Allelochemicals Biologica... - Name

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ANA LUISA ANAYA
is lacking. To test this hypothesis, cucumber seedlings were grown in soil containing
various concentrations of wheat or sunflower tissue. Both tissue types contain phenolic
acids, which have been implicated as allelopathic phytotoxins. The level of
phytotoxicity of the plant tissues was determined by the inhibition of pigweed seedling
emergence and cucumber seedling leaf area expansion. The stimulation of cucumber
seedling rhizosphere bacterial communities was determined by the plate dilution
frequency technique using a medium containing phenolic acids as the sole carbon
source. When sunflower tissue was incorporated into autoclaved soil (to reduce the
initial microbial populations), a simultaneous inhibition of cucumber seedling growth
and stimulation of the community of phenolic acid utilizing rhizosphere bacteria
occurred. Thus, it was possible to observe simultaneous inhibition of cucumber
seedlings and stimulation of phenolic acid utilizing rhizosphere bacteria, and therefore
provide indirect evidence of phenolic acid transfer from plant residues in the soil to
the root surface. However, the simultaneous responses were not sufficiently consistent
to be used as a field screening tool but were dependent upon the levels of phenolic
acids and the bulk soil and rhizosphere microbial populations present in the soil. It is
possible that this screening procedure may be useful for phytotoxins that are more
unique than phenolic acids. Such an inverse relationship between phytotoxicity and
the response of rhizosphere bacterial populations was also observed by Blum et al.
(2000), and such interactions provide indirect evidence for the transfer of
allelochemicals from the plant root to the rhizosphere.
In relation with resistance of weeds to herbicides, Duke et al. (2000) mentioned
that new mechanisms of action for herbicides are highly desirable to fight evolution
of resistance in weeds, to create or exploit unique market niches, and to cope with
new regulatory legislation. Comparison of the known molecular target sites of synthetic
herbicides and natural phytotoxins reveals that there is little redundancy. Comparatively
little effort has been expended on determination of the sites of action of phytotoxins
from natural sources, suggesting that intensive study of these molecules will reveal
many more novel mechanisms of action. These authors gave some examples of natural
products that inhibit unexploited steps in the amino acid, nucleic acid, and other
biosynthetic pathways: AAL-toxin, hydantocidin, and various plant-derived terpenoids.
Natural products have not been utilized as extensively for weed management as
they have been for insect and plant pathogen management, but there are several notable
successes such as glufosinate and the natural product-derived triketone herbicides.
The molecular target sites of these compounds are often unique. Strategies for the
discovery of these materials and compounds are outlined by Duke et al. (2002a).
Numerous examples of individual phytotoxins and crude preparations with weed
management potential are provided by these authors. They described an example of
research to find a natural product solution of a unique pest management problem
(blue-green algae in aquaculture), and mentioned the two fundamental approaches to
the use of natural products for weed management: i) as a herbicide or a lead for a
synthetic herbicide and ii) use in allelopathic crops or cover crops (Duke et al., 2002b).
As it was mentioned, crops may be genetically engineered for weed management
purposes by making them more resistant to herbicides or by improving their ability to