Allelochemicals Biologica... - Name

ALLEOPATHIC ORGANISMS AND
MOLECULES
effects on germination and seedling growth of six weeds. Ferulic acid, gallic acid, pcoumaric
acid, p-hydroxybenzoic acid, vanillic acid, and p-vanillin were bioassayed
in concentrations of 10, 1, 0.1, and 0.01 mM. Equimolar mixtures containing all
these phenolics were prepared at the final total concentration of 10, 1, 0.1, and 0.01
mM to test for possible interactive effects. Chenopodium album, Plantago lanceolata,
Amaranthus retroflexus, Solanum nigrum, Cirsium sp. and Rumex crispus were the
selected target weeds. The highest concentration of the compounds inhibited the
germination of all these weeds, but lower concentrations had no effect or were
stimulatory. However, effects varied with the weed species, the concentration of the
compound tested and the compound itself. In assays with the mixture of phenolics
some additive effects were found (Reigosa et al., 1999).
Reversible sorption of phenolic acids by soils may provide some protection to
phenolic acids from microbial degradation. In the absence of microbes, reversible
sorption 35 days after addition of 0.5-3 mu mol/g of ferulic acid or p-coumaric acid
was 8-14% in Cecil A(p) horizon and 31-38% in Cecil B-t horizon soil materials.
The reversibly sorbed/solution ratios (r/s) for ferulic acid or p-coumaric acid ranged
from 0.12 to 0.25 in A(p) and 0.65 to 0.85 in B-t horizon soil materials. When microbes
were introduced, the r/s ratio for both the A(p) and B-t horizon soil materials increased
over time up to 5 and 2, respectively, thereby indicating a more rapid utilization of
solution phenolic acids over reversibly sorbed phenolic acids. The increase in r/s ratio
and the overall microbial utilization of ferulic acid and/or p-coumaric acid were much
more rapid in A(p) than in B-t horizon soil materials. Reversible sorption, however,
provided protection of phenolic acids from microbial utilization for only very short
periods of time. Differential soil fixation, microbial production of benzoic acids (e.g.,
vanillic acid and p-hydroxybenzoic acid) from cinnamic acids (e.g., ferulic acid and
p-coumaric acid, respectively), and the subsequent differential utilization of cinnamic
and benzoic acids by soil microbes indicated that these processes can substantially
influence the magnitude and duration of the phytoxicity of individual phenolic acids
(Blum, 1998).
Soil solution concentrations of allelopathic agents (e.g., phenolic acids) estimated
by soil extractions differ with extraction procedure and the activities of the various
soil sinks (e.g., microbes, clays, organic matter). This led to the hypothesis that root
uptake of phenolic acids is a better estimator of dose than soil solution concentrations
based on soil extracts. This hypothesis was tested by determining the inhibition of net
phosphorus uptake of cucumber seedlings treated for 5 hr with ferulic acid in wholeroot
and split-root nutrient culture systems. Experiments were conducted with II ferulic
acid concentrations ranging from 0 to 1 mM, phosphorus concentrations of 0.25, 0.5,
or 1 mM, and solution pH values of 4.5, 5.5, or 6.5 applied when cucumber seedlings
were 9, 12, or 15 days old. The uptake or initial solution concentration of ferulic acid
was regressed on ferulic acid inhibition of net phosphorus uptake. Attempts were
made to design experiments that would break the collinearity between ferulic acid
uptake and phosphorus uptake. The original hypothesis was rejected because the initial
ferulic acid solution concentrations surrounding seedling roots were more frequently
and consistently related to the inhibition of net phosphorus uptake than to ferulic acid
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