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 that is characteristic of some plants, algae, bacteria, coral and fungi by which they produce certain biochemicals that influence the growth and development of other organisms. The biochemicals, called allelochemicals can have a beneficial or detrimental effect on neighbouring organisms. They are not required for metabolism, that is the primary life processes (growth, development and reproduction) of the allelopathic organism, thus they are secondary metabolites. The beneficial effect can be called positive allelopathy, the detrimental effect negative allelopathy.

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, but both processes can act together to enhance the survival rate of the plant species.

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.^{[citation needed]} 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.

[edit] History

The term allelopathy, from the Greek words allelo and pathy (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 published in German.^[1] He used the term to describe biochemical interactions that inhibit the growth of neighbouring plants, by another plant.^[2] In 1971 Whittaker and Fenny in an influential paper published in the journal Science, defined Allelochemics as all chemical interactions among organisms.^[1] 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-orgamisms by the liberation of biochemicals into the environment.^[3] 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 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."^[4] In more recent times, plant researches have begun to switch back to the original definition of substances that are produced by one plant that inhibit another plant.^[1] Confusing the issue more, zoologists have borrowed the term to describe chemical interactions between invertebrates like corals and sponges.^[1]

Long before the term allelopathy was used, people observed the negative effects that one plant could have on another. Theophrastus, who lived around 300 B.C. noticed the inhibitory effects of pigweed on alfalfa. In China around the first century A.D. Yang and Tang described 267 plants that had pesticidal abilities, including those with allelopathic effects. The Swiss botanist De Candolle, in 1832 suggested that crop plant exudates 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 affect that happens when two or more organisms attempt to directly use the same resource, allelopathy, on the other hand differs by indirectly affecting other organism after the input of substances into the environment. In the 1970's great effort went into distinguishing competitive and allelopathic effects by some researchers, while in the 1990's others argued that the effects were often interdependent and could not readily be distinguished.^[1]

[edit] 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.^[5] 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.

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.^[6] 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.^[7] A detailed history of this interesting story can be found in Halsey 2004.^[8]

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.^[9]

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.^[10]

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.^[11]

A study of Kochia scoparia in northern Montana by two high school students^[12] 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.^[13] A larger study later showed that Kochia seems to exhibit allelopathy on various crops in northern Montana. ^[14]

[edit] References

[edit] 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.
• 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 bases of the concept of allelopathy. Journal of the History of Biology, 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.
• 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.

[edit] External links

• International Allelopathy Society