Allelochemicals Biologica... - Name

cycles 2 and 3. The results indicate that the use of velvetbean in a cropping system
alters the microbial communities of the rhizosphere and soil, and they are consistent
with the hypothesis that the resulting control of nematodes results from induction of
soil suppressiveness.
6.1. Resistence through natural compounds
Sometimes, natural compounds that confer resistance to a plant against nematode
infestation could be of foreign origin (Reitz et al., 2000). Recent studies have shown
that living and heat-killed cells of the rhizobacterium Rhizobium etli strain G12 induce
in potato roots systemic resistance to infection by the potato cyst nematode Globodera
pallida. To better understand the mechanisms of induced resistance, Reitz et al. (2000)
focused on identifying the inducing agent. Since heat-stable bacterial surface
carbohydrates such as exopolysaccharides (EPS) and lipopolysaccharides (LPS) are
essential for recognition in the symbiotic interaction between Rhizobium and legumes,
their role in the R. etli-potato interaction was studied. EPS and LPS were extracted
from bacterial cultures, applied to potato roots, and tested for activity as an inducer of
plant resistance to the plant-parasitic nematode. Whereas EPS did not affect G. pallida
infection, LPS reduced nematode infection significantly in concentrations as low as 1
and 0.1 mg ml(-1). Split-root experiments, guaranteeing a spatial separation of inducing
agent and challenging pathogen, showed that soil treatments of one half of the root
system with LPS resulted in a highly significant (up to 37%) systemic induced reduction
of G. pallida infection of potato roots in the other half. The results clearly showed
that LPS of R. etli G12 act as the inducing agent of systemic resistance in potato
roots.
In relation with Crotalaria spp. secondary metabolites, it is well known that
these plants produce pyrrolizidine alkaloids and monocrotaline which have high
vertebrate toxicity and could potentially be toxic to nematodes; but it is possible also
that the low C/N ratio of Crotalaria may also contribute to its allelopathic effect
against nematodes. Materials with very low C/N or high content of ammonia will
either result in plasmolysis of nematodes, or proliferation of nematophagous fungi
due to the release of NH4+-N (Rich and Rahi, 1995).
6.2. Soil Amendments
ALLEOPATHIC ORGANISMS AND
MOLECULES
A study was conducted to determine the effects of combinations of organic amendments
and benzaldehyde on plant-parasitic and non-parasitic nematode populations, soil
microbial activity, and plant growth (Chavarria-Carvajal et al., 2001). Pine
bark, velvetbean and kudzu were applied to soil at rates of 30 g/kg and paper waste at
40 g/kg alone and in combination with benzaldehyde (300 mul/kg), for control of
plant-parasitic nematodes. Pre-plant and post-harvest soil and soybean root samples
were analyzed, and the number of parasitic and non-parasitic nematodes associated
with soil and roots were determined. Soil samples were taken at 0, 2, and 10 weeks
after treatment to determine population densities of bacteria and fungi. Treatment
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