Allelopathy: How Plants Influence Others
- Sylvia Rose
- 6 hours ago
- 4 min read
Allelopathy is the natural production and release of chemicals, or allelochemicals, by plants and other organisms. It's only one of many ways plants cooperate or compete to get the best resources.
This biological phenomenon is found in plants, microbes, insects and other animals. They produce biochemicals to enhance or suppress the growth, survival, development and reproduction of nearby organisms.
In plants, allelochemicals are natural herbicides, growth regulators and insect deterrents. They help shape plant communities and ecosystems.
Chemical Power
A diverse group of secondary metabolites, allelochemicals are found in plant tissue such as leaves, stems, roots, seeds, and flowers. They enter the environment by several pathways.
Volatilization: Some allelochemicals are volatile, evaporating from plant surfaces and being carried by the wind to neighboring plants.
Root Exudation: Plants can actively release allelochemicals directly from their roots into the surrounding soil. This is a targeted strategy for influencing the immediate root zone and competing for resources.
Leaching: Rainwater can wash allelochemicals from plant tissues and deposit them in soil. It's common in leaf litter decomposition, where chemicals released affect germination and growth of seedlings.
Decomposition: As plant material decomposes, allelochemicals are released into the soil, potentially affecting subsequent generations of plants or other organisms.
Beneficial Allelopathy: Good Neighbors
Nutrient Cycling: Some allelochemicals, like phenolics, terpenoids, and alkaloids, aid in the decomposition of organic matter. This releases essential nutrients back into the soil to benefit nearby plants.
Phenolic compounds can either stimulate or inhibit breakdown of organic matter in soil. Phenolics slow or accelerate decomposition by promoting microbial activity and production of enzymes such as cellulase.
Pest and Disease Control: Certain allelochemicals have antimicrobial or insecticidal properties, providing natural protection against pests and diseases. These include alkaloids, especially nitrogen-containing compounds like those in opium poppies.
Other examples include terpenoids, such as monoterpenoids and sesquiterpenoids, which are volatile and can deter pests, as well as phenolics like tannins.
Glucosinolates are sulfur-containing compounds and can convert to isothiocyanates to become repellents. They're often found in cruciferous vegetables of the Brassicaceae family, like broccoli and cauliflower. Isothiocyanates may have antioxidant benefits for humans.
Detrimental Allelopathy: Chemical Warfare
Thwarting the Competition: By inhibiting the germination, growth, or nutrient uptake of neighboring plants, allelochemicals allow the producing plant to gain a competitive advantage. This is particularly important in crowded environments where resources are scarce.
Phenolic compounds, like coumarins and flavonoids, can inhibit seed germination and alter nutrient uptake. Terpenoids impair cell division and elongation.
Weed Control: Many agricultural weeds are susceptible to allelopathic compounds released by crop plants or cover crops. Sugar maple roots release allelochemicals to hinder nearby germination of other plants.
Security Against Invasion: Some allelopathic plants are highly effective at preventing the establishment of invasive species, maintaining the integrity of native ecosystems. Goldenrod and bee balm are two examples.
The common mint plant emits volatile organic compounds (VOCs) which help deter pests while promoting growth in companion plants. Once established, mint proves hardy and prolific.
More Examples of Allelopathy
Black Walnut (Juglans nigra): This tree produces juglone, a powerful allelochemical. It inhibits growth of many plants, creating a characteristic "walnut shadow" where few can survive.
Juglone can stunt the growth of over 600 plant species, including tomatoes and potatoes. Gardeners often plant non-sensitive species like roses or certain perennials nearby to avoid negative interactions.
Black walnut enjoys the company of ferns, day lilies, jack-in-the-pulpit, bee balm, phlox, clematis, honeysuckle, elderberry and more. Plants like these brighten up the environs.
Eucalyptus Trees (Eucalyptus spp.): The volatile oils released by eucalyptus inhibit the growth of understory vegetation, reducing competition for resources.
Eucalyptus oil is a complex blend of ingredients, primarily 1,8-cineole (eucalyptol) and alpha-terpineol. Other notable components include alpha-pinene and limonene. Oils are carried by volatilization, dispersal by breeze.
Rice (Oryza sativa): Certain rice varieties have allelopathic properties to suppress the growth of weeds, a natural weed control strategy in rice paddies.
Sunflower (Helianthus annuus): Sunflowers release allelochemicals from their roots to inhibit the growth of many weeds. Sunflowers will also try to annihilate rhubarb, potatoes and beans. Plants with shallow root systems will struggle to survive.
Legumes: As their roots form, legume sprouts like peas and beans release flavonoids into the soil to call nitrogen-fixing bacteria. Plants need nitrogen but can't absorb it directly.
Nodules grow like little roundish houses on roots to protect the bacteria as they perform their vital task. They "fix" nitrogen, turning it into simpler ammonium, which plants can easily take up.
Sorghum (Sorghum bicolor): Sorghum releases allelochemicals able to suppress weeds. It is a favored cover crop, improving soil health while controlling unwanted plant species.
Garlic Mustard (Alliaria petiolata): This invasive plant produces allelochemicals to decrease the growth of native species, allowing it to dominate the forest understory.
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