Allelopathy is defined as any direct or indirect, harmful or beneficial effect of one plant on another through production of chemical compounds that escape into the environment1.
Professor Eiji Tsuzuki is a leading agronomist in Japan. Allelopathy research had been his major activity, besides rice improvement (analysis of characteristics of aromatic rice, breeding aromatic rice cultivars, and application of pyroligneous acid with charcoal mixture to stimulate rice growth). He exploited allelopathic properties of plants for biological weed control to reduce dependence on present herbicides and pesticides in agriculture. He has (a) elucidated the allelopathic characteristics of many crop plants (including buckwheat, lucerne, taro [Colocasia esculenta] and sorghum) by identifying allelochemicals, (b) studied the effects of allelopathic plants on other plants, weeds and plant pathogens, and (c) response of allelopathic plants to soil-incorporated allelopathic materials, and (d) found appropriate methods, time of application, and rate of allelopathic materials for greater benefits to rice farmers2.
Allelopathy (from the Greek, allelon, meaning another and pathos, which means to suffer) is a way some plants deal with competition. Allelopathic plants release chemical substances which literally make other plants suffer. Some of these allelopathic plants store toxins in their leaves. When the leaves fall to the ground, the toxins are released. These toxins leach through the soil and are taken up by other nearby plants. Alternately, allelopathic plants can release chemicals through their roots. These toxins travel through the soil where they can be absorbed by the roots of other plants. Some allelopathic plants use gas warfare by releasing allelochemicals through small pores in their leaves and gassing nearby species3.
Crushed garlic, Allium sativum, produces a bioactive volatile called allicin. Volatiles are compounds that vaporize or evaporate quickly. Intact garlic bulbs or cloves contain a colorless, odorless precursor, allin, which comes in contact with the enzyme, allinase, in injured cells and allicin is released. The color and flavor of garlic develop only after the cells are damaged and release the chemical allicin. Allicin inhibits both the growth of microorganisms and seed germination. Allicin protects the damaged garlic from microorganisms and competition from other monocots for nutrients4.
To observe, record, and analyze the effect that the following volatiles have on celery seed germination and growth:
- effect of garlic volatiles
- effect of broccoli volatiles
Garlic volatiles will inhibit the germination and growth of celery seeds, but broccoli volatiles will not inhibit their germination and growth.
Planning the experiment:
We will conduct 2 separate experiments, but will run them simultaneously. In one experiment, we will compare the effects of garlic volatiles on germination and growth of celery seeds - our experimental group will consist of 20 celery seeds exposed to crushed garlic, our control group will consist of 20 celery seeds not exposed to garlic. In the second experiment, we will compare the effects of broccoli volatiles on germination and growth of celery seeds - in this case, our experimental group will consist of 20 celery seeds exposed to crushed broccoli, our control group will consist of 20 celery seeds not exposed to broccoli.
Based on the literature review, we have made an educated guess that garlic volatiles will inhibit the growth of celery, but we are unsure to what extent their growth will be inhibited. This is what we will find out in our experiment. We guessed that broccoli volatiles will have no effect on the growth of celery. Our experiment will help us to figure out if this is true or not.
MATERIALS AND METHODS
- 4 x 9cm Petri dishes with lids
- Marker to label each Petri dish, so we can differentiate between them
- 4 filter papers to line the base of the Petri dishes
- aluminum foil (approx one A4 size piece, cut into 4 equal pieces)
- Distilled water (2-3ml per dish, equal quantity for each dish)
- Pipette (to measure water and ensure we add equal quantities of water to every Petri dish - at the start and whenever we want to add water)
- Masking tape (to seal each Petri-dish)
- 80 seeds of celery i.e. Apiurn graveolens (20 seeds for each of the 4 Petri dishes)
- 1-2 cloves fresh garlic (and something to crush it with)
- small floret of fresh broccoli (and something to crush it with)
Method (Note: Steps 4 and 9 may be done before step 1):
- Label the lids of the 4 Petri Dishes as: PG1, PG2, PB1, and PB2. These will be used as follows: 'Petri dish with Garlic', 'Petri dish without Garlic', 'Petri dish with Broccoli', 'Petri dish without Broccoli'. Open all 4 dishes, keeping lids with corresponding dish.
- PG1 = Petri dish with crushed garlic (experimental group 1)
- PG2 = Petri dish without crushed garlic (control group 1)
- PB1 = Petri dish with crushed broccoli (experimental group 2)
- PB2 = Petri dish without crushed broccoli (control group 2)
- Line base of all 4 dishes with filter paper.
- Measuring with pipette, put 2 ml of distilled water into each Petri dish. If this doesn't make it wet enough, add a little more, but add the same quantity to every dish. There should be enough water to wet the base without water moving around, as that will cause the seeds to move around.
- Create 4 circular mini-containers out of aluminum foil to be placed in each of the 4 Petri dishes in the center. The outer edge of each container should curve upward by approx 2mm, as it needs to be able to hold garlic crush and broccoli crush. The diameter of each container should be approx 2.5 cm.
- In one mini-container, place crushed garlic (enough to fill the container, equivalent of 1 tsp crushed garlic). Place container carefully in the center of dish PG1.
- Place second mini-container in the center of dish PG2 (this has no garlic).
- In the third mini-container, place crushed broccoli (similar to how garlic was done). Place container carefully in the center of dish PB1.
- Place fourth mini-container in the center of dish PB2.
- Separate the 80 celery seeds into 4 batches of 20 seeds each.
- For each of the dishes (PG1, PG2, PB1, and PB2), carefully add the celery seeds so that they are evenly distributed around the dish. If needed use the back end of a pen or pencil to move the seeds around, spacing them evenly. (Note: be careful not to puncture the filter paper, or you will need to redo that dish.)
- Close dishes with corresponding lids and seal securely with masking tape, going around the open edge, so water will not escape.
- Place all four dishes near a window, not too close to any heat source such as a radiator, otherwise water will dry up. All the 4 Petri dishes must be subjected to the same nuisance variables such as amount of light, and temperature of the room.
- Each day, record the number of seeds that have germinated and grown for PG1, PG2, PB1, and PB2. Make observations and write them down in the Data Sheets.
Process for stabilizing nuisance variables and monitoring the experiment during the observation period:
After we sealed the 4 Petri dishes, we observed them over approx. 2 weeks. We added water twice when the water had dried up. All 4 Petri dishes were always handled in the same manner during the observation period. We opened them at the same time, added same amount of water to each, placed them always next to each other. By doing this, we stabilized the nuisance variables such as light and temperature. If temperature went down on a cold day, all 4 Petri dishes were subjected to it. For the purpose of counting seed germination, we opened each Petri dish on the days we made our observations. We did not make observations on holidays and weekends, but we added water before a 2 day or 3 day break, to ensure the seeds would not die during that time.
Interpretation of data based on the charts:
- Looking at Chart_Garlic: We see that germination and growth of celery seeds is inhibited by presence of garlic volatiles.
- Looking at Chart_Broccoli: It appears that broccoli volatiles may help germination of celery seeds over time. But since the pattern is not consistent, the experiment will need to be repeated.
- Looking at Chart_GarlicNBroccoli: We see that Garlic volatiles inhibit the growth of celery seeds. However, when we compare germination of celery seeds with broccoli to germination of celery seeds without garlic, it appears that the numbers are higher when broccoli is present. This leads us to suspect that broccoli may promote growth of celery seeds.
- In general, the counts for PG2 (celery without garlic) and PB2 (celery without broccoli) should have been similar, but it did not happen as expected. For this reason also, we need to repeat the experiment.
RESULTS - Summary:
Garlic inhibits germination and growth of celery seeds, while broccoli does not. We are unclear if broccoli actually promotes the growth of celery seeds.
It is very obvious that garlic volatiles adversely affected ability of celery seeds to grow. Even those that sprouted seemed to rot quickly.
It is possible that the seeds in Petri dish PG1 (with garlic) rotted toward the end due to the fact that the seeds were enclosed and the volatile was too strong. In real life, seeds will not be enclosed, so maybe they will not rot but they probably will not germinate or flourish.
As far as broccoli, the results are not so clear. Even though the seeds on the dish with broccoli grew slower than the one without broccoli, they did eventually grow and some grew better in broccoli than without broccoli. This part of the experiment would need to be repeated to come to a decision on whether broccoli inhibited the growth of celery seeds or not. It is even possible that broccoli may help celery to thrive. Results were also subject to human error, we got better at counting carefully after a couple days. The data observed on day 2 are not necessarily accurate.
Garlic volatiles definitely inhibit the growth of celery. Broccoli may or may not inhibit their growth.
Future experiments to consider, questions to answer, observations to make in nature:
- View garlic growing in nature. If not crushed, does it inhibit other plants from growing?
- View other volatiles, find some that inhibit and some that promote the growth of plants around them.
- Repeat the experiment with broccoli, to make a better conclusion on whether it inhibited celery seeds or not.
- Repeat the experiment to see how garlic affects lettuce, how carrots affect celery and lettuce etc.