Allelochemicals Biologica... - Name

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SCOTT W. MATTNER
(Putnam, 1985). Initially, the reason why plants devote resources to the production of
these compounds was not understood as they were regarded as functionless waste
products (Mothes, 1955). It is now increasingly accepted, however, that these
compounds function as defensive agents against pathogens, insects and neighbouring
plants (allelopathy). There is an enormous diversity of allelochemicals produced by
plants (Bansal, 1994), with classification based on chemical structure (Whittaker and
Feeney, 1971; Mandava, 1985; Putnam 1985) or on origin and chemical properties
(Rice, 1984). For example, Rice (1984) recognised 14 main categories of
allelochemicals. Of these groups, however, the phenolics are considered by many as
the most important (Putnam and Duke, 1979; Mandava, 1985; Inderjit, 1996).
Putative allelochemicals have been isolated from a variety of different plant organs,
including shoots, roots, flowers, rhizomes, fruit and seed (Rice, 1984). They are
stored in cell vacuoles so as not to interfere with the donor plant itself (Chou, 1989).
Furthermore, secondary metabolites may be bound to sugars as glycosides or occur as
polymers and crystals rendering them ineffective against the donor (Whittaker and
Feeney, 1971). The release of allelochemicals into the environment may occur through
volatilisation, root exudation, leaching or plant residue decomposition (Rice, 1984).
Allelochemicals induce a wide range of symptoms in receiver plants, ranging
from sudden wilting and death (e.g. tomato (Lycopersicon esculentum) grown in the
vicinity of black walnut (Juglans nigra), Hale and Orcutt, 1987), to the more common
subtle changes in growth. Determination of this reaction depends on the mode of
action, the concentration, and the susceptibility of the receiver plant to the
allelochemical. Allelopathic effects may be direct, such as affecting plant metabolism
and growth, or indirect, such as altering of soil properties and nutrient status (Inderjit
and Weiner, 2001). Rizvi et al. (1992) explained 12 plant functions that allelochemicals
may affect, including membrane permeability, stomata function and photosynthesis,
and cytology and ultrastructure.
3. THE INFLUENCE OF PATHOGENS ON INTERFERENCE
AND ALLELOPATHY
Natural plant populations increase in production and number of individuals until
constrained by environmental limitations (Burdon, 1987). The constraint of plant
growth by the environment is often mediated through plant interference. Therefore,
the ability of plants to interfere with their neighbours is important in determining
their abundance in a community. Plant pathogens generally reduce the development,
production and longevity of their hosts. In plant communities, this ‘burden of a
parasite’ may result in a partly unoccupied niche that resistant plants re-inhabit.
Overall, pathogens play a significant role in the ecology of plant communities by
maintaining (Peters and Shaw, 1996) or reducing (Burdon, 1991) species diversity,
driving succession (Van der Putten et al., 1993), ensuring plants do not establish
under their parents (Augsberger, 1984) and help determine the long-term composition
of plant communities (Dobson and Crawley, 1994).