Allelopathy

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(Australian) coastal she oak litter completely suppresses germination of understory plants as shown here despite the relative openness of the canopy and ample rainfall (>120 cm/yr) at the location. Casuarina litter.jpg
(Australian) coastal she oak litter completely suppresses germination of understory plants as shown here despite the relative openness of the canopy and ample rainfall (>120 cm/yr) at the location.

Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms and the community. Allelopathy is often used narrowly to describe chemically-mediated competition between plants; however, it is sometimes defined more broadly as chemically-mediated competition between any type of organisms. The original concept developed by Hans Molisch in 1937 seemed focused only on interactions between plants, between microorganisms and between microorganisms and plants. [1] Allelochemicals are a subset of secondary metabolites, [2] which are not directly required for metabolism (i.e. growth, development and reproduction) of the allelopathic organism.

Contents

Allelopathic interactions are an important factor in determining species distribution and abundance within plant communities, and are also thought to be important in the success of many invasive plants. For specific examples, see black walnut ( Juglans nigra), tree of heaven ( Ailanthus altissima ), black crowberry ( Empetrum nigrum ), spotted knapweed ( Centaurea stoebe ), garlic mustard ( Alliaria petiolata ), Casuarina/Allocasuarina spp., and nutsedge.

It can often be difficult in practice to distinguish allelopathy from resource competition. While the former is caused by the addition of a harmful chemical agent to the environment, the latter is caused by the removal of essential resources (nutrients, light, water, etc.). Often, both mechanisms can act simultaneously. Moreover, some allelochemicals may function by reducing nutrient availability. Further confounding the issue, the production of allelochemicals can itself be affected by environmental factors such as nutrient availability, temperature and pH. Today, most ecologists recognize the existence of allelopathy, however many particular cases remain controversial. Furthermore, the specific modes of action of allelochemicals on different organisms are largely open to speculation and investigation. [1]

History

The term allelopathy from the Greek-derived compounds allilon- (αλλήλων) and -pathy (πάθη) (meaning "mutual harm" or "suffering"), was first used in 1937 by the Austrian professor Hans Molisch in the book Der Einfluss einer Pflanze auf die andere - Allelopathie (The Effect of Plants on Each Other - Allelopathy) published in German. [3] He used the term to describe biochemical interactions by means of which a plant inhibits the growth of neighbouring plants. [4] [5] In 1971, Whittaker and Feeny published a review in the journal Science, which proposed an expanded definition of allelochemical interactions that would incorporate all chemical interactions among organisms. [3] [6] In 1984, Elroy Leon Rice in his monograph on allelopathy enlarged the definition to include all direct positive or negative effects of a plant on another plant or on micro-organisms by the liberation of biochemicals into the natural environment. [7] Over the next ten years, the term was used by other researchers to describe broader chemical interactions between organisms, and by 1996 the International Allelopathy Society (IAS) defined allelopathy as "Any process involving secondary metabolites produced by plants, algae, bacteria and fungi that influences the growth and development of agriculture and biological systems." [8] In more recent times, plant researchers have begun to switch back to the original definition of substances that are produced by one plant that inhibit another plant. [3] Confusing the issue more, zoologists have borrowed the term to describe chemical interactions between invertebrates like corals and sponges. [3]

Long before the term allelopathy was used, people observed the negative effects that one plant could have on another. Theophrastus, who lived around 300 BC noticed the inhibitory effects of pigweed on alfalfa. In China around the first century CE, the author of Shennong Ben Cao Jing , a book on agriculture and medicinal plants, described 267 plants that had pesticidal abilities, including those with allelopathic effects. [9] In 1832, the Swiss botanist De Candolle suggested that crop plant exudates were responsible for an agriculture problem called soil sickness.

Allelopathy is not universally accepted among ecologists. Many have argued that its effects cannot be distinguished from the exploitation competition that occurs when two (or more) organisms attempt to use the same limited resource, to the detriment of one or both. In the 1970s, great effort went into distinguishing competitive and allelopathic effects by some researchers, while in the 1990s others argued that the effects were often interdependent and could not readily be distinguished. [3] However, by 1994, D. L. Liu and J. V. Lowett at the Department of Agronomy and Soil Science, University of New England in Armidale, New South Wales, Australia, wrote two papers [10] [11] in the Journal of Chemical Ecology that developed methods to separate the allelochemical effects from other competitive effects, using barley plants and inventing a process to examine the allelochemicals directly. In 1994, M.C. Nilsson at the Swedish University of Agricultural Sciences in Umeå showed in a field study that allelopathy exerted by Empetrum hermaphroditum reduced growth of Scots pine seedlings by ~ 40%, and that below-ground resource competition by E. hermaphroditum accounted for the remaining growth reduction. [12] For this work she inserted PVC-tubes into the ground to reduce below-ground competition or added charcoal to soil surface to reduce the impact of allelopathy, as well as a treatment combining the two methods. However, the use of activated carbon to make inferences about allelopathy has itself been criticized because of the potential for the charcoal to directly affect plant growth by altering nutrient availability. [13]

Some high profile work on allelopathy has been mired in controversy. For example, the discovery that (−)-catechin was purportedly responsible for the allelopathic effects of the invasive weed Centaurea stoebe was greeted with much fanfare after being published in Science in 2003. [14] One scientist, Dr. Alastair Fitter, was quoted as saying that this study was "so convincing that it will 'now place allelopathy firmly back on center stage.'" [14] However, many of the key papers associated with these findings were later retracted or majorly corrected, after it was found that they contained fabricated data showing unnaturally high levels of catechin in soils surrounding C. stoebe. [15] [16] [17] Subsequent studies from the original lab have not been able to replicate the results from these retracted studies, nor have most independent studies conducted in other laboratories. [18] [19] Thus, it is doubtful whether the levels of (−)-catechin found in soils are high enough to affect competition with neighboring plants. The proposed mechanism of action (acidification of the cytoplasm through oxidative damage) has also been criticized, on the basis that (−)-catechin is actually an antioxidant. [19]

Examples

Garlic mustard (MHNT) Alliaria petiolata - flowers.jpg
Garlic mustard

Plants

Many invasive plant species interfere with native plants through allelopathy. [20] [21] A famous case of purported allelopathy is in desert shrubs. One of the most widely known early examples was Salvia leucophylla , because it was on the cover of the journal Science in 1964. [22] Bare zones around the shrubs were hypothesized to be caused by volatile terpenes emitted by the shrubs. However, like many allelopathy studies, it was based on artificial lab experiments and unwarranted extrapolations to natural ecosystems. In 1970, Science published a study where caging the shrubs to exclude rodents and birds allowed grass to grow in the bare zones. [23] A detailed history of this story can be found in Halsey 2004. [24]

Garlic mustard is another invasive plant species that may owe its success partly to allelopathy. Its success in North American temperate forests may be partly due to its excretion of glucosinolates like sinigrin that can interfere with mutualisms between native tree roots and their mycorrhizal fungi. [25] [26]

Allelopathy has been shown to play a crucial role in forests, influencing the composition of the vegetation growth, and also provides an explanation for the patterns of forest regeneration. [27] The black walnut (Juglans nigra) produces the allelochemical juglone, which affects some species greatly while others not at all. However, most of the evidence for allelopathic effects of juglone come from laboratory assays and it thus remains controversial to what extent juglone affects the growth of competitors under field conditions. [28] The leaf litter and root exudates of some Eucalyptus species are allelopathic for certain soil microbes and plant species. [29] The tree of heaven, Ailanthus altissima, produces allelochemicals in its roots that inhibit the growth of many plants. Spotted knapweed (Centaurea) is considered an invasive plant that also utilizes allelopathy. [30]

Another example of allelopathy is seen in Leucaena leucocephala , known as the miracle tree. This plant contains toxic amino acids that inhibit other plants’ growth but not its own species growth. Different crops react differently to these allelochemicals, so wheat yield decreases, while rice increases in the presence of L. leucocephala. [31] [ unreliable source? ]

Capsaicin is an allelochemical found in many peppers that are cultivated by humans as a spice/food source. [32] It is considered an allelochemical because it is not required for plant growth and survival, but instead deters herbivores and prevents other plants from sprouting in its immediate vicinity. [33] [ dubious discuss ] Among the plants it has been studied on are grasses, lettuce, and alfalfa, and on average, it will inhibit the growth of these plants by about 50%. [33] Capsaicin has been shown to deter both herbivores and certain parasites’ performance. [34] Herbivores such as caterpillars show decreased development when fed a diet high in capsaicin.

Applications

Allelochemicals are a useful tool in sustainable farming due to their ability to control weeds. [35] The possible application of allelopathy in agriculture is the subject of much research. [36] [37] Using allelochemical producing plants in agriculture results in significant suppression of weeds and various pests. Some plants will even reduce the germination rate of other plants by 50%. [33] Current research is focused on the effects of weeds on crops, crops on weeds, and crops on crops. [38] [39] This research furthers the possibility of using allelochemicals as growth regulators and natural herbicides, to promote sustainable agriculture. [40] Agricultural practices may be enhanced through the utilization of allelochemical producing plants. [41] When used correctly, these plants can provide pesticide, herbicide, and antimicrobial qualities to crops. [42] number of such allelochemicals are commercially available or in the process of large-scale manufacture. For example, leptospermone is an allelochemical in lemon bottlebrush ( Callistemon citrinus ). Although it was found to be too weak as a commercial herbicide, a chemical analog of it, mesotrione (tradename Callisto), was found to be effective. [43] It is sold to control broadleaf weeds in corn but also seems to be an effective control for crabgrass in lawns. Sheeja (1993) reported the allelopathic interaction of the weeds Chromolaena odorata (Eupatorium odoratum) and Lantana camara on selected major crops.

Many crop cultivars show strong allelopathic properties, of which rice ( Oryza sativa ) has been most studied. [44] [45] [46] Rice allelopathy depends on variety and origin: Japonica rice is more allelopathic than Indica and Japonica-Indica hybrid.[ citation needed ] More recently, critical review on rice allelopathy and the possibility for weed management reported that allelopathic characteristics in rice are quantitatively inherited and several allelopathy-involved traits have been identified. [47] The use of allelochemicals in agriculture provide for a more environmentally friendly approach to weed control, as they do not leave behind residues. [35] Currently used pesticides and herbicides leak into waterways and result in unsafe water qualities. This problem could be eliminated or significantly reduced by using allelochemicals instead of harsh herbicides. The use of cover crops also results in less soil erosion and lessens the need for nitrogen heavy fertilizers. [48]

See also

Related Research Articles

<span class="mw-page-title-main">Herbicide</span> Type of chemical used to kill unwanted plants

Herbicides, also commonly known as weed killers, are substances used to control undesired plants, also known as weeds. Selective herbicides control specific weed species while leaving the desired crop relatively unharmed, while non-selective herbicides kill plants indiscriminately. The combined effects of herbicides, nitrogen fertilizer, and improved cultivars has increased yields of major crops by 3x to 6x from 1900 to 2000.

<span class="mw-page-title-main">Companion planting</span> Agricultural technique

Companion planting in gardening and agriculture is the planting of different crops in proximity for any of a number of different reasons, including weed suppression, pest control, pollination, providing habitat for beneficial insects, maximizing use of space, and to otherwise increase crop productivity. Companion planting is a form of polyculture.

<span class="mw-page-title-main">Weed control</span> Botanical component of pest control for plants

Weed control is a type of pest control, which attempts to stop or reduce growth of weeds, especially noxious weeds, with the aim of reducing their competition with desired flora and fauna including domesticated plants and livestock, and in natural settings preventing non native species competing with native species.

<span class="mw-page-title-main">Cover crop</span> Crop planted to manage erosion and soil quality

In agriculture, cover crops are plants that are planted to cover the soil rather than for the purpose of being harvested. Cover crops manage soil erosion, soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in an agroecosystem—an ecological system managed and shaped by humans. Cover crops can increase microbial activity in the soil, which has a positive effect on nitrogen availability, nitrogen uptake in target crops, and crop yields. Cover crops reduce water pollution risks and remove CO2 from the atmosphere .Cover crops may be an off-season crop planted after harvesting the cash crop. Cover crops are nurse crops in that they increase the survival of the main crop being harvested, and are often grown over the winter. In the United States, cover cropping may cost as much as $35 per acre.

<span class="mw-page-title-main">Polyculture</span> Growing multiple crops together in agriculture

In agriculture, polyculture is the practice of growing more than one crop species together in the same place at the same time, in contrast to monoculture, which had become the dominant approach in developed countries by 1950. Traditional examples include the intercropping of the Three Sisters, namely maize, beans, and squashes, by indigenous peoples of Central and North America, the rice-fish systems of Asia, and the complex mixed cropping systems of Nigeria.

Chemical ecology is the study of chemically mediated interactions between living organisms, and the effects of those interactions on the demography, behavior and ultimately evolution of the organisms involved. It is thus a vast and highly interdisciplinary field. Chemical ecologists seek to identify the specific molecules that function as signals mediating community or ecosystem processes and to understand the evolution of these signals. The substances that serve in such roles are typically small, readily-diffusible organic molecules, but can also include larger molecules and small peptides.

<i>Centaurea diffusa</i> Species of flowering plant

Centaurea diffusa, also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus Centaurea in the family Asteraceae. This species is common throughout western North America but is not actually native to the North American continent, but to the eastern Mediterranean.

Autotoxicity, meaning self-toxicity, is a biological phenomenon whereby a species inhibits growth or reproduction of other members of its species through the production of chemicals released into the environment. Like allelopathy, it is a type of interference competition but it is technically different: autotoxicity contributes to intraspecific competition, whereas allelopathic effects refer to interspecific competition. Furthermore, autotoxic effects are always inhibitory, whereas allelopathic effects are not necessarily inhibitory–they may stimulate other organisms.

<span class="mw-page-title-main">Living mulch</span> Cover crop grown with a main crop as mulch

In agriculture, a living mulch is a cover crop interplanted or undersown with a main crop, and intended to serve the purposes of a mulch, such as weed suppression and regulation of soil temperature. Living mulches grow for a long time with the main crops, whereas cover crops are incorporated into the soil or killed with herbicides.

<span class="mw-page-title-main">Juglone</span> Chemical produced by walnut trees

Juglone, also called 5-hydroxy-1,4-naphthalenedione (IUPAC) is a phenolic organic compound with the molecular formula C10H6O3. In the food industry, juglone is also known as C.I. Natural Brown 7 and C.I. 75500. It is insoluble in benzene but soluble in dioxane, from which it crystallizes as yellow needles. It is an isomer of lawsone, which is the active dye compound in the henna leaf.

<span class="mw-page-title-main">Push–pull agricultural pest management</span> Intercropping strategy for controlling agricultural pests

Push–pull technology is an intercropping strategy for controlling agricultural pests by using repellent "push" plants and trap "pull" plants. For example, cereal crops like maize or sorghum are often infested by stem borers. Grasses planted around the perimeter of the crop attract and trap the pests, whereas other plants, like Desmodium, planted between the rows of maize, repel the pests and control the parasitic plant Striga. Push–pull technology was developed at the International Centre of Insect Physiology and Ecology (ICIPE) in Kenya in collaboration with Rothamsted Research, UK. and national partners. This technology has been taught to smallholder farmers through collaborations with universities, NGOs and national research organizations.

<span class="mw-page-title-main">Beneficial weed</span> Invasive plant with positive effects

A beneficial weed can be an invasive plant that has some companion plant effect which is edible, contributes to soil health, adds ornamental value, and as well as beneficial also. These plants are normally not domesticated. However, some invasive plants, such as dandelions are commercially cultivated in addition to growing in the wild. Beneficial weeds include many wildflowers, as well as other weeds that are commonly removed or poisoned. Certain weeds that have obnoxious and destructive qualities have been shown to fight illness and are thus used in medicine. Reductions in abundances of weeds which act as hosts may affect associated insects and other taxa which are beneficial. For example, Parthenium hysterophorus which is native to Northern Mexico and parts of the US, has been an issue for years due to its toxicity and ability to spread rapidly. In the past few decades though research has found that Parthenium hysterophorus has been used in traditional medicine to treat inflammation, pain, fever, neurological disorders and diseases like malaria dysentery. It is also known to create Biogas that can be used as a bioremediation agent to break down heavy metals and other pollutants.

Upland rice is rice grown in drier environments. The term describe varieties of rice developed for rain-fed or less-intensely irrigated soil instead of flooded rice paddy fields, or it may describe rice grown outside of paddies.

<span class="mw-page-title-main">Weed</span> Plant considered undesirable in a particular place or situation

A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals. Plants with characteristics that make them hazardous, aesthetically unappealing, difficult to control in managed environments, or otherwise unwanted in farm land, orchards, gardens, lawns, parks, recreational spaces, residential and industrial areas, may all be considered weeds. The concept of weeds is particularly significant in agriculture, where the presence of weeds in fields used to grow crops may cause major losses in yields. Invasive species, plants introduced to an environment where their presence negatively impacts the overall functioning and biodiversity of the ecosystem, may also sometimes be considered weeds.

<span class="mw-page-title-main">Leptospermone</span> Chemical compound

Leptospermone is a chemical compound produced by some members of the myrtle family (Myrtaceae), such as Callistemon citrinus, a shrub native to Australia, and Leptospermum scoparium (Manuka), a New Zealand tree from which it gets its name. Modification of this allelopathic chemical to produce mesotrione led to the commercialization of derivative compounds as HPPD inhibitor herbicides.

<span class="mw-page-title-main">2,4-Dichlorophenoxyacetic acid</span> Herbicide

2,4-Dichlorophenoxyacetic acid is an organic compound with the chemical formula Cl2C6H3OCH2CO2H. It is usually referred to by its ISO common name 2,4-D. It is a systemic herbicide that kills most broadleaf weeds by causing uncontrolled growth, but most grasses such as cereals, lawn turf, and grassland are relatively unaffected.

<span class="mw-page-title-main">Mycorrhizal network</span> Underground fungal networks that connect individual plants together

Mycorrhizal associations have profoundly impacted the evolution of plant life on Earth ever since the initial adaptation of plant life to land. In evolutionary biology, mycorrhizal symbiosis has prompted inquiries into the possibility that symbiosis, not competition, is the main driver of evolution.

<i>Centaurea stoebe</i> Species of flowering plant in the family Asteraceae

Centaurea stoebe, the spotted knapweed or panicled knapweed, is a species of Centaurea native to eastern Europe, although it has spread to North America, where it is considered an invasive species. It forms a tumbleweed, helping to increase the species' reach, and the seeds are also enabled by a feathery pappus.

<span class="mw-page-title-main">Jonathan Gressel</span>

Jonathan Gressel is an Israeli agricultural scientist and Professor Emeritus at the Weizmann Institute of Science in Rehovot, Israel. Gressel is a "strong proponent of using modern genetic techniques to improve agriculture" especially in third world and developing countries such as Africa. In 2010, Gressel received Israel's highest civilian award, the Israel Prize, for his work in agriculture.

Leslie A. Weston FAA, is a plant biologist, who was awarded a Fellow of the Australian Academy of Science in 2023, for her work on weed suppressing ground covers and pest management. She is a professor at Charles Sturt University, at Wagga Wagga, and researches botany, agronomy, weed control and horticulture.

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Further reading