What images come to mind when we think about garlic? We usually experience the pungent odor, the sour bitter taste (when raw) and the sweet aromatic flavor when cooked in our favorite dishes. Block defines, Allium sativum, commonly known as garlic, as a species in the onion family Alliaceae" (Block, 2010). Besides garlic, other species in the Alliaceae group, include, shallots, chives, leeks, and onions. Over the years in history, garlic has been used for culinary, folkloric, and medicinal purposes. Folkloric tales include ones of vampires being kept away if a person had a clove of garlic hanging around their neck, thus believing the smell would drive vampires out. Medicinally garlic has been known to fight off infections when consumed, lower blood pressure, combat yeast infections, prevent cancer, prevent weight gain, and reduce inflammation in the body.
Characteristically, the scent of garlic is one that is distinct, pungent odor that is released when crushed, cut or consumed. When cut, if garlic is fully exposed, there will be a stronger odor. This smell is produced by a volatile, composed of, allicin and a sulfur, containing, "alliin, ajoene, diallysulfide" (Block, 2010) along with a variety of other vitamins, and enzymes. It is uncertain if the volatile will assist or hinder garlic. In most cases volatiles are helpful to garlic because it can be used as a method of defense. When studying biology, plants live in a symbiotic environment in which several different species of plants can live together in harmony. If there is competition for resources of food, nutrients, water, light or space, the end result is for plants to become defensive. It has also been studied, that plants will "emit volatile compounds when they are attacked by herbivorous insects, which may be used by parasitoids and predators to locate their host or prey" (Soler, 2007).
Allelopathy is "any direct or indirect harmful or beneficial effect of one plant...on another through production of chemical compounds that escape into the environment." (Lind, 2009). These allelochemicals can have a positive or negative effect on organisms. In this experiment, we will investigate the effects of allelopathy of garlic on the growth of plants. To further delve into this phenomenon, we will also look at other plants that produce volatiles such as broccoli and observe how their smells affect growth of plants, most, specifically celery. This experiment investigates the effects allelopathy has on plants in a symbiotic environment.
Garlic (Allium sativum) inhibits the growth and germination of celery (alpiurn graveolens) seeds. Broccoli (Brassica oleracea) does not inhibit the celery seed growth and germination.
Due to the knowledge of garlic's history and use, it is our intention to observe at the end of the experiment, if garlic will cease the growth of celery in favor of its own growth. The dependent variable is the growth and germination of celery seeds in the presence of garlic and broccoli. The experimental variables that are being investigated are the presence of broccoli volatiles and the presence of garlic volatiles. The following combination of treatments will be used in this experiment: celery seeds with garlic, celery seeds with broccoli, celery seeds without garlic, and celery seeds without broccoli.
The control group is needed in order to compare and contrast the germination of the celery seeds. It is necessary to numerically determine how many grew, the type of growth pattern, the duration of growth, and how many died. The controls are the two groups of celery seeds without the broccoli and garlic volatiles in the simulated bio dome (Petri dish).
There are several nuisance variables that need to be stabilized in this experiment in order to successfully accomplish the desired germination results in the bio dome. Nuisance variables such as humidity could be controlled by sealing the Petri dish with masking tape. Another nuisance variable, such as inefficient lighting sources could affect the direction and amount of growth a plant exhibits. One way to combat this problem is by keeping the bio dome in the same location, undisturbed. This will be slightly difficult as the lab area is shared with several classes and other students' lab projects as well. There is not a definite method in order to maintain the bio dome's placement is in the same location every time an observation is made. The third nuisance variable that would affect the outcome of the experiment is temperature. By maintaining a homeostatic environment inside and outside of the bio dome temperature is better controlled. One can do this by making sure the masking tape fully covers the Petri dish opening so nothing seeps out. Additionally, by keeping the Petri dish closer to the counter, but away from the window, in a warm, dry place no cold air will be exposed to the sample. There will be a sample size of 20 seeds per experiment in each Petri dish. Each experiment is replicated 20 times by having a large sample size number of seeds per bio dome.
- Garlic (Allium sativum)
- Celery seeds
- (Alpiurn graveolens)
- Broccoli stem (Brassica oleracea)
- Garlic press
- 9 cm Petri dish
- (4 dishes total)
- 2 pieces of Whatman #1 filter paper (per dish)
- Distilled water (3 ml per setup)
- Masking tape
- Triple beam balance
- Aluminum foil
Upon beginning the experiment, 4 Petri dishes were labeled to serve as our bio dome over the 2 week trial period. The control group was labeled PB2 and PG2, in which broccoli (PB2) and garlic (PG2) were not present in the dishes. The experimental group was labeled PB1 and PG1 in which broccoli (PB1) and garlic (PG1) were present in the Petri dishes. The plant materials of garlic and broccoli were crushed using a garlic hand press. Then 1.0 g samples were weighed on a triple beam balance and the material samples were transferred to a tinfoil holder. Each Petri dish contained two pieces of Whatman #1 filter paper and 20 celery seeds. The tinfoil holder that contained the crushed plant material (1 of garlic and 1 of broccoli) was placed on the filter paper and 3 ml of water was added to the Petri dish. Masking tape was placed around the opening of the Petri dish to seal and prevent water from evaporating. The 4 Petri dishes were set on the windowsill of the biology department laboratory classroom, for a 2 week observation. Each day, observations were made to examine the growth of sprouts, length of stems, amount of buds, and any decomposition.
See attached excel spreadsheets: figure 1.1 data sheet, figures 1.3 & 1.4 demonstrate celery seed growth, of PG1,PG2,PB1 & PB2, in which the x axis measures the duration of time in days and the y axis measures the number of sprout growth. Figure 1.2 describes the comparison of PB1& PB2. Figure 1.5 describes the comparison of PG1 & PG2.
The experiment yielded the following results: Day 1 yielded no growth on Petri dishes PG1, PG2, PB1 & PB2. Day 2: PB2 had 17 sprouted, 3 not sprouted. PG2 had 18 sprout and 2 not sprout. PB1 had 13 sprout and 7 not sprout. PG1 had 5 sprout and 15 not sprout. From the 3rd day to the 9th day, PB2 had 17 out of 20 seeds sprouted, and the celery sprout number remained at 17. From the 2nd day to the 3rd day PG2 exhibited a 90% sprout increase and PB1 exhibited a 68% increase of growth. However, from the 3rd day to the 4th day, PG2 exhibited a 65% decrease and PB1 exhibited an 84% decrease of growth. From the 4th day to the 9th day, PG2 sprout number count remained at 13. From the 2nd day to the 7th day, PG1 exhibited slow growth of sprouts and sprout numbers fluctuated from 5, to 8 to 6 sprouts. From the 8th day to the 9th day PG1 exhibited some decomposition while remaining at 6 out of the 20 seeds that sprouted. Day 4 and day 5, PB1 maintained 16 sprouts and 4 not sprouted, while from day 6 to day 9, there were 17 sprouted and 3 not sprouted.
The hypothesis was correct with respect that garlic does inhibit the germination of celery seeds. According to figure 1.5 between the 1st and 3rd days, PG2 showed a higher growth rate (0-20 sprouts) than PG1 which showed a lower growth rate (0-5 sprouts). During the final days of the experiment, (day 7, 8&9) PG1, celery seed count reached a plateau of 6 seeds that grew and 14 seeds that did not grow. It was evident that the volatiles from the garlic as a defensive mechanisms, fought for light, water, and nutrients against celery. The hypothesis was inconclusive that broccoli does not have an effect on celery seed germination.
Broccoli did however, have a minute effect on the growth of the plant. As we can see from figure 1.2 the broccoli inhibited the growth on the first and second days. However as time progressed, the celery continued to grow independently of the presence of broccoli. The highest sprouts of 19 counted on day 3 and from the 4th day to the 9th day, 16-17 sprouts counted. More time is needed to evaluate the different growth patterns of these plants. Some questions to ask ourselves are, do broccoli volatiles over time decrease in potency and therefore indifferent to plant growth? How long will plants survive in the presence of broccoli? That would be another experiment to ponder.
Further research is necessary to fully investigate the allelopathy effects volatiles have on plants. However, if the research was done again what would be different? Performing the experiment in a tightly controlled environment such as a greenhouse, in which, light, temperature, humidity would be observed using scientific instruments, would yield more accurate results. Caretaking of plants by one designated staff member, should be maintained always at the same time, and accurate observations should be made, using scientific measurement devices, and instruments instead of the naked eye. Placing seeds in soil as a nutrient source may yield larger plants and allow more room for growth, instead of relying on filter paper. Better system for watering plants would yield better results: as in our experiment, Petri dishes were watered on the 3rd day and 5th day therefore, the remainder days the bio dome became drier. Watering systems from a timed sprinkler set at a higher frequency, for example, every 2 days may also alleviate dryness and produce larger growth samples. Lastly, the testing should be extended for a month period, as 30 days (or more) would provide a complete evaluation of the experiment and superior outcome.