Allelopathy: Nature’s Chemical Warfare in Plants

Allelopathy & chemical warfare in the plant kingdom

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It’s not just humans that pose the threat of chemical warfare. Many plant species have the ability to produce chemicals that have a significant impact on neighboring plant species in a phenomenon known as allelopathy.

What is Allelopathy?

What is allelopathy?
French Marigolds Release Alpha-Terthienyl, an Allelopathic Compound that Inhibits Root-Knot Nematode Egg Hatching

Allelopathy is a term used to describe a natural phenomenon whereby plants produce phytotoxic chemicals that impact the plants that are growing around them. These impacts can be to do with growth and development or even the behavior of other plants.

The term allelopathy comes from the Greek language with allelon meaning one another and pathy meaning to suffer. Although this is a term that has only been used since the 1930s when a scientist named Hans Molisch wrote a paper called ‘The Influence Of One Plant On Another – Allelopathy’ in which he coined the term. Since then, this paper has shaped modern research on the topic.

Since allelochemicals are phytotoxins, they can have a detrimental impact on other plants, including growth inhibition and reduced seed germination rates. While this poses a challenge for other species, allelopathic plants benefit by experiencing decreased competition for resources. This adaptation is particularly advantageous in environments with limited resources.

As a result, the allelopathic plant can enhance its reproductive success and overall vigor, leading to an increase in abundance within its ecosystem. Consequently, in areas where these plants are prevalent, they wield a significant influence on the shaping of the ecosystem itself.

As I have mentioned, allelochemicals are produced by plants, and the types of chemicals vary depending on the species. For instance, French marigolds produce a terpenoid called alpha-terthienyl, which inhibits the hatching of root-knot nematode eggs. Other common allelochemicals include phenolics and alkaloids. These chemicals are synthesized in response to chemical signals from other species.

But it’s not all bad. In fact, allelopathic plants are often used as a natural weed killer owing to their ability to hinder the growth of pesky plants, decreasing the need for chemical intervention.

Mechanisms of Allelopathy

Mechanisms of allelopathy
Plants Vary in Where they Produce Allelochemicals; While Some Release them from Leaves, Others do so from Roots & Stems

Allelochemicals are released in different ways and have different effects depending on their composition. For example, some plants will release them via the air while others may use their roots.

Chemical Mechanisms

The structure of allelochemicals varies depending on their type. For instance, phenolics have a distinct structure from alkaloids and terpenoids, each exhibiting unique molecular arrangements. Consequently, these different chemical structures result in varying effects on the ‘victim’ plant. Ultimately, the impact of allelochemicals on target plants is intricately linked to their individual structural compositions.

Additionally, it’s important to keep in mind that not all plants produce allelochemicals from the same organ. In fact, while some may produce these chemicals in their leaves, others rely on their roots and stems.

Plants that release chemicals from their roots do so into the soil which the chemicals contaminate the soil so that, when other plants take resources from it, they also absorb the toxins. The speed of the release and its intensity is often affected by factors such as humidity levels and temperature within the soil.

Of course, the density of the roots influences the plant’s ability to store and produce allelochemicals. Those with thicker roots have far greater potential to produce large amounts of toxins compared to those whose roots are more delicate.

On the other hand, the leaves of some plants will fall off, and begin decaying, and only then are the chemicals released into the air.

Once chemicals are released, so begins the warfare between species with chemical signals being sent either through root-to-root communication or via airborne signals.

Interestingly, the production of allelochemicals is often influenced by the lifecycle of the plant. During important periods such as fruiting and flowering, the plant’s chemical production may change. It is believed that during periods of low competition plants will produce more chemicals to fight off competing plants. However, during reproductive stages, the plant is more likely to expend its energy on growing taller.

Mode of Action

There are three main ways in which allelopathic plants influence their competitors; inhibition of germination, suppressing the growth of seedlings, and disrupting the metabolic processes.

In terms of seed germination, the chemical compounds prevent seeds from germinating or hinder their progress. Additionally, some allelochemicals may delay germination. Typically, seeds will germinate when conditions are optimal. If this is delayed, the seed may not have all the necessary nutrients to sprout and grow into a healthy adult plant. A good example of this is the sesame seed whose allelopathic abilities have been shown to reduce germination in other species. In the wild (or indeed, the garden) these allelopathic plants are able to reduce the number of viable seedlings of competitors, giving them an advantage when it comes to resources.

But even when seeds have sprouted and grown into adult plants, the allelopathic effects of the chemicals released by competing plants can still have a significant impact.

For example, some compounds are able to alter the very structure of the roots, preventing them from functioning as they should. This leads to a decreased ability to absorb nutrients which results in hindered growth and development. That said, there are some plant species that have adapted to the presence of allelochemicals and whose roots are better able to cope with the threat.

In many cases, allelopathy is interspecific, with plants releasing chemicals that affect other species. However, there are also several cases of intraspecific allelopathy where the chemicals released impact plants of the same species, again reducing competition. Of course, this can impact the genetic diversity of species within any given ecosystem.

Examples of Allelopathic Plants

In nature, we find many allelopathic plants from trees to grasses and even flowering plants. Let’s take a look at some of the most interesting.

1. Black Walnut (Juglans nigra)

Black walnuts produce Juglone and hydrojuglone, both are allelochemicals.

If you’re an avid gardener, you may have heard horror stories of how it’s nigh on impossible to grow other plants around the black walnut. That’s because this species releases a chemical called Juglone as well as a secondary chemical known as hydrojuglone, both of which are allelochemicals.

While the subject had hardly been intensely studied at the time, the black walnut was one of the first plants to be discovered as damaging to others, in research penned back in 77AD. Planting common garden vegetables (such as the pepper and tomato) and flowers next to black walnuts almost always yields disastrous results. However, things like beans, clematis, and black-eyed Susan seem to be resistant.

By using organic materials in the soil, the levels of juglone can be reduced so it’s still possible to have a thriving garden in the presence of the black walnut.

For those plants that are susceptible to juglone (a naphthoquinone) the results are usually seen as hindered growth and development as the black walnut absorbs the bulk of water and nutrients from the soil as it releases chemicals from its roots. Moreover, this chemical can interrupt the photosynthesis of the victim plants.

There is also the presence of juglone in the stems, fruits, and leaves but this is to a lesser degree.

2. Sorghum (Sorghum bicolor)

Sorghum primarily releases sorgoleone, a hydrophobic quinone.

Sorghum is a common cereal grain known for its moderate persistence, which manifests even after the plant has been removed. The main compound released by this species is sorgoleone, a hydrophobic quinone. Specifically, sorgoleone’s chemical name is 2-hydroxy-5-methoxy-3-[(2Z,6E)-3,7,11-trimethyldodeca-2,6,10-trien-1-yl]-1,4-benzoquinone. Sorgoleone is known to inhibit the proper growth of surrounding plants.

However, this isn’t always to the detriment of the farmer or gardener since this typically affects weeds which can be beneficial. It works when the root hairs of the plant release toxins into the soil which then interferes with plant cell division, germination, and shoot growth as well as processes such as photosynthesis. Plants may also be affected in terms of their ability to produce energy because of the chemical’s potential to interfere with the transportation of electrons within the mitochondria.

Of course, it can also be problematic since the allelochemicals may also have an effect on surrounding crops. That said, there have been studies that have shown that the allelopathic properties of sorghum could be passed onto rice.

Where sorghum poses a problem to other crops, one of the best methods of prevention is regular crop rotation. Otherwise, the impact can clearly be seen on the ecosystem and where sorghum grows, it will likely be a dominant species.

3. Sunflowers (Helianthus spp.)

Sunflowers exhibit allelopathic properties, producing chemicals such as sesquiterpene lactones with a 15-carbon backbone, along with phenolic acids.

The sunflower is a hugely popular species for its unique appearance and impressive growth. But have you ever wondered how it manages to grow so quickly and so large? Well, it may have something to do with its allelopathic properties through which it produces chemicals like sesquiterpene lactones with a 15-carbon backbone as well as phenolic acids.

The complex structures of these chemicals quickly inhibit the growth of surrounding plants like weeds and crops like green beans and potatoes by damaging their cell membranes. While this is bad news for crops, sunflowers are sometimes used as natural weed prevention. As with sorghum, crop rotation can decrease the allelopathic influence of sunflowers. 

They’re also able to reduce seen germination and cause oxidative stress which causes cell damage to neighboring plants. They’re so successful in their allelopathy because the compounds are produced in their roots as well as the stems and leaves of the plant.

Even when sunflowers aren’t actively growing, they’ll still have an impact on the surrounding ecosystem as their allelochemicals remain in the soil long after a plant has perished. In studies, sunflowers were shredded and put into the soil and even their presence this way demonstrated negative effects on new crops.

4. Garlic Mustard (Alliaria petiolata)

Garlic mustard, commonly found in shady spots and wooded areas, can grow vigorously tall due to its allelopathic compounds.

Often found in shady spots and wooded areas, garlic mustard has the potential to grow very strong and tall. This is largely due to the allelopathic compounds it releases called glucosinolates. These chemicals have a very distinctive structure made up of a β-thioglucose group linked to a sulfonated oxime moiety and eventually break down into isothiocyanates which have a variety of effects on neighboring plants.

These effects include root elongation, growth inhibition, development, and germination. What makes them even more powerful is that they’re released from various parts of the plant such as the roots, stems, and leaves.

The problem is that garlic mustard tends to affect fungi that are crucial to the health of the ecosystem, suppressing surrounding vegetation by directly disrupting cellular processes and the ability to absorb nutrients.

It probably comes as no surprise to learn that garlic mustard is considered an invasive species and can be very difficult to control. Even after the plant has been removed, the presence of its allelochemicals can still be found in soil for some time. However, it has been demonstrated that by adding activated carbon to the soil, the threat of the chemicals can be diminished.

5. Spotted Knapweed (Centaurea stoebe)

Spotted Knapweed produces catechin, a phenolic compound that belongs to the flavonoid group and has a benzopyran structure.

Native to eastern Europe, spotted knapweed is considered to be one of the most invasive plants in North America. It produces a phenolic chemical known as catechin which is a flavonoid with a benzopyran structure. It’s found in a lot of plant species but in the case of spotted knapweed, it provides the ability to attack the germination and growth of its neighbors as well as attacking physiological processes that allow plants to uptake water and nutrients. 

This is incredibly problematic for native plant species that are easily outcompeted by spotted knapweed. Once spotted knapweed is established, it can be very difficult to remove and requires an arsenal of methods including chemical, biological, and mechanical removal. This is not only important in landscaping but also in the wild to prevent the plant from taking over ecosystems and the formation of monotypic stands. That said, even once the plant has been removed, its chemicals persist in the environment for a considerable amount of time.

Another issue with spotted knapweed is that the chemicals are not just produced in one part of the plant but rather the leaves and the roots, giving it a greater spread. Of course, the concentration of the chemicals will depend on environmental factors and the strength of the individual plant.

6. Poplar Trees (Populus spp.)

Poplar trees release salicin, salicylates, and various phenolic compounds that can negatively impact the health of neighboring plant life.

Poplar trees don’t have a very long life in comparison to other tree species but they are incredibly fast growing. They’re found in most of the temperate northern hemisphere and produce salicin, salicylates, and other types of phenolic compounds which are detrimental to the health of surrounding plant life.

Interestingly, salicin has a very similar structure to aspirin, whereas salicylates are similar to salicylic acid. They form an impressive attack against neighboring plants, particularly herbaceous species and plants that grow underneath poplars as well as some crops. The effects may vary but include preventing efficient photosynthesis, cell division, seed germination and seedling growth, and the disruption to the ability to absorb nutrients.

It is difficult to stop these effects but choosing plants that tolerate living close to poplars can reduce problems. Even if removal is successful, the chemicals will linger for some time in the environment, continuing to have negative effects and creating allelopathic zones where there are lots of poplar trees.

7. Bermuda Grass (Cynodon dactylon)

Bermuda grass emits phenolic acids and other secondary metabolites that hinder plant life and can impede seed germination.

Bermuda grass is popular in landscaping in warm climates, and although it is very temperature sensitive, it’s very easy to grow and has great drought tolerance. However, gardeners should take care when choosing Bermuda grass as its allelopathy means that it isn’t kind to other grasses and broadleaf species. Many people choose to remove Bermuda grass, but its effects persist in the environment for some time after.  In many ecosystems, it is not uncommon for Bermuda grass to create allelopathic zones where it becomes the most dominant species.

That said, by choosing species that do well around Bermuda grass, such as perennial ryegrass, it is possible to have a harmonious garden.

Bermuda grass releases phenolic acids and various secondary metabolites that are harmful to plant life and can affect the seed germination of things like bread wheat and cotton as well as affect the overall growth of the victim plant. Studies have also shown that peach trees struggle to thrive in areas where they are exposed to Bermuda grass. The problem is that Bermuda grass attacks from all angles since the chemicals are released from the rhizomes, roots, leaves, and stems.

From a physiological point of view, the allelochemicals of Bermuda grass can cause cell division, affect nutrient uptake, and interfere with the plant’s ability to photosynthesize. Of course, this is beneficial to Bermuda grass as it achieves a competitive advantage.

8. Tree of Heaven (Ailanthus altissima)

The Tree of Heaven produces numerous allelopathic chemicals, including quassinoids (a type of triterpenoid), alkaloids containing a nitrogen-containing heterocyclic ring, and phenolic compounds characterized by a benzene ring with hydroxyl groups.

The tree of heaven may sound angelic but this invader from the east of Asia is anything but. In fact, it produces a ton of allelopathic chemicals including quassinoids which are a type of triterpenoid, alkaloids with a nitrogen-containing heterocyclic ring, and phenolic compounds with a benzene ring with hydroxyl groups.

The effects of these chemicals are devastating to herbaceous and woody plants, particularly in the temperate forests of North America. In fact, studies have shown that where the tree of heaven is present, the biodiversity of the ecosystem is significantly reduced.

While the tree of heaven releases its allelochemicals from the roots, leaves, and stems, it has an incredible advantage even over other allelopathic plants; it also releases chemicals from its seeds! These chemicals cause neighboring plants to develop elongated roots as well as impacting seed germination and overall growth. If that isn’t enough, these chemicals also interfere with processes like photosynthesis and nutrient uptake.

In North America, the tree of heaven is considered a highly invasive species and, in areas where it grows, it will quickly outcompete native plants, creating dense stands. Even after the plant is physically removed, the chemicals stay behind, continuing to wreak havoc for a substantial amount of time.

9. Fragrant Sumac (Rhus aromaticus)

Fragrant Sumac produces tannins, terpenoids, and phenolic acids.

Fragrant sumac is certainly a beautiful shrub but it has a dark secret; it produces tannins, terpenoids, and phenolic acids. So, even though this is a native plant, it can cause destruction because it also has a tendency to choke other plant roots with its slow-growing rhizomes.

The tannins produced have complex structures and the terpenoids have multiple isoprene units. The phenolic acids have a much simpler structure with a benzene ring and hydroxyl groups. But what do these chemicals do?

Well, for starters, they’re released from many areas of the fragrant sumac including the flowers, stems, leaves, and roots although their intensity is impacted by environmental factors. They typically target woody or herbaceous plants and cause them to slow in growth and can affect seed germination.

Moreover, these allelochemicals can also affect the physiological processes and their ability to absorb nutrients. Once fragrant sumac has established in an area, the chemicals can persist in the environment for quite some time, even after removal. This means that the biodiversity of an area can be impacted quite significantly.

Because of its beauty, fragrant sumac is highly desired as a landscaping plant, but people can be put off because of its allelopathic properties. However, it is possible to use it in the garden without too many problems by ensuring a good distance between plants, especially considering that sumac is slow-growing. Also, consider that evergreen plants grow quite well around fragrant sumac, so choosing the right companion plants makes a huge difference.

10. Rice (Oryza sativa)

Rice plants release momilactones, diterpenoid compounds with a complex ring structure, along with benzoxazinoids, which feature a benzene ring and an oxazinone ring.

If you’ve ever seen a rice paddy, you’ll know that these plants have an excellent spread and even though these fields are often flooded in the rainy season, the plants have enough height to survive. But this isn’t the only survival tactic of rice plants; they’re also allelopathic.

Rice releases diterpenoid compounds known as momilactones that have a complex ring structure as well as benzoxazinoids with a benzene ring and an oxazinone ring. These chemicals wreak havoc on neighboring plants, although this isn’t necessarily bad news since research has shown that rice can be very effective as a natural weed killer.

When plants are exposed to rice allelochemicals, this affects their seed germination and growth as well as causing elongated roots. This gives rice a massive competitive advantage and is one of the reasons it’s such a successful crop season after season. Getting to grips with the allelopathic potential of rice is important in agriculture since this is one of the most high-demand food crops in the world.

11. Pea (Pisum sativum)

Pea plants possess allelopathic potential that can hinder root elongation, seed germination, and the neighboring plants' nutrient absorption capacity.

Peas are a popular choice for domestic crop growth but you must be careful what you plant them with because they have allelopathic potential that can result in problems with root elongation, seed germination, and the neighboring plant’s ability to absorb nutrients.

Now, this is good in some cases since peas will often target weeds but it is also possible that the pea plant will affect neighboring crops, which isn’t such good news. In fields, where peas are well established, their allelopathic potential means that they can quickly dominate an area season after season, reducing biodiversity. That said, on a commercial level, peas are often used as a natural method of weed control with semi-leafless cultivars being shown to have the most effective results.

So, what are the chemicals responsible for the pea’s allelopathic success? For starters, they produce flavonoids with a basic two aromatic ring structure. They also release coumarins that consist of a benzene ring used to an α-pyrone ring as well as phenolic acids. These chemicals are released from the leaves, roots, and root exudates (secretions from the root.)

Plants that grow well with peas include leafy greens, peppers, and beans and by doing this, there is more chance of growing a harmonious crop garden.

12. Tomato (Solanum lycopersicum)

Tomatoes release nitrogen-containing alkaloids, flavonoids characterized by two aromatic rings connected by a carbon bridge, and phenolic acids featuring a benzene ring with hydroxyl groups.

The humble tomato is a garden crop staple but while these plants may produce tasty fruits, gardeners need to be mindful of their effects on neighboring species. Releasing nitrogen-containing alkaloids, flavonoids with two aromatic rings connected by a carbon bridge, and phenolic acids with a benzene ring and hydroxyl groups, tomatoes can affect everything from germination and growth to root development and nutrient uptake.

In many cases, the affected plants include a variety of weeds so you might think this is good news. However, there are many crop species that are also impacted by the tomato’s allelochemicals including cabbage, broccoli, corn, and eggplant.

Even after tomato plants have been removed, the effects of their chemicals can impact the environment long after. For the best management, it’s a good idea to plant tomatoes with species that can tolerate them such as chives, basil, lettuce, and asparagus.

13. Common Buckthorn (Rhamnus cathartica)

Common buckthorn releases toxins such as emodin and anthraquinones.

Common buckthorn is a tree species that’s natively found in parts of Asia and Africa as well as across Europe. While it has been naturalized in most of North America, it is considered invasive in many areas.

This species releases toxins known as emodin and anthraquinones. A type of anthraquinone, emodin has a tricyclic structure that is made up of one quinone and two hydroxyl groups. This compound targets plants within the woody and herbaceous families and is known for its aggression against native species, hence its title as an invasive plant.

Releasing its allelochemicals from both its stems and leaves as well as the roots, the common buckthorn is easily able to prevent or quash seed germination as well as elongating the roots of neighboring plants and preventing them from flourishing. What’s more, these chemicals have the ability to prevent proper photosynthesis and stop the plant from correctly being able to absorb nutrients.

While removing invasive plants is often a solution, this doesn’t prove to be effective where the common buckthorn is concerned because the chemicals remain in the soil long after the plant has been taken away. Where the species has already invaded, this can have a devastating impact on local plant communities, reducing biodiversity and creating allelopathic zones.

Adding herbicides to the area can help to combat the problem but gaining a knowledge of how allelopathic plants work is usually the best strategy for defeating this pesky invader.

14. Buckwheat (Fagopyrum esculentum)

Buckwheat releases chemicals from its leaves, roots, and flowers, including phenolic acids.

Buckwheat is a species of pseudocereal and it has many health benefits for humans. However, it can be difficult to grow as a crop because of how its allelopathic chemicals affect surrounding plants.

These plants release chemicals from the leaves, roots, and flowers including phenolic acids that have a benzene ring and hydroxyl groups as well as basic flavonoids that contain two aromatic rings connected by a three-carbon bridge. When plants like Russian knapweed, creeping Jenny, and sowthistle are exposed to these chemicals, their seeds struggle to germinate and overall growth is affected.

While this may be good news where weeds are concerned, it’s important to keep in mind that buckwheat can also have similar effects on neighboring crops. It is advisable to use crop rotation to manage the allelopathic effects of buckwheat but also consider planting it with species that are not sensitive such as potatoes, melon, and pumpkins as well as other members of the squash family.

15. Fescue (Festuca spp.)

Fescue utilizes phenolic acids with a benzene ring and hydroxyl groups, along with terpenoids containing multiple isoprene units, to inhibit the germination and growth of neighboring plants.

Fescue is a popular cool season grass but it does come with its pros and cons. Since this is an allelopathic plant, it will work well where weed control is needed. The plant uses phenolic acids with a benzene ring and hydroxyl groups and terpenoids with multiple isoprene units to inhibit the germination and growth of neighboring plants.

However, the problems come when we realize that fescue also exerts a detrimental effect on other grass species. In areas where multiple grass species coexist, especially alongside broadleaf species, fescue resorts to competitive tactics, impairing the victims’ photosynthesis abilities and nutrient absorption capacity. Consequently, fescue typically gains a resource advantage in such environments.

It is so successful because it not only releases two types of allelochemicals but is also able to release these toxins from its stems, roots, and leaves, attacking neighboring species from all angles.

Many people love fescue because it is easy to grow, doesn’t need much fertilizer, and grows well in poor soil. However, if you’re put off the idea because of the allelopathic nature of this grass, consider that you could use a cultivar whose allelopathic properties are not as great.

Moreover, if you’re trying to manage weeds, studies have shown that creeping red fescue and chewings fescue most successfully suppressed weed growth.

16. Black Crowberry (Empetrum nigrum)

In areas where black crowberry is present, it dominates, outcompeting other plant species.

Black crowberry is part of the heather family and is found all over the boreal regions of the northern hemisphere. Neighboring plants, such as the scotch pine, are affected by allelochemicals including phenolic acids and flavonoids.

These are released from many parts of the black crowberry including its roots, stems, and leaves and when other plants are exposed, it can affect seed germination, root development, and overall growth. According to experts, fire is one of the best ways to eliminate black crowberry but even once it has been treated, the allelochemical effects persist in the environment for a good length of time.

In areas where black crowberry is present, there is an obvious stand where this species outcompetes other plants. While it is native to North America, it is still considered difficult to control. That said, the berries do serve as a food source for animals like caribou and bears and humans even use them to make jams and jellies.

17. Goldenrod (Solidago spp.)

The primary allelochemical in goldenrod is sesquiterpene lactone, featuring a complex 15-carbon framework, and belongs to the alpha-methylene-gamma-lactone group.

Goldenrod is a favorite of many flower lovers because it’s a native species and is super easy to grow. But did you know that it could be impacting other plant life in your garden as it releases sesquiterpene lactones, flavonoids, and phenolics?

The most potent allelochemical in goldenrod is sesquiterpene lactone which has a complex 15-carbon framework and is from the alpha-methylene-gamma-lactone group. The flavonoids released by goldenrod have a much less complex structure that consists of just two aromatic rings which are connected by a 3-carbon bridge. Finally, the phenolic element of the goldenrod’s allelochemicals consists of a benzene ring with hydroxyl groups.

Goldenrod is so successful in its allelopathic attempts because of its ability to release toxins from its stem, roots, and leaves. These are known to affect woody plants, particularly crops like raspberry. They also have the ability to strangle the roots of various grasses using their creeping rhizomes so it uses a two-pronged attack.

The chemicals released by goldenrod have an array of effects including messing with the physiological processes of the neighboring plants as well as interfering with germination and development. How intense these effects are depends on the intensity of the allelochemicals but it has been noted that many gardeners actively try to get rid of goldenrod because of how it affects other species. In the wild, this ability certainly gives it an advantage where lots of species are competing for resources.

18. Purple Loosestrife (Lythrum salicaria)

Purple loosestrife (Lythrum salicaria) outcompetes other plants through the release of allelochemicals such as hydrolysable tannins.

Originally introduced to North America as an ornamental plant in the 1800s, purple loosestrife is now considered an invasive species. In some parts of the US, there are heavy restrictions on its sale because of how it affects native species.

It does this because of the release of allelochemicals like hydrolysable tannins that have a polyphenolic structure with a central glucose molecule esterified with gallic acid units. These compounds can affect the seed germination of neighboring wetland plants and inhibit their overall growth. On top of this, the chemicals, released from the roots, leaves, and stems, have the ability to interfere with root development and nutrient uptake. Affected plants include reed canary grass and cattail.

Because of this, purple loosestrife is often the dominant species in ecosystems where it is present and the persistence of its chemicals remains long after the plant has died or been removed. This has a direct impact on the biodiversity within the area and means that purple loosestrife often outcompetes native plants. This also impacts local wildlife which rely on native species, demonstrating that allelopathic plants impact more than just their natural competitors.

Can Plants Coevolve with Allelopathic Plants?

Can plants coevolve with allelopathic plants?
Most Maple Species Naturally Resist Allelochemical Effects

Being constantly attacked by allelochemicals could wipe out a plant species but evolution seems to have swept in and saved the day. In fact, there is plenty of evidence to suggest that some species have developed a resistance to these chemicals and developed adaptations to make them less susceptible to their effects.

Of course, this is a process that takes time but evidence suggests that plants may have altered their germination times, adapted their root morphology, and even developed an ability to produce enzymes that repel allelochemicals.

Research shows that North American plant species that were forced to exist around allelopathic invaders developed a resistance that was not evident in the same species that wasn’t exposed to the chemicals.

Where two species would not usually exist together, there have been further studies that have shown co-evolution is possible and does happen through stabilization and equalization.

Of course, there are also plants that have never been affected by allelochemicals in the first place and have a natural resistance to their effects. Most maple species, apart from silver maple will resist the effects as well as black gum, hickory, witch hazel, corn, quince, and wisteria, among others.

While some plants are resistant to allelochemicals, others may thrive in their presence. For example, plants in the onion and bean family do very well around allelopathic plants. This has been seen in goldenrod dominated areas where the white heath aster now thrives despite the presence of these chemicals.

A great example of coevolution and resistance is the hackberry which appears to have developed a tolerance to juglone produced by the black walnut. Furthermore, eastern gamma grass has been shown to have become far more resistant to the allelopathic properties of the sunflower through evolutionary adaptations. This is similar to the wild radish which is now seen to be able to tolerate growing around the allelopathic black mustard.

Practical Applications of Allelopathic Plants

Practical applications of allelopathic plants
Allelopathic Crops Play a Role in Agroecosystems

When we hear about allelopathic plants, it’s easy to instantly assume the worst. While they can cause problems, there are many examples of how their allelopathic potential could be beneficial.

Weed Control

The use of synthetic pesticides has a devastating impact on the environment and can target insects, plants, and animals for which they are not intended. However, allelopathic plants might just be the answer and are often used as natural weed killers. Not only will this prevent chemical run off but it’s also a much more cost-effective way to control weeds on a commercial level.

Plants that are able to release chemicals that hinder germination and growth can be placed among crops to target invasive weed species. Even the crops themselves may act as a natural weed suppressant and this is commonly seen with barley, sorghum, and rye. This could pave the way to more eco-friendly agriculture and weed management in the near future. 

As I have discussed, crop rotation is vital to the prevention of allelopathic effects. But when used in weed management it can also be beneficial to the health of the soil, making it more nutrient-rich and full of resources for the crops.

Utilization of Allelopathic Crops in Agroecosystems

There are many advantages to using allelopathic crops in the agroecosystem. For starters, using them as a companion plant to non-allelopathic plants creates a balance that ensures minimal negative effects and greater positive ones. For example, planting French marigolds among cucumbers, tomatoes, and radishes can prevent problems with nematode eggs around the roots. 

Ensuring the health of the soil is essential to successful farming and using plants like mustard can decrease the number of fungal pathogens within the soil. What’s more, some other allelopathic plants grow so densely over the ground that the soil is protected from erosion.

Weeds are a huge problem in agriculture but there are lots of allelopathic crops that have a natural ability to suppress weed growth. Wheat, barley, and rye are excellent examples of this and where plants like this are used for weed management, there is a decreased need for chemical herbicides.

Moreover, certain species of allelopathic plants naturally deter pests that may otherwise interfere with crop health. Basil is a great example of this as it is both allelopathic but also repels pests such as carrot flies and tomato hornworms.

With the inclusion of allelopathic plants, the agroecosystem can naturally become more diverse and this results in a more dynamic environment that will allow various species to thrive because of the natural balance. What’s more, the more we learn about using allelopathic plants in agriculture, the more sustainable practices we can develop.

Bioprospecting for Allelopathic Compounds

Humans have used plants for their medicinal properties for thousands of years and nothing has changed. Pharmaceutical companies are using allelochemical compounds for things like wound healing because of their ability to reduce inflammation and promote tissue regrowth.

What’s more, many allelopathic plants have properties such as being anti-microbial, anti-fungal, and analgesic. Even more interesting is the potential anti-cancer properties of certain allelopathic plants and researchers are looking at ways these can be used to treat and prevent cancer. Other studies demonstrate the potential of allelochemicals in the treatment of cardiovascular disease. Of course, there’s a lot of research to do to ensure that side effects are minimal and that these, and other drugs using allelochemicals, are safe for human use.

Bioprospecting is the exploration of natural materials and their potential uses for various applications and this doesn’t just include medicine.

With lots of research into which allelopathic plants contain the right compounds and indeed, how they can be extracted, scientists are finding ways to develop natural herbicides, insecticides, and other products. A great example of this is the use of juglone, produced by the black walnut, which has amazing potential as a natural weed killer. Off the back of this, we are able to find new ways towards a more sustainable future in agriculture and even domestic crop growing.

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