Allelopathy

Allelopathy is the inhibition of growth of a plant due to biomolecules released by another. It is the opposite of symbiotic mutualism. The biomolecules are called "allelochemicals" and are produced by some plants as secondary metabolites. When the allelochemicals are released into the environment, they inhibit the development of neighbouring plants.

By contrast, resource competition involves the reduction of growth factors (such as nutrients, water or light) from the environment.

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.Fact|date=November 2007 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 ("Centaurea maculosa"), Garlic Mustard ("Alliaria petiolata"), and Nutsedge.

Mechanisms of action

There are hundreds of secondary metabolites in the plant kingdom, and many areknown 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. Abiotic and microbial decomposition significantly affect the concentration of allelochemicals reaching other plants.

Proving that allelopathy is occurring is difficult because 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) stupulated six criteria for proving allelopathy:

# pattern of inhibition of one species by another
# production of toxin by the putative aggressor
# 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

And even when relaxed to just three of these conditions, proving allelopathy is rarely if ever accomplished (Blum et al., 1999).

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 targetplants 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, and crops on crops. This research furthers the possibility of using allelochemicals as growth regulators and natural herbicides, to promote sustainable agriculture. A number of such allelochemicals are commercially available or in the process of large-scale manufacture. For example, Leptospermone is a purported 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. [Cornes, D. 2005. Callisto: a very successful maize herbicide inspired by allelochemistry. "Proceedings of the Fourth World Congress on Allelopathy" [http://www.regional.org.au/au/allelopathy/2005/2/7/2636_cornesd.htm] ] It is sold to control broadleaf weeds in corn but also seems to be an effective control for crabgrass in lawns.

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. [Muller, C.H., Muller, W.H. and Haines, B.L. 1964. Volatile growth inhibitors produced by aromatic shrubs. "Science" 143: 471-473. [http://www.sciencemag.org/cgi/content/abstract/143/3605/471] ] 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. [Bartholomew, B. 1970. Bare zone between California shrub and grassland communities: The role of animals. "Science" 170: 1210-1212. [http://www.sciencemag.org/cgi/content/abstract/170/3963/1210] ] A detailed history of this interesting story can be found in Halsey 2004. [Halsey, R.W. 2004. In search of allelopathy: An eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral. "Journal of the Torrey Botanical Society" 131: 343-367. The [http://www.californiachaparral.org/ California Chaparral Institute] also offers a PDF-format version of this paper. [http://www.findarticles.com/p/articles/mi_qa4017/is_200410/ai_n11850358] ]

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. The black walnut "(Juglans nigra)" produces the allelochemical juglone, which affects some species greatly while others not at all. "Eucalyptus" leaf litter and root exudates are allelopathic for certain soil microbes and plant species. The tree of heaven, "(Ailanthus altissima)" produces allelochemicals in its roots that inhibit the growth of many plants. The pace of evaluating allelochemicals released by higher plants in nature has greatly accelerated, with promising results in field screening. [ Khanh, T.D, Hong, N.H., Xuan, T.D. Chung, I.M. 2005. Paddy weed control by medical and leguminous plants from Southeast Asia .Crop Protection [doi:10.1016/j.cropro.2004.09.020] ]

Many crop cultivars show strong allelopathic properties, of which rice ("Oryza sativa") has been most studied. Rice allelopathy depends on variety and origin: Japonica rice is more allelopathic than Indica and Japonica-Indica hybrid. 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. [ Khanh, T.D, Xuan, T.D.and Chung, I.M.2007. Rice allelopathy and the possibility for weed management. Annals of Applied Biology [doi:10.1111/j.1744-7348.2007.00183.x] ]

Garlic mustard is an invasive plant species in North American temperate forests. Its success may be partly due to its excretion of an unidentified allelochemical that interferes with mutualisms between native tree roots and their mycorrhizal fungi. [Stinson, K.A., Campbell, S.A., Powell, J.R., Wolfe, B.E., Callaway, R.M., Thelen, G.C., Hallett, S.G., Prati, D., and Klironomos, J.N. 2006. Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms. "PLoS Biology" [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040140] ]

A study of kochia ("Kochia scoparia") in northern Montana by two high school students [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.] showed that when kochia precedes spring wheat ("Triticum aestivum"), it reduces the spring wheat's performance. Effects included delayed emergence, decreased rate of growth, decreased final height, and decreased average vegetative dry weight of spring wheat plants. [ M.C. Overcast, J.J. Brimhall. 2000. Allelopathic Effects of Selected Weed Exudates on Germination and Early Growth of Triticum aestivum in Northern Toole County, Montana. [http://agnotes.org/AgNotes/docs/255.htm] ] A larger study later showed that kochia seems to exhibit allelopathy on various crops in northern Montana. [ M.C. Overcast, D.R. Cox. 2001. Effects of Allelochemicals Produced by Kochia scoparia on Selected Crops Grown in North Toole County (NTC), Montana.]

References

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*Blum U., S. R. Shafer, and M. E. Lehman. 1999. Evidence for inhibitory allelopathic interactions involving phenolic acids in field soils: concepts vs. an experimental model. "Critical Reviews in Plant Sciences", 18(5):673-693.
*Einhellig, F.A. 2002. The physiology of allelochemical action: clues and views. "In": "Allelopathy, from Molecules to Ecosystems", M.J. Reigosa and N. Pedrol, Eds. Science Publishers, Enfield, New Hampshire.
*Harper, J. L. 1977. "Population Biology of Plants". Academic Press, London.
*Jose S. 2002. Black walnut allelopathy: current state of the science. "In": "Chemical Ecology of Plants: Allelopathy in aquatic and terrestrial ecosystems", A. U. Mallik and anon. (Inderjit), Eds. Birkhauser Verlag, Basel, Switzerland.
*Mallik, A. U. and anon. (Inderjit). 2002. Problems and prospects in the study of plant allelochemicals: a brief introduction. "In": "Chemical Ecology of Plants: Allelopathy in aquatic and terrestrial ecosystems", Mallik, A.U. and anon., Eds. Birkhauser Verlag, Basel, Switzerland.
*Muller C. H. 1966. The role of chemical inhibition (allelopathy) in vegetational composition. "Bull. Torrey Botanical Club", 93:332-351.
*Reigosa, M. J., N. Pedrol, A. M. Sanchez-Moreiras, and L. Gonzales. 2002. Stress and allelopathy. "In": "Allelopathy, from Molecules to Ecosystems", M.J. Reigosa and N. Pedrol, Eds. Science Publishers, Enfield, New Hampshire.
*Rice, E.L. 1974. "Allelopathy". Academic Press, New York.
*Webster 1983. "Webster's Ninth New Collegiate Dictionary". Merriam-Webster, Inc., Springfield, Mass.
*Willis, R. J. 1985. The historical basis of the concept of allelopathy. "J. Hist. Bio.", 18: 71-102.
*Willis, R. J. 1999. Australian studies on allelopathy in "Eucalyptus": a review. "In": "Principles and practices in plant ecology: Allelochemical interactions", anon. (Inderjit), K.M.M. Dakshini, and C.L. Foy, Eds. CRC Press, and Boca Raton, FL.