Students investigate how light affects plant growth by observing changes in a plant’s growth and movement as light availability is altered through an experiment. Grades 3-5

phototropism: a plant’s bending and growing towards a light source

Although plants don’t have the ability to move from their rooted position, they do have the ability to respond to stimuli such as temperature, animals, moisture, gravity, and light. Tropisms are plant growth movements toward or away from a specific stimulus in nature. They help plants achieve optimal growth. Tropism comes from the Greek word, “to turn.”

Phototropism, photo meaning light, is the growth of a plant toward light. For plants, this light source is the sun, but artificial alternatives can also stimulate phototropism. This ability is very useful for plants, enabling them to position their leaves and flowers to efficiently receive the light energy they need for photosynthesis.

Plants have special receptors made of chemical pigments known as phytochromes. When phytochromes absorb visible wavelengths of light they emit a chemical signal that produces a hormone known as auxin. Auxins cause the cells on the shaded side of a plant to elongate more than cells on the sunny side. The growth of cells on the light-receiving side of the plant is inhibited. As a result, plants bend and twist towards the light.

In this lesson, we will focus on phototropism. However, there are a couple of other types of tropisms displayed by plants that are also important. Gravitropism causes stems of plants to grow up and roots to grow down. Hydrotropism causes plant roots to grow towards water.

1. Show students a handful of sunflower seeds. Use a picture if actual sunflower seeds aren't available.
2. As you show the students the sunflower seeds, ask them if they can tell you what they are. Once the sunflower seeds are identified, see if the students can tell you how sunflower seeds are produced. Use the following facts to teach your students about the production, processing, and use of sunflower seeds:
   • Sunflowers produce seeds which can be eaten, pressed for oil, or used in birdseed.
   • Sunflower seeds are produced mostly in North and South Dakota, Minnesota, Kansas, Colorado, Nebraska, Texas and California.¹
   • The type of sunflowers which are grown for seeds can grow to be 10 feet tall.
   • The head of a mature sunflower is usually about 15" in diameter.
   • Sunflowers are typically planted in the spring and harvested in the fall.
3. Summarize the lifecycle of the sunflower using pictures to illustrate.
4. Now that students have a basic knowledge about sunflowers, show them the YouTube video clip, Time Lapse: Sunflower Following the Sun. As the students watch the video, ask them to watch the leaves and the bud of the sunflower. Can they guess what the sunflower bud and leaves are following? Explain that it is the sun.
5. Define the word, phototropism. Break the word down into smaller pieces, explaining that “photo” means “light” and “tropism” means “to turn.” Draw on prior knowledge and remind students that plants receive their energy from the sun. Some plants move so that the surface of their leaves receive the most sun rays.
6. Tell students that in this lesson, they will design an experiment so they can observe phototropism in action.

1. Distribute the Tropism Twist activity sheet to each student. Ask students to write a hypothesis for the testable question, “Does light affect the direction that a seedling will grow?” in the appropriate place on their worksheet.
2. Divide students into lab groups consisting of 3-4 students. Distribute shoebox, scissors, duct tape, and cardboard. Instruct students to write their names on the bottom of the shoebox.
3. Show students a completed tropism testing box and guide them through the steps of creating their own boxes. Use the diagrams to guide students through the construction process.
   • Carefully draw and cut out a two-inch square from the middle section of one end of the box. Students may need help from the teacher and the teacher’s utility knife. It is recommended that only the teacher be equipped with a utility knife.
   • Place the lid on the front of the box. Hold the box up to the light. Look through your two-inch hole and make certain that this hole is the only source for light to get into the box. Carefully duct tape over any other cracks or crevices that may be letting light in. Do not tape the box shut.
   • Using paper to create a pattern, cut two pieces the height of the inside of the shoebox and half the width. Trace the pattern on stiff cardboard and cut them out. Tape them into the box as shown.
4. After tropism boxes are complete, instruct students to use the designated planting station to plant two bean seeds for their group experiment. The planting station should be supplied with newspaper, 6-ounce plastic cups, potting soil, bean seeds, water spray bottles, craft sticks, masking tape, and markers for labeling.
5. Place planted seeds in a lighted area and wait for the seeds to germinate. When the seedlings are approximately two inches tall, place the watered seedlings into the shoebox as shown.
6. Close the box, tape it, and place it by a sunny window so the square hole on the top can be exposed to the light.
7. After five days, carefully shine a flashlight through the square hole to observe the plant growth. It is best not to disturb plants during this testing period. It can alter the final outcome.
8. In another 3-5 days, check to see if the plant has grown enough to reach the top of the box. Remove the shoebox lid once the plant has reached the top of the shoebox. Have students record their observations and answer the questions on their Tropism Twist worksheet.

Variations

• Have students plant the bean seeds, then build the tropism boxes on another day while you are waiting for seeds to germinate.
• Using different kinds of seeds, test to see if different kinds of seedlings display phototropism more than others. Do some seedlings bend and twist the moment they germinate? Do other seedlings show no sign of phototropism? Compare and contrast growth rate and angle of growth rate between seedlings.

After conducting these activities, review and summarize the following key concepts:

• Sunflowers are an agricultural crop grown by farmers. Their seeds are produced for food, oil, or animal feed.
• Because plants receive their energy from the sun through photosynthesis, some plants "move" in order to face the sun and receive the most energy.

1. http://www.sunflowernsa.com/

This lesson was originally developed in 1993 through a partnership between the California Department of Food and Agriculture, California Farm Bureau Federation, Fertilizer Inspection Advisory Board, Fertilizer Research and Education Program and the California Foundation for Agriculture in the Classroom. It was updated in 2013 with funding from the California Foundation for Agriculture in the Classroom and a grant from the California Department of Food and Agriculture’s Fertilizer Research and Education Program.

Original Author: Pamela Emery
Executive Director: Judy Culbertson
Illustrator: Erik Davison
Layout and Design: Nina Danner

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