Allelopathy

Allelopathy describes those situations and events where chemicals produced by higher plants, algae, fungi, or microorganisms cause some effect, either inhibitory or stimulatory, on other members of the plant or microbial community . Unlike competition for a resource, the central principle in allelopathy arises from the fact that plants and microorganisms collectively produce thousands of chemicals, and many of these chemicals are released from the producing organism by leaching, exudation , volatilization , or decomposition processes. Subsequently, some of these compounds (known as allelochemicals) alter the growth or physiological functions of organisms that encounter them during growth. For example, almost pure droplets of sorgoleone (a quinone) are exuded from the roots of Sorghum species, and sorgoleone inhibits growth in plants that contact it by blocking photosynthesis and respiration. While the word "allelopathy" was first used in the 1930s, the phenomenon that it describes was suggested by natural philosophers more than two thousand years ago as they observed that some plants did not grow well near other kinds of plants.

Research conducted in the last half of the twentieth century demonstrated cases of growth inhibition by allelochemicals that influenced vegetational patterns, rate and sequences in plant succession, weed abundance, crop productivity, and problems in replanting fruit and other crops. Investigators have focused on identifying the producing plants and the chemicals they give off, the physiological effects on receiving species, and how climatic and soil conditions change the action of allelochemicals. Cinnamic and benzoic acids, flavonoids , and various terpenes are the most commonly found allelochemicals, but several hundred chemicals have been identified, including many other classes of secondary plant compounds. A few allelo-chemicals have been developed as herbicides and pesticides, and it may be possible to genetically engineer a crop to produce its own herbicides.

see also Flavonoids; Interactions, Plant-Plant.

Frank A. Einhellig

Bibliography

Inderjit, K., M. M. Dakshini, and Frank A. Einhellig, eds. Allelopathy: Organisms, Processes, and Applications. Washington, DC: American Chemical Society, 1995.

Putnam, Alan R., and Chung-Shih Tang, eds. The Science of Allelopathy. New York: John Wiley & Sons, 1986.

Rice, Elroy L. Allelopathy, 2nd ed. Orlando, FL: Academic Press, 1984.

allelopathy The release into the environment by an organism of a chemical substance that acts as a germination or growth inhibitor to another organism. Typical substances include alkaloids, terpenoids (see TERPENE), and phenolics (see PHENOL). The phenomenon was described originally for heath and scrub communities, notably the Californian chaparral, but is now thought to be a widespread anti-competition mechanism in plants (e.g., barley inhibits competing weeds by means of root secretions). It is, however, extremely difficult to demonstrate in natural ecosystems. Allelopathy is also found in other organisms (e.g., antibiotics may be produced by fungi to inhibit competing bacteria, when the term ‘antibiosis’ may be used).

allelopathy The release into the environment by an organism of a chemical substance that acts as a germination or growth inhibitor to another organism. Typical substances include alkaloids, terpenoids, and phenolics. The phenomenon was described originally for heath and scrub communities, notably the Californian chaparral, but is now thought to be a widespread anti-competition mechanism in plants (e.g. barley inhibits competing weeds by means of root secretions). It is, however, extremely difficult to demonstrate in natural ecosystems. Allelopathy is also found in other organisms (e.g. antibiotics may be produced by fungi to inhibit competing bacteria, when the term ‘antibiosis’ may be used).

Allelopathy

Derived from the Greek words allelo (other) and pathy (causing injury to), allelopathy is a form of competition among plants. One plant produces and releases a chemical into the surrounding soil that inhibits the germination or growth of other species in the immediate area. These chemical substances are both acids and bases and are called secondary compounds. For example, black walnut (Jugans nigra ) trees release a chemical called juglone that prevents other plants such as tomatoes from growing in the immediate area around each tree. In this way, plants such as black walnut reduce competition for space, nutrients, water, and sunlight.

allelopathy The secretion by plants of chemicals, such as phenolic and terpenoid compounds, that inhibit the growth or germination of other plants, with which they are competing. For example, the aromatic oils released by certain shrubs of the Californian chaparral pass into the soil and inhibit the growth of herbaceous species nearby. Some plants produce chemicals that are toxic to grazing herbivorous animals.