All plants need a number of things in order to survive; nutrients, sunlight, water and space are just a few of these essentials. Nutrients and water come to a plant from many outside sources, but in order to prevent overcrowding certain plants have developed a defense mechanism that helps fend off weeds and other plants, resulting in a more healthy life (Fimple, 2006), the area of research that focuses on these effects is called Allelopathy.
Allelopathy focuses on chemical effects of plants upon each other (Biology 13 Laboratory Manual, 2009). Scientists have been studying the negative effects that one plant can have upon another for thousands of years. In 200 B.C. Theophrastus, a successor of Aristotle and Plato, documented the effects of pigweed on alfalfa, and in the first century A.D. Yang and Tang documented and described 267 plants that had "pesticidal abilities" (Wikipedia, 2009). But, the first scientist to coin the term Allelopathy was the Austrian Professor Hans Molish in the book Der Einfluss einer Pflanze auf die andere - Allelopathie published in Gernam. The term comes from the Greek words allelo, meaning mutual harm, and pathy, meaning suffering (Wikipedia, 2009).
Allelopathy can be displayed in three different ways, volatilization, leaching and exudation (Ferguson, 2009). This conducted experiment focused on volatilization of a plant that has an extensive reputation for many things, including its negative effects on things like vampires and its positive effects on things like the plague (Mendham, 2008). The plant in question is Allium sativum, or Garlic.
Anyone who has used garlic while cooking or eaten at an Italian restaurant knows that Garlic has a particularly aromatic smell. Some people love it, others hate it, but none can deny the particular scent that it produces when a clove of garlic is crushed. The strong smell is produced by a volatile, which is a chemical that evaporates into the air (Biology 13 Laboratory Manual, 2009). In order to determine what effect this evolutionary trait has on other plants we tested the effects of a crushed garlic clove on the germination of lettuce seeds.
Because we were so curious about the effects of garlic, we set up a second experiment to compare garlic's affects on lettuce to that of another plant, Brassica oleracea (Broccoli). This cruciferous vegetable's allelopathic abilities have not been so extensively studied, but there are a few experiments that have concluded that Broccoli affects the growth of other cruciferous crops (Ferguson, 2009). In my personal experience, Broccoli's aromatic affects are best noted when broccoli is cooked, so I doubt that a raw stalk will have any effect on the lettuce seeds.
Because of Garlic's reputation, the documentation on its allelopathic affects and our personal experience with Garlic, we hypothesize that Garlic will greatly affect the germination of lettuce seeds, resulting in little to no germination during the course of the experiment. Because Broccoli has little smell when raw, we hypothesize that Broccoli will not affect the germination of the lettuce seeds and should result in the same amount of germination as the control seeds.
- Garlic clove (Allium sativum)
- Broccoli stem (Brassica oleracea)
- Lettuce seeds (Lactuca sativa)
- 9 cm Petri Dish
- Whatman filter paper
- Distilled Water
- Masking Tape
- Garlic Press
- Set up each of the 4 Petri dishes. Each dish included a piece of filter paper lining the bottom.
- Crush up garlic clove with the garlic press and set in a small piece of aluminum foil. The Aluminum foil allows the garlic's potential smell (volatile) to project into the air, but does not allow the lettuce seeds to come in contact with the garlic.
- Place the aluminum foil with the garlic into the middle of a petri dish, label the petri dish "A."
- Cut a small piece of broccoli off the stalk and place the sample in a small piece of aluminum foil. The foil allows the broccoli's potential smell to project into the air, but does not allow the lettuce seeds to come in contact with the broccoli.
- Place the aluminum foil with the broccoli into the middle of a petri dish, label the petri dish "B."
- Count out 80 lettuce seeds and divide them into sections of 20 seeds.
- Place 20 lettuce seeds around the aluminum foil in Petri dishes marked "A" and "B."
- Place 20 lettuce seeds in a third petri dish and label the dish "C." This is the first control dish.
- Place 20 lettuce seeds in a fourth petri dish and label the dish "D." This is the second control dish.
- Add approximately 3 ml of water to petri dishes A, B, C, and D.
- Close the petri dish and seal it with masking tape.
- Place each petri dish near a window, allowing light to come in contact with the seeds.
In order to conduct the procedure, the petri dishes will be closely monitored for a scope of two weeks. Observations will include the stem length of the germinated lettuce seeds (measured in mm), the color of the seeds, and the aroma of the garlic and broccoli by asking the questions; "Over time does the volatile decrease?" "Does the rate of germination decrease with the intensity of the volatile?"
The experiment on the affects of garlic and broccoli on the germination of lettuce seeds comes with a set of nuisance variables (variables that must be controlled or they will affect the outcome). These nuisance variables will be controlled to the best of our ability. The first nuisance variable we encountered is temperature, as it can affect the germination of seeds as well as the rate of release of the volatile by broccoli and garlic (the hotter the temperature the more the plant releases the volatile). We will control this by setting all petri dishes (A-D) in the same room in the same area. Temperature in the laboratory room will be monitored and controlled by the school to maintain a specific temperature and should have little deviation.
The second nuisance variable we identified is light. Lack of light can result in death of plants, as they convert sunlight to energy in a process called photosynthesis. Light is as difficult to control as temperature, but will be controlled by ensuring that all petri dishes are in the exact same environment. The petri dishes are placed near a window so they will all receive the same amount of natural light.
- The graph above reflects the number of seeds that germinated in the control group vs. the number of seeds that germinated in the presence of garlic.
- The above graph shows the length of germination in the control group vs. the length of germination in the seeds in the presence of garlic.
- The above graph shows the number of seeds germinated in the control group vs. the number of seeds that germinated in the presence of broccoli.
- The above graph shows the length of germination in the control group vs. the length of germination in the presence of broccoli.
- The above graph shows the number of germinated seeds in the presence of garlic vs. the number of germinated seeds in the presence of broccoli.
- The above graph shows the length of the germinated seeds in the presence of garlic vs. the length of the germinated seeds in the presence of broccoli.
- We also observed the color of germination of seeds, and that variable varied proportionately with the length of the germination. Meaning, the more the seeds germinated, the greener they were. All lettuce seeds started out brown.
- Lastly, we noted the aroma of the garlic and the broccoli. Broccoli became slight more aromatic as it began to rot, but garlic remained strong throughout the entire process.
Discussion and Conclusions
As you can see from the data, all of the seeds in the garlic control group germinated immediately, and the number of geminated seeds in the presence of garlic is very few, at the end of the two weeks only 4 had germinated. Put that together with the fact that of those four seeds that germinated they resulted in less than 1mm of length. We conclude that our hypothesis was correct, garlic has an effect on the germination of lettuce seeds, resulting in either a very slow germination or none at all. Garlic's volatile remained strong throughout the experiment, partially due to the fact that the volatile was trapped in the petri dish, ensuring that it was present the entire time.
The broccoli was a different story, all of the broccoli seeds germinated, and the length of the germination was consistent with the control group in the first week and was slightly ahead of the control group in the second week. These results support our hypothesis that broccoli has very little affect on the germination of lettuce seeds. Broccoli's volatile got slightly more aromatic as the experiment went on, this could possibly lead to the increased growth of the lettuce seeds toward the very end.
The last two graphs compare the results of broccoli vs. garlic, supporting our hypothesis once more, that lettuce seeds in the presence of garlic are negatively affected and result in little to no germination and lettuce seeds in the presence of broccoli are not affected.
The experiment was a success; the only future experiments I would perform would be regulating the seeds in the control group a bit closer to eliminate some of the nuisance variables, and conducting the experiment for longer, to see if the garlic completely prohibited the germination of the lettuce seeds and eventually ended up killing them or to see if the germination of the seeds in the presence of garlic continued to speed up as time elapsed.