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and resource competition [30]. The allelopathic inhibition has been traced to
compounds in glandular trichomes on the leaf surface, and one compound (5-
methoxy-3,3'-dihydroxystilbene, batatasin III) has been found to account for
28% of the observed activity [28].
Diffuse knapweed
Centaurea diffusa
Lam. is a Eurasian species that has become invasive follow-
ing its introduction to North America. Callaway and Aschehoug [33] grew
C.
diffusa
in competition with grass species from its native Eurasia and closely
related species from North America. Experiments were conducted with and
without activated carbon to reduce the effects of allelochemicals including 8-
hydroxyquinoline released by
C. diffusa
.
C. diffusa
had much stronger
inhibitory effects on the growth of the North American grass species, and these
effects were greatly reduced by the incorporation of activated carbon, again
strongly supporting an allelopathic mechanism.
Spotted knapweed
Since its introduction into North America from Eurasia,
Centaurea maculosa
Lam., spotted knapweed, has invaded millions of hectares in the Pacific
Northwest. Ridenour and Callaway [34] found that root exudates of
Centaurea
were inhibitory to
Festuca idahoensis
Elmer, and that this effect could be
reduced by the addition of activated carbon. In further studies [35-36], it was
found that
C. maculosa
roots exude a mixture of (±)-catechin. The (+)-cate-
chin enantiomer shows activity against microbial root pathogens, while (-)-
catechin is phytotoxic. Racemic catechin was isolated from field soil [35, 37]
at concentrations shown to be inhibitory in bioassays. Using
Arabidopsis
thaliana
(L.) Heynh., a susceptible species, it was shown that exposure to (-)-
catechin triggers intracellular generation of reactive oxygen species, sequen-
tially followed by a Ca
2+
signaling cascade that results in extensive changes in
gene expression and finally cell death [37]. Tests of (-)-catechin against native
species displaced by
C. maculosa
have not yet been reported.
Recent studies suggest a more complex situation in the field. Carey et al.
[38] report greenhouse experiments showing that
Centaurea maculosa
can
parasitize carbon from neighboring
Festuca idahoensis
, a native bunchgrass,
via soil arbuscular mycorrhizal (AM) fungi. They estimated that as much as
15% of the aboveground carbon in
C. maculosa
plants is obtained through car-
bon parasitism. Callaway et al. [39] explored interactions of
C. maculosa
with
six North American native species in a field study with and without the appli-
cation of the fungicide benomyl to reduce AM colonization of roots. Effects
were complex, and
C. maculosa
growth was either stimulated or inhibited
depending on the specific combination of fungicide and competitor. Finally, as
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