for yourself by reproducing their experiment. Repeat the original experiment. When the coleoptiles break through the surface of the soil, use fingernail scissors to snip off the tips of the coleoptiles from half of the seedlings (see Figure 13.3). Position the pot so that plants with and without the tips receive direct sunlight from a window.
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