A Little Bit More
The name for this inhibiting chemical effect on surrounding plants is called allelopathy (al-eel-OP-ah-thee). Allelopathy is any direct or indirect effect of a plant, either harmful or beneficial, on other plants through the release or escape of chemicals into the environment. Chemical substances released from the roots of allelopathic plants are absorbed by surrounding seeds or plants and their germination or growth may be prevented or greatly depressed; in some cases, these chemicals can even prevent photosynthesis.
As the unit poster points out, this is a chemical process that a plant uses to reduce seed germination and/or seedling growth of other plants “in their space” although some researchers have broadened this definition to include interactions between plants, animals, and other organisms as well. Even the inhibiting effect of lots of fallen pine needles beneath a tree, is considered to be an example of allelopathy.
The Black Walnut is probably our best known allelopathic plant but sunflowers, Tree of Heaven, tobacco, peas, rice and many other plants have allelotoxins too. Many plants have been suspected of having allelopathic properties but not enough studies have been conducted to determine which ones they are. It’s interesting too that some plants are especially immune to these toxins and are even able to thrive in places where others cannot – now there’s a great adaptation! As the unit poster points out, although tomatoes and peppers are adversely affected by walnut toxins, several other common vegetables seem unaffected so maybe that Black Walnut tree can remain next to your garden after all?
Activities
Science – Sunflowers
Objectives: See whether sunflowers really do effect other plants
Materials: Sunflower seeds, small pots, soil and a variety of other seeds
We’ve learned that Black Walnut is an allelopathic plant but we also learned that sunflowers are allelopathic too. Sunflower seeds are easy enough to obtain so let’s use these to set up an experiment to see what might happen if we plant sunflowers along with some other kinds of seeds.
1. Obtain an assortment of small seeds (probably easiest to get those in packets from the store) along with sunflower seeds (there are several varieties but any kind will do)
2. I’d recommend at least 4-6 pots (insulated foam cups work well) be set up although you may want to do a few more. Fill each pot with soil and be sure there is a small hole in the bottom for drainage.
3. Place a marker on the rim of each pot that will represent the top of the pot and we’ll call this “12 o’clock”
4. Plant two sunflower seeds in the middle of half of your pots (not too deep, but should be covered with soil) Be sure to mark these so you know there are sunflower seeds in each of these.
5. Now plant just one kind of your seeds in the 1 0’clock position in each of all of your cups (both those WITH sunflowers and those WITHOUT), a second kind of seed in the 3 o’clock position in each of your cups and then a third and fourth on around in the 6 and 9 o’clock positions. The “clock” positions for the seeds will let you know within each cup, what kind of seed does or does not germinate and grow.
6. Once all of your seeds are planted, you should have half of your cups with a seed from all of the different kinds of seeds on hand PLUS sunflowers and half of your cups with ONLY the additional seeds, no sunflowers
7. Once all the seeds are in place, line up the pots and water so the soil is moist. Over the next few days, keep the pots moist – not wet!- and warm.
8. Once the seeds begin to germinate (different species will probably germinate at different times) keep an eye on which kinds are growing and which kinds are not. You’ll have to use some judgment here. You will want to look for any differences between those cups with sunflowers and those cups without sunflowers to see what effect the sunflowers are having on the other seeds.
The cups that had NO sunflowers were our “control group” while cups WITH sunflowers were our experimental group. The only differences (our only variable) that should be made is whether we had sunflowers in the same pot with the other seeds or not. We were looking to see if the sunflowers had any effect on the growth of the seeds planted in the same cup. Remember that some kinds of plants may be affected while other may not. Good observation should help you decide whether the sunflowers had an adverse effect on your plants.
Key Concepts
Structures and Functions, Scientific Method/Approach, Growth and Development, Behavior and Regulation, Adaptations and Diversity
Questions
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