Allelochemicals Biologica... - Name

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ANA LUISA ANAYA
and accessibility to external inputs -seeds, crop cultivars, machinery, etc.) and socioeconomic
constraints (market, tenure status, attitude towards entrepreneurial risk,
etc.). The use of cover crops and organic amendments, via the promotion of diversity
in insect, fungal, bacterial or mychorrhyzal communities, may alter antagonist or
competitive effects to the benefit of crops and to the detriment of weeds. Once factors
driving these effects are better understood, it might be possible to use this knowledge
to improve organic weed management systems locally. It would also be helpful to find
indicators of “functional biodiversity”, where weed species abundance is assessed on
the role that they have in the agroecosystem (e.g. strong /weak competitors, promoters
of the presence of beneficial arthropods, etc.). Management of allelopathy is another
potential tool in the arsenal of the organic farmer (Barberi, 2002). In the United
States, the rate of increase of organic growers was estimated at 12% in 2000. However,
many producers are reluctant to undertake the organic transition because of
uncertainty of how organic production will affect weed population dynamics and
management. The organic transition has a profound impact on the agroecosystem.
Changes in soil physical and chemical properties during the transition often impact
indirectly insect, disease, and weed dynamics. Greater weed species richness is usually
found in organic farms but total weed density and biomass are often smaller
under the organic system compared with the conventional system. The improved weed
suppression of organic agriculture is probably the result of combined effects of several
factors including weed seed predation by soil microorganisms, seedling predation by
phytophagus insects, and the physical and allelopathic effects of cover crops (Ngouajio
and McGiffen, 2002).
2. ALLELOPATHY
Increasing attention has been given to the role and potential of allelopathy as a management
strategy for crop protection against weeds and other pests. Incorporating
allelopathy into natural and agricultural management systems may reduce the use of
herbicides, fungicides, nematicides, and insecticides, cause less pollution and diminish
autotoxicity hazards. There is a great demand for compounds with selective toxicity
that can be readily degraded by either the plant or by the soil microorganisms.
Plant, microorganisms, other soil organisms and insects can produce allelochemicals
which provide new strategies for maintaining and increasing agricultural production
in the future. Compounds with allelopathic activity may provide novel chemistry for
the synthesis of herbicides, insecticides, nematicides, and fungicides that are not based
on the persistent petroleum derived compounds which are such a public health concern
(Waller and Chou, 1989; Waller, 1999).
Several crops (some of which can be used as cover crops) have been proved to
release allelopathic compounds in the soil (Jimenez-Osornio and Gliessman, 1987;
Blum et al., 1997; Inderjit and Keating, 1999; Anaya, 1999), many of which have
been chemically characterized (Pereda-Miranda et al., 1996; Inderjit, 1996; Seigler,
1996; Waller et al., 1999). The idea of exploiting these compounds as natural herbicides
is therefore very attractive (Putnam, 1988; Weston, 1996; Duke et al., 2000).