How Do Roots Grow When the Direction of Gravity Changes?

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


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.

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 panes of glass. The second set will be germinated between horizontal panes of glass, so the roots will be blocked from growing in the same direction as the gravitational field. The last set will be rotated 90° 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


* 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?

Materials and Equipment

* Blotting paper, cut to fit the glass panes (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.)
* Six panes of glass from 5"x7" photo frames (Alternatives to glass: Plexiglas® from your local hardware store, clear plastic sheet protectors, or plastic baggies)
* Rubber bands
* Permanent marker
* Modeling clay
* Three shallow pie plates (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. For the first "seed sandwich," moisten the blotting paper.
2. Place several seeds on the moist paper, in a grid pattern, several inches apart. Avoid edges because they are more likely to dry out.
3. Place the blotting paper between two glass panes (or into the plastic sheet protector or plastic baggie).
4. 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 steps 1–4 for two other seed "sandwiches."

Plant Biology Science Project: Growing rootlets between glass panes

5. Mark the four edges on the front side of each glass pane with a permanent marker (or tape and a pen): label them Up, Down, Left, and Right.
6. Using the modeling clay to hold it in place, set one glass sandwich (#1) upright in a 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.
7. Place the second glass sandwich (#2) horizontally in another shallow pie plate, holding it in place with the modeling clay.
8. Start the third seed sandwich (#3) off vertically in the modeling clay on the third pie plate, but rotate it 90° every few days to see how this changes the direction of the root growth. Keep careful records of when you rotate this third seed sandwich.
9. Keep the seeds moist by using the eyedropper or squirt bottle to slowly and carefully add a little water to each seed sandwich. Be careful not to dislodge the seeds.
10. Watch for the seeds to germinate and the rootlets to start growing.
11. Observe how the roots grow under the different conditions.
12. 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. 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°; directly up as 180°; and to the right as 270°.
13. Make a table of your data, showing the length of each rootlet at different times. Graph this data. Did they grow faster when growing in the direction of gravity (#1) vs. growing horizontally (#2)?


* Investigate how rotating the seeds 180° 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° (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. What happens if you rotate the glass 180° every 24 hours?
* Make a seed sandwich in which the seeds are germinated while held at a 45° angle to the vertical. Use four plastic triangles, 90° x 45° x 45°, and the modeling clay to hold the seeds at 45°. Question: does reducing the angle from 90° 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° or more, away from the vertical.
* Graph the results of your experiments on polar graph paper

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