Allelopathy

thumb|[[Casuarina equisetifolia 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. Allelochemicals are a subset of secondary metabolites, which are not directly required for metabolism (i.e. growth, development and reproduction) of the allelopathic organism.

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 nut grass (Cyperus rotundus).

Allelopathy is classified as a biotic factor, as it involves chemical interactions between living organisms, most commonly among plants. In allelopathic interactions, certain species release chemical compounds into the environment that inhibit the germination, growth, or reproduction of neighboring organisms. This process provides a competitive advantage to the allelopathic species by directly interfering with the development of potential competitors.

Allelopathy is frequently mistaken for resource competition, another biotic factor in which organisms compete for limited abiotic resources such as sunlight, water, and soil nutrients. 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. 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. 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." 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. 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. 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. 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.

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. 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.'" 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. 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. More recently, a 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. The use of allelochemicals in agriculture provides for a more environmentally friendly approach to weed control, as they do not leave behind residues. Currently used pesticides and herbicides leak into waterways and result in unsafe water quality. 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.

Mechanisms

Allelochemical interactions between plants can be performed through various mechanisms, which continue to be studied and refined through ongoing research. Evidence indicates that these compounds can influence plant growth by inhibiting germination, suppressing growth, and disrupting reproductive processes through toxic substance emissions.

Germination Inhibitor

A germination inhibitor is a chemical compound that prevents seed sprouting by disrupting the signals required for germination. (−)-Catechin is a naturally occurring antioxidant released by spotted knapweed (Centaurea stoebe) and is an example of a potential germination inhibitor. This species produces significantly higher levels of (−)-catechin compared to other plants, facilitating its competitive advantage over native vegetation, including forbs and grasses.

In addition to (−)-catechin, plants such as big sagebrush (Artemisia tridentata) emit volatile compounds including camphor, monoterpene, cineole, and methyl jasmonate (MeJA), all of which have shown qualities to inhibit seed germination. Methyl jasmonate (MeJA), in particular, is highly effective at preventing the germination of native tobacco seeds. Furthermore, when sagebrush is subjected to herbivory, it releases up to 1000 times more MeJA, which further suppresses the germination of nearby plant species. This phenomenon demonstrates how plants use chemical signals to influence interspecific competition and improve their chances of survival. Although these studies mentioned have shown effects on plants when reviewed in a laboratory environment, it continues to be reviewed as research of allelopathic seed germination is difficult to identify and conclude as the determining factor as competition and other a biotic factors cannot be reasoned out as the contributing factor.

Growth and Reproduction Suppressor

Allelopathic plants release chemical compounds that specifically inhibit the growth and reproductive processes of neighboring plant species. A well-known example is Johnson grass (Sorghum halepense), which synthesizes the allelochemical sorgoleone. This compound plays a critical role in the plant's competitive ability by suppressing the growth and reproductive success of other species. Research has demonstrated that johnson grass significantly affects the distribution of neighboring plants by inhibiting both their growth and reproductive functions.

Growth chamber experiments have shown that leachates from the shoots and roots of Johnson grass substantially reduce the growth and reproductive output of little bluestem (Schizachyrium scoparium), demonstrating the direct effects of allelopathy on plant community dynamics. This inhibition of growth and reproduction promotes the dominance of Johnson grass in areas where it occurs, thereby altering the composition of local plant communities.

References

External links

Allelopathy Journal
International Allelopathy Society

Mechanisms

Germination Inhibitor

Growth and Reproduction Suppressor

See also

References

Further reading

External links