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Project Summary

Difficulty	4  –  8
Time required	Long (a couple of weeks)
Prerequisites	None
Material Availability	Readily available
Cost	Low ($20 - $50)
Safety	No issues.

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Sponsor

Sponsored by a generous grant from Monsanto Fund

Objective

In this science project, you will study how growing rootlets respond to gravity.

Introduction

Geotropism (also called gravitropism) is the directional growth of an organism in response to gravity. Roots display positive geotropism when they grow downward, while shoots display negative geotropism when they grow upward. Among the first scientists to study geotropism was Charles Darwin, who, along with his son Francis, published The Power of Movement in Plants in 1880. Despite a long history of studying this subject, there are many questions about how geotropism actually works.

The process can be broken down into three phases: perception, transduction, and response.

Perception is the sensing of environmental stimuli. Perception allows organisms to gain information about properties and elements of the environment that are critical to their survival. The perception of gravity by root tips is thought to be mediated by special cells called statocytes (see Figure 1). These cells contain small bodies that sink to the bottom of the cells in response to gravity. The term for this type of body is statolith. This perception process that takes place in the statocyte is essentially the same as dropping a rock to determine which way is down. The statocyte cell senses where the statolith touches inside the cell; now it "knows" which way is down.

The next step is for the statocyte to communicate this information to other parts of the root. The goal is to tell the part of the root that is growing which direction to go. To do this, it has to convert the signal generated by the falling statolith into a chemical signal. The term used for converting information from one form to another is transduction.The process of signal transduction is very important in biology. For example, your eyes transduce light energy into electrical signals that are sent to the brain for processing, which allow you to see your environment. In the case of the statocyte, the signal from physical contact of the statolith is "transduced" into a chemical signal that is able to communicate with other cells.

The final step is the response of the growing cells to the signal indicating which way is down. Signal transduction from the statocytes to the root-tip cells results in a specific response: growth in the direction of the gravitational field. Molecular genetics, fine-laser ablation (this destroys very specific regions of the root to see how this affects growth), and zero-gravity environment experiments are some of the tools that are used to tease out the molecular mechanism by which plants sense and respond to gravity.

Figure 1. This diagram of a plant root illustrates the regions of gravity perception. Within the root cap at the apex (red), some cells develop into the gravity-sensing cells called statocytes (yellow). These cells contain statoliths (small white dots) that move in response to the direction of the gravity (toward the bottom). (Drawing by David Whyte, 2008).

In this science project, you will be watching and recording three sets of seeds to see how the growing root tips respond to the change in the direction of gravity. The first set will be germinated while held vertically between two plastic panes. The second set will be germinated between horizontal plastic panes, so the roots will be blocked from growing in the same direction as the gravitational field. The last set will be rotated 90 degrees to observe how quickly the roots respond to the change in direction of gravity (this could be every day, or at longer intervals, depending on how fast the root tips are growing).

Terms, Concepts and Questions to Start Background Research

To do this science project, you should know what the following terms mean. Have an adult help you search the Internet or take you to your local library to find out more.

• Geotropism
• Statocyte
• Statolith
• Signal transduction
• Stimulus perception
• Cellular response

Questions

• Does geotropism cause the root to grow faster, or does it just determine the direction of growth?
• Do roots grow in random directions when they are blocked from growing downward?
• What other types of tropism are displayed by plants?
• How would plants develop in a zero-gravity environment?

Bibliography

You might want to start your research by checking out a botany book from your local library and reading what it has to say about germination and tropism. Some other sources of information are listed below.

• Kiss, J.Z. (2006). Up, down, and all around: How plants sense and respond to environmental stimuli. The National Academy of Sciences of the U.S.A., Volume 103 pages 829-30. Retrieved December 26, 2007 from http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1348016&blobtype=pdf
• Johnson, Gregory B. "Tropism", World Book Encyclopedia. 1999.
• These websites investigate plant tropisms and offer instructions for building a box to view roots in growing plants:
  • National Gardening Association. (2004). Curriculum Connections: Investigating Plant Tropisms. Retrieved March 17, 2008 from the National Gardening Association's kidsgardening.com website: http://www.kidsgardening.com/growingideas/projects/july04/pg2.html
  • National Gardening Association. (2004). Creating Root-View Boxes. Retrieved March 17, 2008 from the National Gardening Association's kidsgardening.com website: http://www.kidsgardening.com/growingideas/projects/july04/pg1.html#box

Materials and Equipment

• Blotting paper, cut to fit the plastic panes; available online, from suppliers such as Carolina Biological: www.carolina.com (Alternatives: felt cloth, paper towel)
• Seeds (1 package) (Note: Radish seeds are good because they germinate quickly. Tomato, basil, and thyme seeds are also suitable.)
• CD "jewel" cases (18). You will use 6 cases for each procedure. You'll need 18 cases to run three trials concurrently. (Alternatives to CD "jewel" cases: panes of glass from 4" X 6" photo frames, Plexiglas® from your local hardware store, clear plastic sheet protectors, or plastic baggies)
• Rubber bands
• Permanent marker
• Modeling clay
• Shallow pie plates (6) (Alternatives: cardboard boxes lined with foil or wax paper)
• Eyedropper or squirt bottle filled with water
• Lab notebook
• Ruler and protractor
• Graph paper

Experimental Procedure

To start this experiment, you should bring all of your materials together on a flat work surface. The surface might get a little wet, so have some paper towels handy.

1. Begin preparing your "seed sandwiches." Cut the blotting paper to fit into the CD jewel cases. You can also use a folded paper towel.
2. Place blotting paper in six of the CD cases, (or glass panes, plastic sheet protector or plastic baggie).
3. Moisten the blotting paper. It should not be dry or dripping wet.
4. Place four seeds on the moist paper. You can vary the number of seeds if desired.
5. Close the CD cases. If you are using the glass panes, loop two rubber bands top to bottom, and two rubber bands right to left around the glass panes. If you are using a plastic sheet protector, trim the edges with scissors and use paper clips or staples to hold it together. If you are using a baggie, close it carefully, leaving a small amount of air in the bag. Repeat the steps above for the other seed "sandwiches."

   Figure 2. Growing rootlets between glass panes.

6. Label the backs of all six seed sandwiches 1, 2, 3, 4, 5, and 6 with a permanent marker (or tape and a pen). Mark the four edges on the front side of each sandwich: Up, Down, Left, and Right.
7. Using the modeling clay to hold them in place, set two sandwiches (#1 and #2) upright, each in their own shallow pie plate, with Up on the top edge. If you are using the plastic sheet or the baggie, you can keep the seed sandwiches upright by pinning them to a cork board or taping them to a surface, such as your refrigerator.
8. Place two different sandwiches (#3 and #4) horizontally (flat) in two more shallow pie plates. These sandwiches lie flat so that the rootlets cannot grow in the direction of gravity. You may want to use modeling clay under the sandwiches to lift them off the surface of the pie plate, allowing water that you'll be adding and that might drip into the pie plate to dry.
9. The final two seed sandwiches (#5 and #6) should set vertically in the modeling clay on two more pie plates, upright with the label "Up" at the top. You will rotate these sandwiches 90 degrees clockwise every 2 days to see how this changes the direction of the root growth. Keep careful records of when you rotate these seed sandwiches #5 and #6.
10. Keep the seeds moist by carefully opening the CD cases and using the eyedropper or squirt bottle to slowly and carefully add a little water to each seed sandwich, if necessary, each day. Be careful not to dislodge the seeds.
11. Watch for the seeds to germinate and the rootlets to start growing.
12. Observe how the rootlets grow under the different conditions.
13. Record the length of root growth for each seedling in your lab notebook. To compare the results quantitatively, record the lengths of the roots at various times. Draw or take pictures of the seeds for your report. Note: As a more advanced option, you could also record the angle of growth at various times. Use the direction directly toward the bottom edge, "down," as zero degrees; to the left as 90 degrees; directly up as 180 degrees; and to the right as 270 degrees.
14. Make a data table showing the length of each rootlet at different times. Graph this data. Did they grow faster when growing in the direction of gravity vs. growing horizontally? How quickly did the growing root tips respond to the change in the direction of gravity in sandwiches 5 and 6?
15. Repeat the entire experiment, with fresh materials, at least two more times. Label the sandwiches for the second trial 7–12 and for the third trial 13–18. Space out the start of each trial by a few days.

Variations

• Investigate how rotating the seeds 180 degrees for various times affects root-tip growth. Make five sandwiches with germinated roots. Keep #1 vertical for the length of the experiment. Rotate plates #2–#5 by 180 degrees (so that the root tips are pointing up) for various periods of time each day. During this trial time, the direction of gravity relative to the direction of root-tip growth is reversed. After this period of "upside-down" growth, rotate the panes again so that the root tips are once more pointing downward. Try this for several different time intervals. For example, 10 minutes, 1 hour, and 6 hours per day.
• Make a seed sandwich in which the seeds are germinated while held at a 45-degree angle to the vertical. Use four plastic triangles, 90° x 45° x 45°, and the modeling clay to hold the seeds at 45 degrees. Question: does reducing the angle from 90 degrees increase the "scatter" in the data for the direction of growth? Why or why not? Try other angles and graph the scatter for each angle. A simple measure of scatter might be the number of roots growing at 30 degrees or more, away from the vertical.
• Graph the results of your experiments on polar graph paper. (Polar graph paper can be downloaded here). Figure 3 below shows the length and direction for three roots, taken at three different times. What trends do your graphs demonstrate?

  Figure 3. Polar graph of rootlet growth. (Drawing by David Whyte, 2008).

• Perform the experiment so that one set of seeds is protected from light and the other is exposed to light. Does the light affect your results?
• You might try to use a scanner to record the positions of the root hairs. Be very careful not to break the glass or cause any water damage to the scanner.

• For more science project ideas in this area of science, see Plant Biology Project Ideas.

Credits

David Whyte, PhD, Science Buddies

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Last edit date: 2009-01-27 13:40:00