Allelochemicals Biologica... - Name

IMPACT OF PATHOGENS ON PLANT INTERFERENCE AND
ALLELOPATHY
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1988; Grace and Tilman, 1990; Casper and Jackson, 1997) and allelopathy (Putnam
and Duke, 1978; Rice, 1984; Inderjit et al., 1995; Anaya, 1999) already occur in the
literature.
The view of competition in the present chapter is ‘the interaction between
individuals brought about by the shared requirements for a resource in limited supply,
and leading to a reduction in the survivorship, growth and/or reproduction of the
individuals concerned’ (Begon et al., 1986). Plants mostly compete for the resources
of light, water and nutrients (Donald, 1963), and less frequently for carbon dioxide,
oxygen and space (Trenbath, 1974). In the exact sense of the definition, plants do not
compete so long as these resources are in excess of the needs of both. When plants do
compete, however, the outcome always reduces the growth of at least one of the
competitors. The outcome of competition between two plants depends on the ability
of each species to secure and utilise resources, i.e. their competitiveness. A highly
competitive plant may be one that has a high rate of uptake of a particular resource or
a low requirement for that resource (Grace and Tilman, 1990). Plants may differ in
their ability to compete for individual resources, while environmental differences may
also influence their overall competitive ability. Differences in competitive ability, in
turn, help to structure the composition of mixed plant communities.
2.2. Allelopathy
Molisch first advanced the term allelopathy in 1937. It derives from the Greek words
‘allelon’, meaning of each other, and ‘pathos’, meaning to suffer (Mandava, 1985).
Despite the origin of its root words, Molisch used the term to refer to the chemical
interactions between all plants (higher plants and microorganisms), including
stimulatory as well as inhibitory effects. Some authors have considered that the concept
covers only inhibitory effects (Rice, 1974; Putnam, 1985; Boyette and Abbas, 1995),
yet others exclude microorganisms from the definition (Putnam and Duke, 1978;
Putnam, 1985; Pratley, 1996). Many inhibitory chemicals produced by plants, however,
stimulate growth at low concentrations (Liu and Lovett, 1993; Pratley, 1996), and
microorganisms can mediate allelopathy (Rice, 1992; Bremner and McCarty, 1993).
For these reasons, the definition of allelopathy used in this chapter closely follows
that of Molisch’s, ‘the beneficial and detrimental chemical interaction among [plant]
organisms including microorganisms’ (Rice, 1984). Although some authors have
confused allelopathy with competition, the distinguishing feature is that allelopathy
involves an addition of a chemical to the environment, whereas competition involves
the shared utilisation of some limited factor required for growth (Muller, 1969; Rice
1984).
Any chemical produced by a plant (donor) that stimulates or inhibits the growth
of a neighbour (receiver or receptor) is broadly termed an allelochemical. Typically,
allelochemicals are secondary metabolites (Whittaker and Feeney 1971; Rice, 1984;
Rizvi et al., 1992), produced as by-products of the acetate and shikimic acid pathways.
They may also form as degradation products from the action of microbial enzymes