Allelopathy is an important mechanism of plant interference mediated by the addition of plant-produced phytotoxins to the plant environment and competitive strategy of plants (Muller 1969, Chou and Lin 1976, Rice 1984, Fischer et al. 1994, Langenheim 1994). Allelochemicals are released from plant tissue in a variety of ways including emission of volatile substances from living plant parts, exudation from roots, or leaching from above ground parts by rain, dew, fog, etc. (Rice 1984). Many researchers have found that inhibitory substances involved in allelopathy are terpenoids, and phenolic substances (Carballeira 1980, Muller 1965, Kil and Yim 1983, Weidenahmer et al. 1994, Seigler 1996). A wide array of biologically active constituents is produced by plants in the genus Artemisia (Marco and Barbera 1990). The volatile essential oil of Artemisia species resulted in reduction in seedling survival (Lydon et al. 1997, Kil et al. 1992). The volatile oil of Artemisia afra has been reported to have several biological activities, notably antibacterial, antifungal and anti-oxidative properties. Monoterpene vapours may cause anatomical and physiological changes in plant seedlings and exposure to volatile terpenes can lead to accumulation of lipid globules in the cytoplasm, reduction in organelles including mitochondria and disruption of membranes surrounding mitochondria and nuclei (Lorber and Muller 1976). The root tip cells subjected to the alkaloids gramine and hordenine caused damages to the cell walls, disorganization of organelles, increase cell vacuoles, and the appearance of lipid and globules, showing food reserves (Liu and Lovett 1993). Large amounts of monoterpene hydrocarbons and/or sesquiterpenes are found to lower the antimicrobial activity of essential oils (Chalchat et al. 1997).
