BioEd Online

Teacher Background

Generally, leaves and stems grow upward, toward light sources, while roots grow downward. But plants do not have nervous systems or sensory organs—no eyes, ears, or vestibular system like animals have. So, how do plants “know” which way is up?

Plants sense and respond to their environments in a number of ways. Receptor molecules within plant cells perceive changes in external conditions, such as light, and initiate internal signaling pathways that enable the plant to react. Communication inside plants occurs hrough hormones, chemical substances produced in one part of the plant that have a developmental or physiological effect elsewhere in the plant. There are seven major kinds of plant hormones, and one, auxin, is primarily responsible for directional growth responses.

Light is important for plant development, including flowering and seed germination. It also is essential for photosynthesis, and can stimulate plant growth in a particular direction (toward or away from a certain wavelength of light). A plant’s growth response to light is called phototropism, from the Greek words trope (for “turn”) and photo (for “light”). A phototropic response involves the detection of a light wavelength by receptor molecules in plant cells, and transduction (i.e., conversion) of that signal into biochemical responses that lead to altered growth patterns.

Charles Darwin, the great evolutionary biologist, investigated grass seedlings’ growth responses to blue light (about 460 nanometers in wavelength) as early as 1881. He already knew that growing plants would bend toward light coming from a single direction. However, he found that when he covered the tips of grass seedlings with a foil cap, the seedlings no longer tilted toward the light source. Normal bending occurred when he covered the seedling tips with a glass tube and when he covered the stem below the tip with an opaque collar. Darwin and his coinvestigator son, Francis, proposed that the seedlings were bending toward light in response to an “influence” that was transported down the stem from the growing tip.

In 1926, Fritz Went, a Dutch scientist, identified the chemical messenger that causes cells on the shaded side of a shoot to elongate and grow faster than cells on the lighted side, thereby bending the stem toward the light source. He called this messenger hormone auxin. Today, synthetic auxins play important roles in agriculture as weed killers, and in preventing fruit from dropping off trees and bushes before it can be harvested.

Because stems grow toward a source of blue or white light (which, of course, contains wavelengths of light in the blue range), they are said to have a “positive” phototropic response. Conversely, roots have a weak response in the opposite direction. Because they grow away from a source of blue or white light, roots are said to have a “negative” phototropic response.

Note: For in-depth information regarding the role auxins play in plant growth and development, and about Brassica rapa, please download the Plants in Space Teacher's Guide.

Objectives and Standards

Inquiry

• Ask a question about objects, organisms and events in the environment.

• Plan and conduct a simple investigation.

• Use appropriate tools and techniques to gather data and extend the senses, and analyze and interpret data.

• Use data to construct a reasonable explanation.

• Think critically and logically to make the relationships between evidence and explanations.

• Use mathematics in all aspects of scientific inquiry.

• Communicate investigations and explanations.

Life Science

• Reproduction is a characteristic of all living systems; because no individual organism lives forever, reproduction is essential to the continuation of every species.

• All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.

• Behavior is one kind of response an organism can make to an internal or external stimulus.

• An organism’s behavior evolves through adaptation to its environment. How a species moves, obtains food, reproduces, and responds to danger are based in the species’ evolutionary history.

Earth and Space Science

Gravity is the force that keeps planets in orbit around the sun and governs motion in the solar system. Gravity alone holds us to Earth’s surface and explains the phenomenon of the tides.

Materials and Setup

For complete list of materials, material options, safety issues and setup information, please download "How Does Light Affect Root Growth?"

Materials per Student Group or Student

• Brassica rapa seeds (Wisconsin Fast Plants^®)

• Prepared flasks with media (see "Preparing Plant Growth Media and Flasks," p. 7 )

• Prepared experiment chambers, (see "Making a White-Light Seed Growth Chamber," p. 9)

• Logbook (class or individual)

• Marker pens

• Metric rulers (mm)

• Pencils with eraser tip

• Petri dishes (or shallow containers)

• Tweezers or forceps

• Access to electrical outlets, away from windows, if possible.

Setup

1. Wisconsin Fast Plants^® germinate within 24 hours, so schedule student investigations accordingly. (Planting on a Monday might be advisable, unless students have access over the weekend.) 

2. Plant seeds do not have to be oriented in any particular direction for lighting experiments on Earth since the roots will grow downward because of gravitropism.

3. Prepare media, flasks and experiment chambers. If grade-level appropriate, have students prepare media, flasks and build chambers.

Procedure and Extensions

1. Provide opportunities for students to learn about Brassica rapa plants.

2. Have students design their experiments (type of media and density used, number of seeds per flask, number of flasks, codes used to identify flasks, number of experiment repetitions, etc.; see “Repeating the Experiment,” left sidebar, p. 10.)

3. Instruct students to code the flask lids, and record flask codes, medium density (or densities if layered), date/time seeds are planted, etc., in a logbook.

4. Tell students to gently pour Brassica rapa seeds into the Petri dish.

5. Instruct students to pick up one seed with tweezers and place it centered and on top of the medium in a flask. Have them use the eraser end of the pencil to gently press the seed one or two millimeters into the medium (repeat if more than one is desired).

6. Have students seal the flask with its lid, place it inside of an experiment chamber, cover the chamber with its lid (lights over flasks), then turn on the LED lights.

7. Have students record observations once or twice per day for five to seven days (or more).

8. Have students compare their results with those of STS-134, available online at www.bioedonline.org/.

Handouts and Downloads

Download color images from the Plants in Space Teacher's Guide.