Allelopathy means that one plant harms another with specific biomolecules. While it is sometimes considered the opposite of symbiosis, it is also a component of symbiosis.
Conceptually, biomolecules (specifically termed allelochemicals) produced by a plant are released into the environment and subsequently influence the growth and development of neighbouring plants. It is important to keep in mind that allelopathy involves the addition of a chemical compound or compounds (secondary metabolites) into the environment, while resource competition involves the removal or reduction of some factor or factors in the environment (such as nutrients, water, or light).
Although allelopathic science is a relatively new field of study, there exists convincing evidence that allelopathic interactions between plants play a crucial role in both natural and manipulated ecosystems. These interactions are undoubtedly an important factor in determining species distribution and abundance within some plant communities. Allelopathic interactions are also thought to be an important factor in the success of many invasive plants. For specific examples, see Spotted Knapweed, Garlic Mustard (Alliaria petiolata), and Nutsedge.
Mechanisms of action
There are hundreds of secondary metabolites in the plant kingdom, and many are known to be phytotoxic (Einhellig, 2002). Allelopathic effects of these compounds are often observed to occur early in the life cycle, causing inhibition of seed germination and/or seedling growth. The compounds exhibit a wide range of mechanisms of action, from effects on DNA (alkaloids), photosynthetic and mitochondrial function (quinones), phytohormone activity, ion uptake, and water balance (phenolics). Interpretations of mechanisms of action are complicated by the fact that individual compounds can have multiple phytotoxic effects (Einhellig, 2002).
Demonstrating allelopathy in nature
The vast majority of allelopathy research attempts to focus on direct negative plant-plant interactions caused by allelochemicals. One of the greatest challenges of this approach is showing that the effect is direct, since allelochemicals can have indirect effects on plant species through interaction with biotic (e.g. mycorrhizae) and/or abiotic soil factors (e.g. nutrient availability; anon., 2002). In terrestrial systems, the soil plays an important role as the matrix through which potential allelochemicals pass. Both abiotic and microbial decomposition will have significant effects on the concentration of allelochemicals reaching other plants.
Proving that allelopathy is occurring is difficult for the reason that it is difficult to separate the effects of allelopathy from those due to resource competition (e.g., for space, light, water, nutrients or CO2). Controlled greenhouse studies that allow for examination of a single independently varying factor may be of little interest since the factors do not vary independently in nature. Willis (1985) required that six criteria be met, and even when maximally relaxed to just three, proving allelopathy is rarely if ever accomplished (Blum et al., 1999).
- pattern of inhibition of one species by another
- putative aggressor must produce a toxin
- known mode of release of this toxin
- toxin transport or accumulation in the environment
- toxin affects the metabolism of neighbouring plants
- observed pattern of inhibition cannot be solely explained by physical competition, relative fitness for the environment, or other factors
Role of plant stress
Allelopathy also interacts with plant stress, because stressed source plants often release a greater array and concentration of allelochemicals, and stressed target plants may be more susceptible to allelochemicals (Reigosa et al., 2002). Measurement of the effects of allelochemicals along stressor gradients should help to elucidate the relationship between allelopathy and stress.
Examples of allelopathy
One of the most studied aspects of allelopathy is the role of allelopathy in agriculture. Current research is focused on the effects of weeds on crops, crops on weeds (Pheng et al. 1999), and crops on crops. This research furthers the possibility of using allelochemicals as growth regulators and natural herbicides (a number of them are either commercially available or in the process of large-scale manufacture) to promote sustainable agriculture. Leptospermone is a purported allelochemical in lemon bottlebrush (Callistemon citrinus). It was investigated as a possible commercial herbicide but was found to be too weak. However, a chemical analog of leptospermone was found to be an effective herbicide. The analog is mesotrione, tradename Callisto. It is sold to control broadleaf weeds in corn but also seems to be an effective control for crabgrass in lawns.
One of the most famous cases of purported allelopathy is in desert shrubs. Salvia leucophylla was one of the most widely known early examples because it was on the cover of the journal Science in 1964. 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. A detailed history of this interesting story can be found in Halsey 2004.
In other studies allelopathy has been demonstrated to play a crucial role in forests, influencing the composition of the vegetation growth, while also providing an explanation for the patterns of forest regeneration. The black walnut (Juglans nigra) produces juglone, an allelopathic substance that interferes with the growth of other plants. Juglone is somewhat selective, with certain species greatly affected by it and others not affected at all. Eucalyptus leaf litter and root exudates are allelopathic for certain soil microbes and plant species. The tree of heaven, (Ailanthus altissima) produces allelopathic substances in its roots that inhibit the growth of many plants. Furthermore, the pace of evaluating allelochemicals released from higher plants in nature has greatly accelerated, with promising results in field screening..
Many crop cultivars showed strong allelopathic properties, of which rice (Oryza sativa) has been most studied. Rice allelopathic activity is variety dependent and origin dependent, where Japonica rice shows greater allelopathic activity than Indica and Japonica-Indica hybrid. More recently, critical review on rice allelopathy and the possibility for weed management of Khanh et al.reported that allelopathic characteristics in rice are quantitatively inherited and several allelopathy-involved traits have been identified.
Plant species Garlic mustard is an invasive plant in North American temperate forests. Its success may be partly due to its excretion of a not yet identified allelochemical that interferes with mutualisms between native tree roots and their mycorrhizal fungi.
A study of kochia (Kochia scoparia) in North Toole County (NTC), Montana by two high school students showed that when kochia precedes spring wheat (Triticum aestivum), it reduces the spring wheat's performance. Among these effects are delayed emergence, decreased rate of growth, decreased final height, and decreased average vegetative dry weight of spring wheat plants. This small study was followed by another which further showed that kochia does seem to exhibit allelopathic effects on various crops grown in northern Montana.  For their work in this area, Overcast & Cox were awarded a first place team prize at the International Science and Engineering Fair (ISEF) in 2001.
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