Don’t Come Closer!
How Plants Protect their own Space

Have you ever felt like you needed your own space? Sometimes we all need to be alone. Plants and trees need to have their own space, too. How do you get your own space? You can walk away or shut your door, but what can plants do? They can't walk or run anywhere. Plants have a different way of getting their own space. They use allelopathy.

Allelopathy is a chemical process that a plant uses to keep other plants from growing too close to it. Some pine trees are allelopathic. When their needles fall onto the ground and they keep unwanted plants from growing near the pine tree.
Some other plants that use allelopathy are black walnut trees, sunflowers, wormwoods, sagebrushes, and trees of heaven.

How Does Allelopathy Work?
Allelopathy is a chemical process that a plant uses to keep other plants out of its space. There are several types of chemical alleopathy

• Plants release chemicals that affect other plants growth from their roots into the ground. The plants trying to grow near the allelopathic plant absorb those chemicals from the soil and are unable to live.
• Some plants release chemicals from their roots that will slow down or stop any plants nearby from the process of photosynthesis. The chemicals actually change the amount of chlorophyll a plant produces. This affects the amount of food the plant can make. If a plant can’t make food it will die.
• Sometimes plants release chemicals in the form of gasses. These gasses are released through the small pores in the plant’s leaves. Other plants absorb the gas and are stunted or die.
  

Nature’s Impact: Allelopathy means "Power"
Can you picture what the ground under pine trees looks like? Remember that each year the pine tree drops some of its needles to the ground around the tree. These pine needles cover the ground under the tree which keeps other plants from growing there. The pine needles contain acid, which goes into the soil when the needles start to decompose. This acid doesn't hurt the pine tree, but discourages or kills other plants growing near it.

Scientists study allelopathy and use their research to find natural, healthier herbicides and pesticides to stunt or kill specific unwanted plants and insects, but not kill the plants we want.

Experiment 1: Look for Signs of Allelopathy in Your Neighborhood
For this experiment you will need:

• a marker
• plastic bags
• a notebook

Choose several of your favorite trees and collect about 10 leaves from each of them. Look around the trees and try to determine if they are allelopathic. (At the base of the tree is there little or no vegetation?)

Then place the leaves in a plastic bag and label the bag with any known fact about the tree, such as its location, whether it shows signs of allelopathy or other distinguishing features. Try to identify the plant but if you are having trouble go to your local library and ask the librarian for a field guide about trees. Or visit a local plant nursery and ask their plant experts. If you do not use the leaves immediately, place the bag in the refrigerator for up to 2 days and identify and analyze them later.

If you suspect that the trees show signs of allelopathy you can test out your theory in Experiment 2.

Experiment 2: Testing whether this Tree is Allelopathic?
For this experiment you will need:

• glass jars
• a spoon
• radish seeds
• some paper towels (plain with no ink design)

Collect two or three leaves from a few different trees (you could use the same leaves collected in Experiment 1) and tear them into small pieces. Put the pieces from each tree into separate glass jars with some water. Each of the mixtures should have more plant matter than water. When you make up the jars, make sure you repeat the same procedures for all of them.
Label the jars with the name of the tree from which the tree leaves came and any other information you think is important. Swirl the jars several times a day for two to three days with your spoon.

Now you are ready to start the main part of the experiment.

• Place 5 radish seeds inside a piece of folded paper towel with a mixture of leaf bits and water from each of your glass jars.
• Enclose each of the paper towels inside separate plastic bags and place them in a dark room.
• Wait seven to ten days and analyze the growth of the plants.
  • Which of the radish plants grew?
  • Which did not?
  • Does the growth of the radish help you see which of the leaves had allelopathic chemicals in them?
  • What traits did those seeds demonstrate?

As a side note, one paper towel should be dampened with plain water to serve as the control. You can use the control to compare with your experimental seeds. Using one paper towel with plain water is also important in case something goes wrong with the experiment. One possible problem might occur if the chemicals in your water are killing the seed instead of the leaf chemicals killing the seed.

Analyze your Results:

• Look very closely at the seeds. Check all the seedlings' parts for damage that may have been caused by toxins. In what ways were the seedlings affected when compared to the control?
• Make a table showing the name of each tree, the number of radish seeds that sprouted, and any interesting observations you noted.
• Graph your results.

Make a Conclusion:

• Did the leaves you suspected being allelopathic have any influence on the radish seeds? How would you describe the influence?
• What ways could you further study this phenomenon?

Experiment ideas courtesy of Dragonfly Pages.

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