Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the 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. 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 herbivoryPlant defense against herbivory or host-plant resistance describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants use several strategies to defend against damage caused by herbivores...
.
Allelopathy is characteristic of certain plants,
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
,
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
,
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
, and fungi. Allelopathic interactions are an important factor in determining
species distributionSpecies distribution is the manner in which a biological taxon is spatially arranged. Species distribution is not to be confused with dispersal, which is the movement of individuals away from their area of origin or from centers of high population density. A similar concept is the species range. A...
and abundance within plant
communitiesA biocoenosis , coined by Karl Möbius in 1877, describes the interacting organisms living together in a habitat . This term is rarely used in English, as this concept has not been popularized in Anglophone countries...
, and are also thought to be important in the success of many invasive plants. For specific examples, see Spotted Knapweed (
Centaurea maculosaCentaurea maculosa, the spotted knapweed, is a species of Centaurea native to eastern Europe.It has been introduced to North America, where it is considered an invasive plant species in much of the western United States and Canada. In 2000, C. maculosa occupied more than in the US.Knapweed is a...
), Garlic Mustard (
Alliaria petiolata), and
NutsedgeCyperus rotundus is a species of sedge native to Africa, southern and central Europe , and southern Asia. The word cyperus derives from the Greek "κύπερος" and rotundus is from Latin, meaning "round"...
.
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.
History
The term allelopathy, from the
GreekAncient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
-derived compounds
allelo- and
-pathy (meaning "mutual harm" or "suffering"), was first used in 1937 by the Austrian professor
Hans MolischHans Molisch was a Czech-Austrian botanist.He taught as a professor at the German University of Prague , Vienna University , Tohoku Imperial University , and a university in India.From 1931 to 1937 he acted as the vice-president of the Austrian...
in the book
Der Einfluss einer Pflanze auf die andere - Allelopathie (The Effect of Plants on Each Other) published in German. He used the term to describe biochemical interactions that inhibit the growth of neighbouring plants, by another plant. In 1971, Whittaker and Feeny published a study in the journal
Science, which defined allelochemicals as all chemical interactions among organisms. In 1984, Elroy Leon Rice in his
monographA monograph is a work of writing upon a single subject, usually by a single author.It is often a scholarly essay or learned treatise, and may be released in the manner of a book or journal article. It is by definition a single document that forms a complete text in itself...
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 environmentThe natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....
. Over the next ten years, the term was used by other researchers to describe broader chemical interactions between organisms, and by 1996 the International Homeopathy Society 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
invertebrateAn invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s like
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s and sponges.
Long before the term allelopathy was used, people observed the negative effects that one plant could have on another.
TheophrastusTheophrastus , a Greek native of Eresos in Lesbos, was the successor to Aristotle in the Peripatetic school. He came to Athens at a young age, and initially studied in Plato's school. After Plato's death he attached himself to Aristotle. Aristotle bequeathed to Theophrastus his writings, and...
, who lived around 300 BC noticed the inhibitory effects of
pigweedPigweed can mean any of a number of weedy plants which may be used as pig fodder:* Amaranthus species** Amaranthus palmeri, the 'pigweed' resistant to glyphosate in the US southeast.* Chenopodium album * Portulaca species...
on
alfalfaAlfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
. In China around the first century AD, the author of
Shennong Ben Cao Jing described 267 plants that had pesticidal abilities, including those with allelopathic effects. In 1832, the Swiss botanist De Candolle suggested that crop plant
exudateAn exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation. It can apply to plants as well as animals. Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood...
s were responsible for an agriculture problem called soil sickness.
Allelopathy has not been universally accepted among ecologists, and up to the early part of this century many ecologists argued that the effects of competition could not be distinguished from so called allelopathy. Competition is a negative effect which presents when two or more organisms attempt to directly use the same resource. Allelopathy, on the other hand differs by indirectly affecting other organisms after the input of substances into the environment. 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.
Examples of allelopathy
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
herbicideHerbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s, 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 thermochemical in lemon bottlebrush (
Callistemon citrinusCallistemon citrinus, also known as Crimson Bottlebrush is a shrub in the family Myrtaceae. It is native to the states of Queensland, New South Wales and Victoria in Australia, occurring in the vicinity of rocky streams and near-coastal swamps....
). Although it was found to be too weak as a commercial herbicide, a chemical analog of it,
mesotrioneMesotrione is an herbicide sold under the brand names Callisto and Tenacity. It is a synthetic analog of leptospermone developed to mimic the effects of this natural herbicide. Mesotrione works by inhibiting the plant enzyme 4-hydroxyphenylpyruvate dioxygenase which is necessary for carotenoid...
(tradename Callisto), was found to be effective. It is sold to control broadleaf
weedA weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
s in corn but also seems to be an effective control for crabgrass in
lawnA lawn is an area of aesthetic and recreational land planted with grasses or other durable plants, which usually are maintained at a low and consistent height. Low ornamental meadows in natural landscaping styles are a contemporary option of a lawn...
s. Sheeja (1993) reported the allelopathic interaction of the weeds
Chromolaena odorataChromolaena odorata is a species of flowering shrub in the sunflower family, Asteraceae. It is native to North America, from Florida and Texas to Mexico and the Caribbean, and has been introduced to tropical Asia, west Africa, and parts of Australia. Common names include Siam Weed, Christmas Bush,...
(
Eupatorium odoratum) and
Lantana camaraLantana camara, also known as Spanish Flag or West Indian Lantana, is a species of flowering plant in the verbena family, Verbenaceae, that is native to the American tropics. It has been introduced into other parts of the world as an ornamental plant and is considered an invasive species in many...
on selected major crops.
Many crop
cultivarA cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...
s show strong allelopathic properties, of which rice (
Oryza sativaOryza sativa, commonly known as Asian rice, is the plant species most commonly referred to in English as rice. Oryza sativa is the cereal with the smallest genome, consisting of just 430Mb across 12 chromosomes...
) 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.
Many invasive plant species interfere with native plants through allelopathy. A famous case of purported allelopathy is in
desertA desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
shrubA shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
s. One of the most widely known early examples was
Salvia leucophyllaSalvia leucophylla is an aromatic sage native to the southern coastal mountain ranges of California and Baja California.-Taxonomy:The plant's specific epithet, leucophylla, describes the light grayish leaves...
, because it was on the cover of the journal
ScienceScience is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
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.
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 walnutJuglans nigra, the Eastern Black walnut, is a species of flowering tree in the hickory family, Juglandaceae, that is native to eastern North America. It grows mostly in riparian zones, from southern Ontario, west to southeast South Dakota, south to Georgia, northern Florida and southwest to central...
(Juglans nigra) produces the allelochemical
jugloneJuglone, also called 5-hydroxy-1,4-naphthalenedione or 5-hydroxynaphthoquinone, is an 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...
, which affects some species greatly while others not at all.
EucalyptusEucalyptus is a diverse genus of flowering trees in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia...
leaf litter and root exudates are allelopathic for certain soil microbes and plant species. The
tree of heavenAilanthus altissima , commonly known as tree of heaven, ailanthus, or in Standard Chinese as chouchun , is a deciduous tree in the Simaroubaceae family. It is native to both northeast and central China and Taiwan. Unlike other members of the genus Ailanthus, it is found in temperate climates rather...
,
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.
Garlic mustardGarlic mustard is a biennial flowering plant in the Mustard family, Brassicaceae. It is native to Europe, western and central Asia, and northwestern Africa, from Morocco, Iberia and the British Isles, north to northern Scandinavia, and east to northern India and western China...
is an invasive plant species in North American
temperate forestTemperate forests correspond to forest concentrations formed in the northern hemisphere. Main characteristics include: wide leaves, big and tall trees and non seasonal vegetation...
s. 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.
A study of
Kochia scopariaBassia scoparia is a shrub which is native to Eurasia. It has introduced populations in many parts of North America, where it is found in grassland, prairie, and desert shrub ecosystems...
in northern
MontanaMontana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
by two high school students showed that when
Kochia precedes spring wheat (
Triticum aestivum), it reduces the spring wheat's growth. Effects included delayed emergence, decreased rate of growth, decreased final height and decreased average vegetative dry weight of spring wheat plants. A larger study later showed that
Kochia seems to exhibit allelopathy on various crops in northern Montana.
Corn gluten meal (CGM) is a natural preemergence weed control used in turfgrass, which reduces germination of many broadleaf and grass weeds.
Further reading
- 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.
- 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.
- 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. 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.
- 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.