Allelopathy: Effects of volatiles from crushed garlic and crushed broccoli plant materials on seed germination and growth.
by Milton Rosa-Ortiz
Kingsborough Community College
Evolution, the change in the genetic material of a population of organisms through successive generations (Wikipedia.com), is a major concept in the study of biology. As part of evolution, it is not uncommon for organisms to compete amongst their own species, as well as with other species. They compete for space, light and nutrients in order to survive and reproduce. One way plants compete is by producing chemicals that protect them from predators, or from other plants (Beeber, et al; General Biology Lab Manual,).
The area of research that focuses on chemical effects of plants upon each other is called alleleopathy. It is defined as the release into the environment by an organism of a chemical substance that acts as a germination or growth inhibitor to another organism (Rice,1984).
As anyone that has spent time in an Italian kitchen can tell you, garlic is a plant with a very strong scent and flavor. When garlic is crushed substances known as volatiles, are expelled (Wikipedia, 2010). Field observations seem to indicate that these substances have an effect on the germination and growth of plants in it's close proximity. The intent of this experiment was to asses whether any of these garlic volatiles indeed inhibit the germination and growth of other seeds planted nearby.
Commonly known as garlic, Allium sativum, is a species in the onion family Allicae. Garlic has been used throughout recorded history for both culinary and medicinal purposes. It has a characteristic pungent, spicy flavor that mellows and sweetens considerably with cooking. A bulb of garlic, the most commonly used part of the plant, is divided into numerous fleshy sections called cloves. The cloves are used as seed, for consumption (raw or cooked), and for medicinal purposes. The leaves, stems (scape), and flowers (bulbils) on the head (spathe) are also edible and are most often consumed while immature and still tender. The papery, protective layers of "skin" over various parts of the plant and the roots attached to the bulb are the only parts not considered palatable (McGraw-Hill Concise Encyclopedia of Science and Technology, 2005).
When crushed, garlic yields allicin, a powerful antibiotic and antifungal compound (phytoncide). It also contains the sulfur containing compounds alliin, ajoene, diallylsulfide, dithiin, S-allylcysteine, and enzymes, vitamin B, proteins, minerals, saponins, flavonoids, and maillard reaction products, which are non-sulfur containing compounds. The composition of the bulbs is approximately 84.09% water, 13.38% organic matter, and 1.53% inorganic matter, while the leaves are 87.14% water, 11.27% organic matter, and 1.59% inorganic matter. The phytochemicals responsible for the sharp flavor of garlic are produced when the plant's cells are damaged. When a cell is broken by chopping, chewing, or crushing, enzymes stored in cell vacuoles trigger the breakdown of several sulfur-containing compounds stored in the cell fluids. The resultant compounds are responsible for the sharp or hot taste and strong smell of garlic (Allaby, Michael. A Dictionary of Plant Species. Oxford University Press, 2006).
For comparison, we also sat up a contiguous experiment to assess whether crushed broccoli has a similar effect on other seeds, planted nearby.
Broccoli can be described as a cool season biennial crucifer (Brassica oleracea var. italica) of Mediterranean origin, belonging to the plant order Papaverales. Broccoli is grown for it's thick branching lower stalks which terminate in clusters of loose green flower buds. Stalks and buds are cooked as a vegetable or may be processed in either canned or frozen form.
Volatiles from crushed garlic (Allium sativum) inhibit the germination of celery seeds, as well as their sprouting and subsequent growth. Volatiles from crushed crucifers such as broccoli (Brassica oleracea var. italica), on the other hand, do not inhibit the germination of celery nor do they have an effect on their sprouting and subsequent growth.
Materials and Methods
- 4 Petri dishes
- 8 pieces of Whatman filter paper.
- 1 tbsp of crushed garlic
- 1 tbsp of crushed broccoli
- 80 celery seeds
- Aluminum foil
- Masking tape
In this experiment we exposed 20 seeds to garlic (Allium sativum) volatiles, 20 seeds to broccoli (Brassica oleracea) volatiles and, as a control, 20 more seeds planted without exposure to any volatiles. Subsequently, we observed, if any, developmental differences, over the course of two weeks ( 8 days of observations). Specifically, we paid careful attention to the number of seeds that germinated and, how much they grew.
Four 9 cm Petri dishes were lined with two pieces of Whatman filter paper.
Garlic was crushed and placed in an aluminum boat in the middle of one of the Petri dishes. This dish was labeled BG1 (as in Broccoli-Garlic 1). Twenty celery seeds were placed on the filter paper, surrounding the aluminum boat. Care was taken so that the seeds would not come into contact with the contents of the boat; to ensure that they were exposed only to the volatiles from the crushed plant material. Then, 25 ml of water were added to the bottom of the filter paper, and the dish was sealed with masking tape.
A control dish was set up without any crushed garlic. This dish was labeled BG2.
The same procedure was followed with crushed broccoli. The sample was labeled BG3.
Another control dish was set up without any crushed broccoli. This dish was labeled BG4.
For this experiment, the dependent variable was the germination and growth of celery seeds. The experimental variable was the presence of garlic/broccoli volatiles. The treatment was celery with garlic and celery without garlic; as well as, celery with broccoli and celery without broccoli. The size of the study was 20 seeds in each Petri dish. The study was replicated 20 times. The nuisance variables for this experiment were:
- Humidity; which we addressed by sealing the Petri dish with masking tape.
- Light; which was addressed by exposing all four dishes to the same light.
- Temperature; which was addressed by keeping the dishes in the same location through the length of the experiment.
Data and Results
(See Table: SPROUTING AND SUBSEQUENT GROWTH OF CELERY SEEDS EXPOSED TO GARLIC OR BROCCOLI, included)
Within the first four days of the experiment we observed that the seeds exposed to the garlic volatiles (BG1) exhibited minimal (5 seeds) germination and very minimal growth. A week after the experiment had begun, all sprouted seedlings exposed to garlic had died and rotted. None of the other seeds in this Petri dish sprouted over the course of the experiment.
The seeds that were not exposed to any garlic volatiles (BG2), on the other hand, exhibited significant sprouting (18 seeds) and the seedlings grew healthily. The seeds that were exposed to the broccoli volatiles (BG3) exhibited above average (13 seeds) sprouting and healthy growth. And, the seeds that were not exposed to any broccoli volatiles (BG4) followed right behind BG2 (with 17 seeds) in sprouting and healthy growth.
Over the course of the experiment all seeds in the Petri dish of samples BG2, BG3, and BG4 sprouted and grew healthily whereas BG1 rotted and died.
From the observations recorded during the experiment we would conclude that our hypothesis was correct. The volatiles released by crushed garlic indeed inhibited the germination of celery seeds, as well as their subsequent growth. It is uncertain at this time which is the volatile that inhibits the growth of other seeds in it's close proximity. Further experiments should be conducted to determine the nature of that volatile.
We would also conclude that broccoli may or may not have had an effect on the germination of celery seeds. Initially it appeared as if broccoli had slowed down the germination of some seeds. But it did not have any observable effect on the growth of the seedlings since eventually all the seeds sprouted and grew healthily. A more detailed and specific study should be performed to determine whether broccoli slows down significantly the germination of celery seeds, or any other seeds.