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. Allelochemicals are a subset of secondary metabolites, which are not required for metabolism (i.e. growth, development and reproduction) of the allelopathic organism. Allelochemicals with negative allelopathic effects are an important part of plant defense against herbivory.
The production of allelochemicals are affected by biotic factors such as nutrients available, and abiotic factors such as temperature and pH.
Allelopathy is characteristic of certain plants, algae, bacteria, coral, and fungi. 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), black crowberry (Empetrum nigrum), spotted knapweed (Centaurea stoebe), garlic mustard (Alliaria petiolata), Casuarina/Allocasuarina spp., and nutsedge.
The process by which a plant acquires more of the available resources (such as nutrients, water or light) from the environment without any chemical action on the surrounding plants is called resource competition. This process is not negative allelopathy, although both processes can act together to enhance the survival rate of the plant species.
The term allelopathy from the Greek-derived compounds allelo- (ἀλληλο-) and -pathy (παθός) (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. He used the term to describe biochemical interactions by means of which a plant inhibits the growth of neighbouring plants. 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. Confusing the issue more, zoologists have borrowed the term to describe chemical interactions between invertebrates like corals and sponges.
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. 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 competition which results when two (or more) organisms attempt to use the same limited resource, to the detriment of one or both. Allelopathy is a direct negative effect on one organism resulting from the input of substances into the environment by another. 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.
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 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 c. 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 affects 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.'" 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. maculosa.  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.
Many invasive plant species interfere with native plants through allelopathy. 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. 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 story can be found in Halsey 2004.
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.
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. The concentration of juglone by which the black walnut produces is not seen to have an affect on other plants. The leaf litter and root exudates of some Eucalyptus species 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.
The possible application of allelopathy in agriculture is the subject of much research. 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 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. 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. 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.
- Stamp, Nancy (March 2003), "Out of the quagmire of plant defense hypotheses", The Quarterly Review of Biology, 78 (1): 23–55, doi:10.1086/367580, PMID 12661508, S2CID 10285393.
- Fraenkel, Gottfried S. (May 1959), "The raison d'Etre of secondary plant substances", Science, 129 (3361): 1466–1470, Bibcode:1959Sci...129.1466F, doi:10.1126/science.129.3361.1466, PMID 13658975.
- Willis, Rick J. (2007). The History of Allelopathy. Springer. p. 3. ISBN 978-1-4020-4092-4. Retrieved 2009-08-12.
- Roger, Manuel Joaquín Reigosa; Reigosa, Manuel J.; Pedrol, Nuria; González, Luís (2006), Allelopathy: a physiological process with ecological implications, Springer, p. 1, ISBN 978-1-4020-4279-9
- Whittaker, R. H.; Feeny, P. P. (1971). "Allelochemics: Chemical Interactions between Species". Science. 171 (3973): 757–770. Bibcode:1971Sci...171..757W. doi:10.1126/science.171.3973.757. ISSN 0036-8075. JSTOR 1730763. PMID 5541160. Retrieved 2020-10-20.
- Rice, Elroy Leon (1984), Allelopathy, (first edition, november 1974 by the same editor) (Second ed.), Academic Press, pp. 422 p, ISBN 978-0-12-587058-0
- Roger, Manuel Joaquín Reigosa; Reigosa, Manuel J.; Pedrol, Nuria; González, Luís (2006), Allelopathy: a physiological process with ecological implications, Springer, p. 2, ISBN 978-1-4020-4279-9
- Chang-Hung Chou, "Introduction to allelopathy", 2006, Part 1, 1-9, doi:10.1007/1-4020-4280-9_1
- Liu D and Lovett J (1994) Biologically active secondary metabolites of barley I Developing techniques and assessing allelopathy in barley Journal of Chemical Ecology 19:2217-2230.
- Liu D and Lovett J (1994) Biologically active secondary metabolites of barley. II. Phytotoxicity of barley allelochemicals Journal of Chemical Ecology 19:2231-2244.
- Nilsson, Marie-Charlotte (1994). "Separation of allelopathy and resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup". Oecologia. 98 (1): 1–7. Bibcode:1994Oecol..98....1N. doi:10.1007/BF00326083. ISSN 0029-8549. PMID 28312789. S2CID 21769652.
- Lau, Jennifer A.; Puliafico, Kenneth P.; Kopshever, Joseph A.; Steltzer, Heidi; Jarvis, Edward P.; Schwarzländer, Mark; Strauss, Sharon Y.; Hufbauer, Ruth A. (2008). "Inference of allelopathy is complicated by effects of activated carbon on plant growth". New Phytologist. 178 (2): 412–423. doi:10.1111/j.1469-8137.2007.02360.x. ISSN 1469-8137. Retrieved 2020-11-29.
- Yoon, Carol Kaesuk (2003-09-09). "Forensic Botanists Find the Lethal Weapon of a Killer Weed - The New York Times". New York Times. Archived from the original on 2019-12-12. Retrieved 2020-11-29.
- Brendan Borrell. "NSF investigation of high-profile plant retractions ends in two debarments". Retraction Watch. Retrieved 29 November 2020.
- Shannon Palus. "Sample tampering leads to plant scientist's 7th retraction". Retraction Watch. Retrieved 29 November 2020.
- Science, A. A. for the A. of. 2010. Corrections and Clarifications. Science 327:781–781. American Association for the Advancement of Science.
- Perry, L. G., G. C. Thelen, W. M. Ridenour, R. M. Callaway, M. W. Paschke, and J. M. Vivanco. 2007. Concentrations of the Allelochemical (+/-)-catechin IN Centaurea maculosa soils. J Chem Ecol 33:2337–2344.
- Duke, S. O., F. E. Dayan, J. Bajsa, K. M. Meepagala, R. A. Hufbauer, and A. C. Blair. 2009. The case against (–)-catechin involvement in allelopathy of Centaurea stoebe (spotted knapweed). Plant Signaling & Behavior 4:422–424. Taylor & Francis.
- Craig, Murrell ; Gerber Esther ; Krebs Christine ; et al. 2011. INVASIVE KNOTWEED AFFECTS NATIVE PLANTS THROUGH ALLELOPATHY. AMERICAN JOURNAL OF BOTANY 98(1):38-43 doi:10.3732/ajb.1000135
- Douglass, Cameron H., Leslie A. Weston, and David Wolfe. 2011. Phytotoxicity and Potential Allelopathy in Pale (Cynanchum rossicum) and Black swallowwort (C. nigrum) Invasive Plant Science and Management 4(1):133-141
- Muller, C.H., Muller, W.H. and Haines, B.L. 1964. Volatile growth inhibitors produced by aromatic shrubs. Science 143: 471-473. 
- Bartholomew, B. 1970. Bare zone between California shrub and grassland communities: The role of animals. Science 170: 1210-1212. 
- 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 California Chaparral Institute also offers a PDF-format version of this paper. 
- 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 
- Cipollini, D. 2016. A review of garlic mustard (Alliaria petiolata, Brassicaceae) as an allelopathic plant. tbot 143:339–348. Torrey Botanical Society.
- Cheng, Fang; Cheng, Zhihui (2015-11-17). "Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy". Frontiers in Plant Science. 6: 1020. doi:10.3389/fpls.2015.01020. ISSN 1664-462X. PMC 4647110. PMID 26635845.
- Chalker-Scott, Linda (March 2, 2019). "Do black walnut trees have allelopathic effects on other plants?" – via research.wsulibs.wsu.edu. Cite journal requires
- K. Sasikumar, C.Vijayalakshmi and K.T. Parthiban 2001. Allelopathic effects of four eucalyptus species on redgram (Cajanus cajan L.)
- 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
- Kong, C.H., Li, H.B., Hu, F., Xu, X.H., Wang, P., 2006. Allelochemicals released by rice roots and residues in soil. Plant and Soil, 288: 47-56.
- Kong, C.H., Hu, F., Wang, P., Wu, J.L., 2008. Effect of allelopathic rice varieties combined with cultural management options on paddy field weeds. Pest management Science, 64: 276-282.
- Chen, X.H., Hu, F., Kong, C.H., 2008. Varietal improvement in rice allelopathy. Allelopathy Journal, 22: 379-384.
- Cornes, D. 2005. Callisto: a very successful maize herbicide inspired by allelochemistry. Proceedings of the Fourth World Congress on Allelopathy 
- Pheng, S., Olofsdotter, M., Jahn, G. and Adkins, S. W. (2009-12) Potential allelopathic rice lines for weed management in Cambodian rice production. Weed Biology and Management, 9 4: 259-266.
- Pheng, S., Olofsdotter, M., Jahn, G., Nesbitt, H. and Adkins, S.W. (2009-12) Allelopathic potential of Cambodian rice lines under field conditions. WEED BIOLOGY AND MANAGEMENT, 9 4: 267-275.
- Pheng, S., M. Olofsdotter, G. Jahn and S. Adkins. 2010. Use of phytotoxic rice crop residues for weed management. Weed. Biol. Manage. 10: 176-184.
- 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
- anon. (Inderjit). 2002. Multifaceted approach to study allelochemicals in an ecosystem. In: Allelopathy, from Molecules to Ecosystems, M.J. Reigosa and N. Pedrol, Eds. Science Publishers, Enfield, New Hampshire.
- Bhowmick N, Mani A, Hayat A (2016), "Allelopathic effect of litchi leaf extract on seed germination of Pea and lafa", Journal of Agricultural Engineering and Food Technology, 3 (3): 233-235.
- Blum U, Shafer SR, Lehman ME (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, doi:10.1016/S0735-2689(99)00396-2.
- 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 CH (1966), "The role of chemical inhibition (allelopathy) in vegetational composition", Bulletin of the Torrey Botanical Club, 93 (5): 332–351, doi:10.2307/2483447, JSTOR 2483447.
- 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.
- Sheeja B.D. 1993. Allelopathic effects of Eupatorium odoratum L. and Lantana camara, L. on four major crops. M. Phil dissertation submitted to Manonmaniam Sundaranar University, Tirunelveli.
- Webster 1983. Webster's Ninth New Collegiate Dictionary. Merriam-Webster, Inc., Springfield, Mass.
- Willis, R. J. (1985), "The historical bases of the concept of allelopathy", Journal of the History of Biology, 18: 71–102, doi:10.1007/BF00127958, S2CID 83639846.
- 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.
|Look up allelopathy in Wiktionary, the free dictionary.|