Jan. 28, 2004

Solar Photography

Plant parts are of various colors depending on the type and amount of color pigment they contain. Leaves get their green color from a light-sensitive substance found in special plant cells, or chloroplasts.

In this project, you will determine how the color pigment found in plant leaves is affected by sunlight.

Getting Started

Purpose: To determine the effect of sunlight on the color of leaves.

Materials

• scissors
• black construction paper
• geranium plant
• making tape

Procedure

1. Use scissors to cut two sections from the black construction paper that are large enough to cover one leaf on the geranium plant. Note: The paper pieces should be slightly larger than a leaf on the plant.
2. Cut out a heart-shaped piece from the center of one of the paper pieces and save the outline (not the heart).
3. Sandwich a leaf between the two paper pieces, being very careful not to damage the leaf.
4. Tape the paper pieces together along their edges (see figure below).
5. Repeat the procedure attaching papers with heart-shaped cutouts on three or four leaves.
6. Place the plant in a sunny area.
7. Wait seven days.
8. Carefully remove the papers without damaging the leaves.
9. Observe the areas of the leaves that are covered by the papers.

Results

In areas covered by the black paper, the leaves change from dark green to pale green, yellow, or even white. There is a dark green heart where the leaf was exposed to light.

Why?

A chemical called chlorophyll (chloro = green and phyll = leaf) gives leaves their green color. Chlorophyll is found in special plant cells, or chloroplasts (site where the energy-producing reaction of photosynthesis occurs). Light is necessary for the development of chlorophyll.

Before a germinating seed breaks through the surface of the soil to receive light, the parts of the developing plant are pale. Colorless structures called proplastids are present. The presence of light changes proplastids to green chloroplasts. In the absence of light the leaf changes from dark green to a paler color as chloroplasts change to colorless plastids (organelles formed from proplastids that usually contain pigments, but some are colorless).

The opaque paper blocks the sun's light; thus, in the covered areas, the chloroplasts change to colorless plastids. Light shines through the cutout sections; thus, the chloroplasts remain a dark green.

Try New Approaches

1. How long does it take for the green chloroplasts to change? An exact time for the conversion of one structure of chloroplast cannot be determined with this experiment, but a general time can be determined from observing color changes during a measured time period. Repeat the experiment using enough plants to provide 14 healthy leaves. Cover the 14 leaves with solid pieces of black construction paper instead of using pieces with heart shapes cut from their centers. Prepare the leaves at night so that you can begin the experiment with the rising of the sun on the first day. Remove one paper patch after the covered leaf has received one-half of the light scheduled for that day. Take a color photograph to obtain the best record of the leaf's color. Remove the second paper patch at the end of the first day and photograph the leaf again. Continue this schedule of removing the patches. Compare the photographs to determine the time required to make noticeable color changes in the leaves. Note: Light duration and intensity vary with seasons. If possible, repeat the experiment during different seasons and compare the results. Make note of the season in which you perform the experiment.
2. Do the faded areas of a leaf return to their original green color if exposed to light? Use any of the plants that have faded leaves produced by covering the leaves with opaque paper. Place the plants where they will receive light. Watch them daily for seven days and use photographs to record any daily color changes. Science Fair Hint: Use the photographs to represent procedures and results.
3. Is light energy received on all of a leaf's surfaces? Repeat the original experiment covering different leaves in three different ways: front and back, only the front, only the back.

From Janice VanCleave's A+ Projects in Biology: Winning Experiments for Science Fairs and Extra Credit by Janice Pratt VanCleave. Text © 1993 by Janice Pratt VanCleave (janicevancleave.com). This material is used by permission of John Wiley & Sons, Inc.

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