NCSU-NSCORT Outreach - Phototropism Experiments

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Subject Introduction
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 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.

Questions:
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:

35mm black film canisters with lids 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.

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.
Add about 2 inches of potting soil and moisten the soil.
Sprinkle the contents of a packet of radish seeds on the top of the soil.
Add a thin layer of soil on the top.
Water again lightly to make sure that you don't uncover the seeds.
Place Saran wrap on top of the container until the seedlings emerge (this may take a day or two).
Keep a pan of water under the container and water if needed from the top.

Preparing the Chambers

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.
Stick a square piece of double-sided tape on the top of one of the spare lids.
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.
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

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.
Wet a piece of paper toweling (again, a 1.5 cm square) by dipping in water, and then squeezing to remove the excess water.
Place this square onto the double-sided tape (which is on the inside of the film canister lid).
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.
Stick the cotyledons of the seedling onto the piece of paper toweling in the lid
Carefully place the lid onto the canister so that the stem extends straight out into the canister.
Set up half of your canisters so that the "window" is on the side and half with the "window" facing downward.
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.
Leave the plants in this position for a few hours, then carefully open the chamber and look at the seedlings.
Rewet the paper towel if necessary, close the chamber and observe again after one day, two days, three days etc.
While you are waiting to observe the results, you canmake predictions about what you expect to happen.

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.

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 per ception and response are not yet fully understood.

Acknowledgements:

The laboratory exercises are modified versions of "Bottle Biology and Wisconsin Fast Plants Projects" by Drs. Coe and Paul Williams, Department of Plant Pathology, University of Wisconsin, Madison.