Introduction to allelopathy

Abstract

Theophrastus (372-285, BC), a disciple of Aristole, reported an example of the inhibitory effect of pigweed on alfalfa (Jelenic, 1987). Yang and Tang (1988) made an extensive review on plants used for pest control as described in Shengnong Ben Tsao Jing in 25-220A.D. in China. They described 267 plants considered to have pesticidal activity, and many of them also exhibited allelopathic potential. Lee Shi-Jen (1518-1593, AD), a famous Chinese pharmacologist, wrote a book on Chinese medicinal herbs, illustrating the toxic and nutritious natures of chemical constituents to organisms, particularly to humans. He also indicated that the plant constituents might be affected by habitats. In 1832, De Candolle, a Swiss botanist, suggested that the soil sickness problem in agriculture might be due to exudates of crop plants (see Rice, 1984). Later, Hoy and Stickney (1881) reported a deleterious effect of black walnut trees on the growth of plants nearly. Schreiner and Reed (1907,1908) found some soil organic acids, which were originally released by plant roots, suppressed the growth of some crops. Years later, Molisch (1937) coined a term Allelopathy from two Greek words of "Allelo" and "pathy" meaning "mutual harm", expressing that a natural phenomenon of one plant releases inhibitory substance which inhibits the growth of other plant sharing the same habitat. Rice (1984) further defined the allelopathy as both stimulatory and inhibitory effects of one plant upon another including microorganisms. Among other terms, antibiotic refers to a chemical produced by a microorganism and effective against other microorganisms; marasmin concerns a chemical produced by a microorganism and active against a higher plant; and Koline is for chemicals produced by higher plants and effective against higher plant (Grummer, 1955). On the other hand, autointoxication is one plant produces toxic substance(s) which inhibit the growth of its own (Chou and Lin, 1976; Chou, 1999). In the early 20th century, both allelopathy and autointoxication received a great attention in agricultural productivity, in particular to a continuous monoculture causing yield reduction of crops (Bode, 1940; Börner, 1960; Evenari, 1949; Havis and Gilkason, 1947; Patick, 1955). Until the late 1960, the allelopathic concept was firstly applied to plant ecology in elucidating the mechanism of plant interference, such as plant dominance, succession, and climax formation (Muller, 1969). Salvia leucophylla, a California soft chaparral produced monoterpenes, namely α-pinine, β-pinine, cineole, camphore, which suppressed the growth of many herbaceous plants nearby, resulting in a unique bare, inhibition, and normal growth zones (Muller, 1966). Muller and his students also contributed significant findings on other chaparral shrubs, such as Adenostoma fasciculatum (McPherson and Muller, 1969), Arctostaphylos glandulosa var. zacaensis (Chou and Muller, 1972). Muller rather used plant interference, including both competition and allelopathy, and defined competition to mean that one plant takes up necessary substances form a habitat so as to have a harmful effect on the growth of other plant that required the same substances. On the other hand, allelopathy is the process that plant releases phytotoxic compounds into the environment to inhibit the growth of plant sharing the same habitat. Whittaker and Feeny (1971) published a classic paper entitled "Allelochemics: chemical interaction between species", and stated that "chemical agents are of major significance in adaptation of species and organization of communities." Allelopathy thus plays a significant role in plant dominance, succession, formation of plant communities and climax vegetation, and crop productivity (Muller, 1969; Rice 1984; Rizvi and Rizvi, 1992; Chou, 1999). Furthermore, a term "allelochemical" derived from allelochemics was firstly used by Chou and Waller (1983), and has become popular in agricultural science, dealing with the mechanism of chemical interactions among organisms, such as plant-plant, plant-insect, insect-insect, plant-microorganism, and microorganism-microorganism. Finally, the definition of allelopathy is confirmed by the International Allelopathy Society in 1996 meant that "any process involving secondary metabolites produced by plants, algae, bacteria, and fungi that influences the growth and development of agriculture and biological systems". In other words, allelopathic compounds may regulate plant growth and development, involving photosynthesis, respiration, transpiration, biochemical metabolism and even in molecular basis of protein and nucleic acid synthesis.