Plant tropisms and growth hormones

Introduction: (Initial Observation)

Look at the picture. Why all pansy flowers are faced the same direction? I checked them in the morning, noon time and even in the afternoon. It seems they are all looking at the sun.

Even though I first noticed that in the Pansy flowers, it is not limited to pansies and viola. All flowers move. Some are just more noticeable.

When I first noticed such a movement, I simply thought plants are smart and will decide which way to face. Later I thought it might be the power of sun that makes them move. But now I want to find out real reasons behind such peculiar movement.
Plants move is not just a response to light. Even if you knock over a plant, soon the leaves will have turned upward and the roots downward. They respond to touch, too – if you touch a plant on the same side every day, it will grow away from that side, trying to get away from objects and toward open space.

So what makes the plants grow up? are they growing towards light or they are growing against gravity?

Information Gathering:

Find out about plants and their moves. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.

The question of how or why Plant move? is not a new question. In my Internet search I found a document (http://archive.showmenews.com/2001/Feb/20010216News029.asp ) that shows even Darwin was wondering and studying about plants movement.

More research showed that such involuntary response of an organism, or part of an organism, involving orientation toward or away from something is called tropism.

If you do more Internet search using the keywords tropism and plant, you will find that depending on the type of stimuli that causes tropism, we have different types of tropism. For example plant movement cause by light is called phototropism and plant movements caused by gravity is called gravitropism. Also search to see what is thigmotropism?

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to see why and how do plants move and grow at certain directions. Some specific questions (that can be studied using scientific method) are:

Do plants grow towards light?

Do plants grow against gravity?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

The effects of light, gravity and touch can be tested in different experiments.

For example if you want to test the effect of gravity, following is how you define variables.

Independent variable or manipulated variable is the direction of gravity.

Dependent variable or responding variable is the direction of plant growth.

Constants are the type of plant, size of plant and direction of light.

Controlled variables are water, nutrients, temperature and other environmental conditions.

If you want to study the effect of light on the direction of plant growth, this is how you define variables.

Independent variable or manipulated variable is the direction of light.

Dependent variable or responding variable is the direction of plant growth.

Constants are the type of plant, size of plant, amount of light, and direction of gravity.

Controlled variables are water, nutrients, temperature and other environmental conditions.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

For each of the questions that you want to study, suggest one specific hypothesis. For example for the effect of gravity on the direction of plant growth this is a sample hypothesis:

Plants root grow towards gravity while plants stem grows against the direction of gravity. My hypothesis is based on my observation of plants growing at home in glass test tubes. such plants get equal amounts of light from all directions, so their growth direction is affected by gravity.

Note that hypothesis does not have to be correct. Often the results of your experiments may make you reject your own hypothesis.

For the effect of light in the direction of plant growth this is a sample hypothesis:

My hypothesis is that plants grow towards light. under certain light conditions, one side of the plant will enlarge or shrink and that will make the plant tilt toward the shrunken side or opposite the enlarged side. For example in the morning, the side of the plant that is toward sun, will lose some water and molecules will shrink, as a result the plant tilts toward that side.

Your further studies will show that shrinking or growing one side of stem is not caused by lose of water molecules, but it has to do with growth hormones.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

As your first experiment, you want to build a model that shows how plants move (This is called Tropism).

You will need the following materials:

• Your hands

Procedure (what to do):

1. Clasp your hands together in front of you, keeping your elbows together and your wrists nice and relaxed.
2. Keeping your right hand still, push upwards with your left.
3. Remember to keep your wrists relaxed

Q: What happened?

• Did your hands tilt to the right?

Now do the opposite, keep your left hand still and push up with your right.

• Did your hands tilt to the left?

Observations:

You have just done what plants do every time they want to grow somewhere!
They stop growing on the side of the stem where they want to turn and keep on growing on the other side. This causes them to tilt in the direction they want to grow, just like your hands!

Experiment 1:

Building a Model of a Plant to see how plants grow.

You will need the following materials:

• A slinky
• Strong Cardboard
• Scissors
• “Duck Tape” (or other strong adhesive tape)
• A Balloon
• An elastic band
• A straw (or a length of rubber tubing)
• Some wire twists (used for closing plastic bags or tying up flowers)

Procedure (what to do):

1. Attach the balloon to the straw (or rubber tubing) using the elastic band.
2. Try to make the balloon more supple by blowing into it.
3. Cut the cardboard into two squares or circles – cut them large enough to cover the ends of the slinky.
4. Punch a hole into the center of one of the pieces of card. The hole needs to be big enough to squeeze the balloon through.
5. Tape the pieces of card to each end of the slinky
6. Stuff the balloon into the slinky and place the slinky vertically on one end.

Now you are ready to make your plant grow!

7. Inflate the balloon by blowing into the straw.

Q: If you keep blowing what happens?

The whole slinky will extend more or less upwards, just like a growing plant.

Now that you’ve got your “plant” to grow, try to make it bend!

8. Using one of your wire twists, hook the wire round several of the slinky’s hoops and inflate the balloon again by blowing into it.

• Q: What do you notice?

Just as in a real plant, the slinky will bend to the side that does not grow (where the wire is hooked round the hoops of the slinky)

Observations:

The great thing about this model is that it also represents the main force used by plants to move, that is internal pressure acting on a semi-rigid box.

In the case of the balloon, air pressure is the force that drives the rings of the slinky apart. Real plants use water pressure, or in scientific terms, “hydrostatic pressure”, to get their cells to elongate.

Phototropism

As we’ve already seen, plants change their direction of growth and they do so in response to different stimuli in their environment (light, soil, and water etc.)

Q: Why do you think this is important to a plant?
Can a plant get up and walk to a sunny spot if it gets shaded by a tree, or jump back up if knocked over by the wind?

Many organisms, including humans and plants, use light and gravity to provide them with orientation and guidance in their environments.

As we’ve already seen while collecting information and in the earlier activities, plants can change their direction of growth (tropism). Plant roots grow away from the light (called “negative phototropism”) but toward gravity (called “positive gravitropism”), whereas plant shoots grow toward light (called “positive phototropism”) and away from gravity (called “negative gravitropism”).

Q: Why do you think it is important for these different parts of the plant to respond in these different ways?
Think about the function of the roots and shoot and the resources they need to carry these functions out.

Experiment 2:

How does a plant respond to light?
Which is stronger – a plant’s response to gravity or light?

To help you answer these questions we’ve included two simple experiments you can do to test how a stem segment (called the “hypocotyl”) of a young seedling responds to light and gravity and to test which is the stronger response.

You will need the following materials:

• Ten 35mm black film canisters with lids
• An additional lid for each canister
• A single hole punch or cork borer
• Clear tape
• Paper toweling (pieces cut into 1.5cm squares)
• Double-sided foam sticky tape (pieces cut into 1.5cm squares)
• 3-4 day old radish seedlings
• A permanent marker pen

Procedure (what to do):

First you’ll need to grow some seedlings for the experiments.
If you’ve never done this before, it is very easy to do and seedlings can be grown in a greenhouse or in relatively warm and well-lit areas, such as near a window.

1. Place a layer of paper toweling at the bottom of a container (a seed tray or flowerpot) and place a layer of paper toweling at the bottom.
2. Add about 2 inches of potting soil and moisten the soil.
3. Sprinkle the contents of a packet of radish seeds on the top of the soil.
4. Add a thin layer of soil on the top.
5. Water again lightly to make sure that you don’t uncover the seeds.
6. Place Saran wrap on top of the container until the seedlings emerge (this may take a day or two).
7. Keep a pan of water under the container and water if needed from the top.

Preparing the Chambers

1. To make a “window’ in your chamber, punch a hole in the side of the film canister about 1/3 of the way down from the top.
2. Stick a square piece of double-sided tape on the top of one of the spare lids.

3. Peel the top off the tape, and sit the canister on this lid, to act as a pedestal to stop the canister from rolling about.

4. Set up half of your canisters so that the window is on the side and half with the window facing downward, placed over the double-sided tape.

Setting up the Experiment

1. Place a piece of double-sided tape (a 1.5cm square) onto the inside of another film canister lid and peel off the backing, to expose the second sticky side.

2. Wet a piece of paper toweling (again, a 1.5 cm square) by dipping in water, and then squeezing to remove the excess water.

3. Place this square onto the double-sided tape (which is on the inside of the film canister lid).

4. Pinch or cut the stem of a 2-3 day old radish seedling, just above the soil. Try to pick a straight seedling whose 2 seed leaves (called “cotyledons”) have a span of no more than 2 cm across.

5. Stick the cotyledons of the seedling onto the piece of paper toweling in the lid

6. Carefully place the lid onto the canister so that the stem extends straight out into the canister.

7. Set up half of your canisters so that the “window” is on the side and half with the “window” facing downward.

8. Close the canister and with the marker pen, mark a dot on the lid at the same side as the “window” and an arrow facing upward.

9. Leave the plants in this position for a few hours, then carefully open the chamber and look at the seedlings.
10. Rewet the paper towel if necessary, close the chamber and observe again after one day, two days, three days etc.
11. While you are waiting to observe the results, you can make predictions about what you expect to happen.

Canister Number	Window Position	Moved towards gravity(Place X)	Moved Towards light (Place X)
1	Side	X
2	Side
3	Side
4	Side
5	Side
6	Down
7	Down
8	Down
9	Down
10	Down

Making Predictions

Instead of just waiting to see what the plants will do, you can make predictions about the possible outcomes.

In the chambers with the “window” on the side

• Q: Do you think the stem will grow toward or away from the “window” (light source)
• or, will the stem move toward gravity?

In the chambers with the covered “window”
Q: Do you think the stem will grow toward or away from gravity, that is move down or up?
Or do you think the stem will remain straight out toward the other end of the canister?

Q: Do you think the response to gravity will be stronger than to light?

Observations:

You can design additional experiments on your own to test other hypotheses. For example, will the seedling still respond to light if it is a certain color? To test this hypothesis, different colors of cellophane can be placed over the window in the canister.

Note:
In phototropism, molecules in the plant (called “receptors”) perceive certain colors, or wavelengths, of light (primarily blue light). The receptor is activated which leads to a change in the direction of growth through a series of steps. These steps appear to involve the plant hormone auxin. Auxin moves from the lighted side of the stem to the darkened side, where it stimulates cell elongation. It may surprise you to know that the details of the steps between light perception and response are not yet fully understood.

Gravitropism

Now we want to see the effect of gravity on plant movement (gravitropism).

Experiment 3:

Question: Where is Gravity Perceived in the Root?

In this experiment we want to see the effect of gravity on movement of plant root.

You will need the following materials:

• Pea,
• Broadbean
• Soya bean or maize seeds

(These are all good to use for this activity because they have lots of reserves and so can be grown on wet paper towels).

• Paper Towels
• Water

Procedure (what to do):

1. Fold a piece of paper towel in half to get a crease down the middle of the towel.
2. Place the seeds down the middle of the towel, along the crease.
3. Fold the paper towel over and sit in a dish of water to keep the towel moist.
4. Put the paper towel in a tray.
5. Place the tray as near to vertical as is possible.
6. Grow the seeds until the root is approximately 3cm long.
7. Use some roots as your control (that is roots which are handled in the same way but which have not had their root caps taken off)
8. Flick or cut off the very tip of 3 or 4 roots (if you have used maize seeds the root cap is the pinkish region at the very tip of the root)
9. Turn the roots horizontally and leave for 3-4h.

• Q: Did all the roots bend?
• Q: Which ones did?
• Q: Which ones did not?
• Q: Is the root cap necessary for a plant to bend?
• Q: Is starch necessary for plants perception of gravity (“Graviresponse”)?

Question:

Is starch necessary for plants perception of gravity (“Graviresponse”)?

You will need the following materials:

• 6-7 day old Seedlings (radish or mint )
• Dark box or cupboard

Procedure (what to do):

1. First you need to grow some seedlings for the experiment. If you’ve never done this before, it’s very easy to do.
2. Put half of your total number of plants in the dark for 24hrs to deprive the plants of light. The plant will use up its starch reserves.
3. Set the light deprived plant (starchless) plants and normal plants in the dark and turn them horizontally onto their sides.
4. It is important to remember to place the plants in the dark for two reasons. It will stop the re-synthesis of starch in the light deprived plant and will also prevent any possible movement in response to light.
5. Leave the plants in this position for 2 hours.
   • Q: When do the normal plants start to bend?
   • Q: When do the starchless plants start to bend?

6. Leave the plants in the dark overnight and take another look at them in the morning.

• • Q: Have all the plants bent?
  • Q: What effect do you think starch has on gravitropic bending?

Question: Where does the bending occur?

You will need the following materials:

• 2 potted plants with long stems (such as mint)
• Dark box or cupboard Marker Pen

Procedure (what to do):

1. Mark the stem of your plant at regular intervals (such as one mark every centimeter).
2. Place it on its side in the dark box or cupboard and wait 2-4h.
   • Q: Where did the plant bend? At the tip of the stem, or further back?

Now let the plant grow vertically overnight, and repeat this experiment.

• • Q: Where did the plant bend a second time?

If plants bend at the tip of the stem, it’s probably due to cell division. Further back, it is due to cells stretching out.

Materials and Equipment:

List of material can be extracted from the experiment section.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

No calculation is required.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

References:

List of References
http://archive.showmenews.com/2001/Feb/20010216News029.asp

http://www2.kenyon.edu/Depts/Biology/edwards/project/steffan/b45sv.htm

http://www.plant-hormones.info/auxins.htm

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPLANTHORM.html