Bending Plant Roots with Gravity

Background Information for Teachers

Geotropism (also called gravitropism) is the directional growth of an organism in response to gravity. Roots display positive geotropism when they grow downward (in the direction of gravity), while shoots display negative geotropism when they grow upward (opposite the direction of gravity). 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. Watch this time-lapse video to see a corn seedling root display geotropism.

This time-lapse video shows a corn seedling root bend down in response to gravity.

https://www.youtube.com/watch?v=zpbjnGzT_7Q

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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.

Image Credit

The cells in a plant root are protected by an outer layer called the root cap that is colored in red. The inner layer of cells are called statocytes (colored in yellow) and in each statocyte contains smaller structures called statoliths (colored in white) that move in the direction of 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 (light green). These cells contain statoliths (small dots) that move in response to the direction of the gravity (toward the bottom). (Drawing by David Whyte, 2008).

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. 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 lesson plan, you and your students 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 in a plastic bag. The second set will be germinated horizontally in a plastic bag, 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.

Additional Background Links

• Up, down, and all around: How plants sense and respond to environmental stimuli, Proceedings of the National Academy of Sciences by Kiss, J.
• Plant Tropic Responses, Plants-In-Motion by Hangarter, R.
• The Advantages of Geotropism in Roots, hunker.com by Jacob J. Wright
• Gravitropism vs. Phototropism, Biology Teaching Greenhouse by Joshua Culberson and Drew Highsmith

Prep Work (0 minutes)

Besides making sure that all necessary materials are available, this lesson plan does not require additional prep work.

Engage (20 minutes)

1. Before starting the experiment, show your students some plant seeds and a growing plant and then ask them the following questions.

   Have you ever planted a seed and watched it grow? What happens when the plant starts to grow? Do you know which part of the plant comes out of the seed first?

   Students might say that the leaves or shoots come out first, since this is the first part they see breaking through the soil surface. Let them know that when a seed is planted, it first grows roots to anchor the plant to the ground. Then the shoots, the part of the plant that becomes the stem or stalk, start to develop and the plant breaks through the soil surface. Only when the plant starts to see sunlight will it open its first leaves and start to grow.

2. Show your students this video demonstrating how corn seeds start growing. Tell them that this time-lapse video shows how corn seeds germinate (start to grow). First they develop roots, and then the shoots start to grow. Once they have seen the video, continue with the next question.
   

   https://www.youtube.com/watch?v=iFCdAgeMGOA

   When the roots and shoots came out of the seeds, did you notice which direction they grew in?

   The roots grew downwards and the shoots grew upwards. You can see that, even if the root and shoot were not pointing in the correct direction initially, they would turn to go down or up respectively.

   Why do the roots or shoots not grow sideways into the soil? Does anyone have an idea of how the roots and shoots of the plant know which way is up or down?

   Gather all the ideas that the students come up with. The important aspect here is that plants are able to "sense" the direction in which they are growing. If your students struggle with this question, point them into the right direction by asking "How do we know which way is up and down?", "Which parts of our body are always facing up or down?", "Do we use specific parts of our body to figure out which way is up or down?" It might be difficult for students to think of how plants can "sense" which way is up or down. Ask them for ideas and prompt them to think about how we know which way is up or down. For example, how can cues from our environment tell us which way is up (the sky is up and the ground is down, the ceiling is up and the floor is down, etc.)? Can we still tell which way is up with our eyes closed or in a totally dark room? What senses do we use?

3. Explain to your students that plants have developed a way to know which way is up and down, called geotropism or gravitropism, which is based on gravity. Gravity is an invisible force that pulls objects towards each other. Earth's gravity is what keeps us on the ground and what makes things fall to the ground. Plants can sense the gravitational force using specific cells in their root tips (as shown in Figure 1). These cells contain small particles that sink to the bottom of the cells in response to gravity. This is essentially the same as dropping a rock to determine which way is down. The cell senses where the particles touch inside the cell; now it "knows" which way is down. This information is transmitted to the plant roots or shoots, and once the plant knows which way is up and down, it can respond by growing the shoots upwards and the roots downwards.

   Why do you think geotropism is important for plants? What would happen if their roots grew up into the air or the leaves grew underground?

   A plant's leaves grow up into the air because they need to absorb sunlight, and the roots grow down into the soil so they can absorb nutrients and water. So, if the leaves grew into the soil, they would not be able to absorb sunlight, and if the roots grew up into the air, they could not absorb nutrients and water, and the plant would not survive.

4. Tell your students that now that they know that roots always grow downwards and shoots always grow upwards, they will do an experiment to find out how growing root tips respond to the change in the direction of gravity. They will plant radish seeds in a plastic bag and watch them grow throughout the course of one week while rotating them by 90 degrees every other day to observe how quickly the roots respond to the change in direction of gravity.

   Optionally, you can also show your students this introductory video to this experiment:

   

   https://www.youtube.com/watch?v=FSMbrMxCyYU

   What do you think will happen if you let the roots come out of the seed, grow for a bit, and then turn them to the side so that what used to be upwards now is sideways? Do you think the roots will react to this change?

   Do not give the answer to this question but ask them to formulate their own hypothesis and write it down on their worksheet. Then proceed to the experimental part.

Explore (Active: 60 minutes; Total: 7 days)

Have your students follow these steps:

Day 1

1. Prepare a large cardboard box to make sure that no light can get inside. If the box has any cracks where light can penetrate, seal them with opaque tape, such as duct tape. Do not tape the lid of the box shut yet so that you can easily put the zip-lock bags inside later.
2. Label three plastic zip-lock sandwich bags using a permanent marker (or a pen and tape). Label all four edges on one side of each bag: "Up," "Down," "Left," and "Right" as shown in Figure 2.
3. For each bag, fold a full-sheet paper towel in half twice so it fits in the bag (or stack two half-sheet paper towels on top of each other and fold them in half once).
4. Put the folded paper towels in plastic bags, making sure an open edge (not a folded edge) is at the top of the bag. Make sure the bags can zip closed.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 2. Folded paper towel in a zip-lock bag.

5. Lightly moisten the paper towels in each bag with water so they are damp but not soaked. Pour a little water in the bag (about two tablespoons), close it, and swish the water around. If some of the paper towel is still dry, add a little more water. If there is excess water in the bag, pour it out.
6. Carefully open each bag and peel apart the paper towels so there are two layers on each side, as shown in Figure 3.
7. Place five seeds in the middle of the bags as shown in Figure 3. The seeds should be evenly spaced from each other. None of the seeds should be close to the sides of the bag. Press down on the paper towels so the seeds stick to them.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 3. Five seeds evenly spaced inside the bag.

8. Carefully zip the bags closed, leaving some air in each so the seeds have enough oxygen to grow. Holding each bag up to a light, look at the middle of the bags to make sure the seeds are still in place.
9. Use the strong tape to carefully attach two of the seed bags to a vertical side in the large cardboard box. Secure the bags using the area above the zipper and make sure the "Up" label is at the top. The third bag should be taped to the bottom surface in the cardboard box as shown in Figure 4.
10. Write a "V" on one of the vertical bags to stand for "vertical." On the other vertical bag, write an "R" for "rotate." This bag will be rotated 90 degrees clockwise every two days, starting in two days. On the bag lying flat, write an "H" for "horizontal".
11. Close the box and make sure that no light can get in (tape the lid shut if necessary). Put the box at a place where it can be undisturbed for two days.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 4. Zip-lock bags taped inside the cardboard box.

Day 3

12. Two days after putting the seeds in the bags, carefully remove each bag from the box, one at a time. Do not leave any bag exposed to the light for more than 10 minutes. Be careful to not rip tape off of the bags as it might tear them. Briefly hold the bags up to a light to look at the seeds to check for root formation and root growth direction. You can use your fingers to feel if there are any roots between the paper and which way they are growing.
13. Write down your observations in your worksheet, such as how many seeds sprouted and the direction the roots are growing, for each bag. Include a drawing of how the roots grew.
14. If water has collected in any bag, carefully open it and pour the water out. If any paper towels got too dry in any bag, add more water. Seal the bag so it is flat, but be careful not to squeeze all of the air out, as the seeds will still need some oxygen to grow. Then put the bag back in its original location. You can use new tape to secure the bags if needed. Rotate the bag marked with the "R" 90 degrees clockwise (so that "Left" is on top and "Right" on bottom) before securing it in place.

Day 5

15. Repeat the same steps as on day 3 (steps 12–14), and again rotate the bag marked with the "R" 90 degrees clockwise (so that "Down" is on top and "Up" is on the bottom) before securing it in place.

Day 7

16. Six days after putting the seeds in the bags, carefully remove each bag from the box and hold it up to a light again to look at the seedlings. Feel the roots through the paper with your fingers if it is difficult to see them.
17. Then, open each bag and take out the paper towel, making sure to keep its orientation the same as it was inside the bag. Carefully open the paper towels to expose the sprouted seeds, trying not to break the fragile roots or stems.
18. Write down your observations again, such as the direction the roots are growing in each bag. Again, include a drawing of how the roots grew.

Reflect (30 minutes)

Once the experiment is done (after 7 days) and students have collected their data, gather the class to discuss their observations and reflect on their results. You can use following questions for your discussion.

What were your observations at Day 3 of the experiment? Did any of the radish seeds start growing? How many? Can you think of a reason why some did not sprout?

On day 3, most, or at least half, of the radish seeds should have sprouted. The time needed for seeds to grow depends on the environmental conditions around them, such as the availability of water, oxygen supply, or surrounding temperature.

Did you see any roots or shoots developing yet? Which way did they point?

On day 3, there most likely is already some root growth happening for some of the seeds. At this point, as the bag has not been rotated yet, the roots for the "V" and "R" bag should both point downwards. The roots in the "H" bag should grow randomly in all directions or form coils.

How do the roots grow in the bag labeled "V" after 5 days? Is the growth direction different from the roots in the "R" bag that you rotated on day 3?

On day 5, the seedling roots in the "V" bag should mostly point toward the "Down" label, in the "R" bag they should have bent and mostly point towards the "Right" label (which should be at the bottom). The roots in both bags should grow in the direction of gravity, which is downwards. However, as you rotated the "R" bag, the right part of the bag is now at the bottom, which is why the roots should point towards the "Right" side of the bag.

Which direction did the roots of the seedlings in the rotated bag grow after 7 days after you rotated the bag once more on day 5? Do they look different from the roots in the "V" bag?

On day 7, the roots in the "V" bag should look similar to how they did on day 5, except longer. In the "R" bag, the root near the root tip should be bent towards the "Up" label (which should now be at the bottom), while the root in the middle should be horizontal, running straight between the "Left" and "Right" labels. The initial part of the root that grew in the first three days should still face towards the "Down" label. An example is shown in Figure 5. This is because the root tip guides the growth toward gravity, and as the direction of gravity changed when the bag was rotated, the root tip changed the direction of the new root growth.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 5. Root growth in "R" bag after 7 days.

What did you observe in the bag that was lying on the flat surface, labeled "H"? Did the roots of the seedlings in the horizontal bag grow slower than the ones that could grow down in the vertical bag?

The roots in the "H" bag should point in all different directions because they cannot grow downwards. They also should get longer over time. Although the root tips can sense which way is down, they are physically prevented from growing in that direction. Thus, the roots will grow randomly to the sides or form coils.

Looking at your results, how do you think plant roots would grow in space, where there is no gravity (assuming they are still in a cardboard box with no light)?

Get your students involved in a discussion. A great resource to answer this question can be found at How Plants Work. Without the presence of light and gravity, the roots as well as the shoots get disoriented as they cannot sense where is up and down. As a result, they will grow in random directions or form coils similar to the results in the "H" bag.

Can you think of another experiment to demonstrate geotropism? Think about other parts of the plants that respond to the direction of gravity.

Let your students come up with some ideas. Remind them that not only root growth, but also growth of the shoots responds to gravity. In contrast to the roots, they will always grow upwards, so If you let a plant grow and then turn it to its side, for example by rotating the pot to the side, the shoots of the plants will bend to grow upwards again. You can show students this video as a demonstration.

https://www.youtube.com/watch?v=r7uyPrvljqg

Besides gravity, what other factors do you think can affect plant growth?

Light, moisture, temperature, humidity, soil quality, pollution, or wind are some examples your students might come up with. Most of them are climate-related. Some additional factors that you can mention are genetic factors, which include adaptation to droughts, resistance to diseases, or stress tolerance.

What tests could you do to find evidence for how environmental conditions other than gravity affect the growth of plants or different organisms?

Let your students discuss this question and design some experiments to find evidence for cause-effect relationships such as how soil moisture, temperature, or light conditions affect plant growth.

Assess

You can use this quiz to assess student learning after the activity:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (PDF)

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Plant Scientist

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Career Profile

Plant scientists study plants to find out how they react to environmental changes. This allows them to make crops more resistant to droughts, help protect endangered plant species, or see how plants react to pollution and climate change. They may do experiments to investigate which plants grow the fastest or to find the best environmental conditions for them to grow. For example, they could test how the growth of plant roots and shoots reacts to pesticides or drought conditions. Read more

Agricultural Technician

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Career Profile

Agricultural technicians conduct research on plants, food, or other agricultural products. They do experiments to find out how plants react to environmental changes due to climate or genetic factors. This helps them to improve the yield and quality of crops or make them more resistant to diseases and pests. Read more

Lesson Plan Variations

• Take the investigations of geotropism "above ground" and instead of observing root growth, explore how the plant shoots react to a change in gravity direction. To do this, use young plants that are not too big yet and first let them grow straight. Then, turn the pot to the side and observe how the shoots will change their growth direction to point upwards again.
• In addition to geotropism, let your students research phototropism: the movement of plants towards light. Design an experiment in which your students can investigate how plants react to a change in the direction of light (similar to sunflowers), while the direction of gravity remains constant. How does the plant grow if the light source is at one side and then switched to the opposite side? Instructions on how to do this experiment are given in the Science Buddies project Plants on the Move! Experiments with Phototropism.
• Let your students explore how other environmental factors such as humidity, temperature, or soil quality affects plant growth.