References

27 Root exudation and rhizosphere biology 411
27.3.5
The Role of (±)-Catechin in C. maculosa Invasion
For (±)-catechin to facilitate C. maculosa invasion, it must inhibit the North
American grassland species with which C. maculosa competes. Effects of
(±)-catechin on seedling growth and survival have been examined for more
than 25 North American grassland species that are native to the types of
plant communities invaded by C. maculosa (Bais et al. 2003; Weir et al. 2003;
Perry et al. 2005a). (±)-Catechin concentrations as low as 50 µg ml −1 reduce
thegrowthofsensitiveNorthAmericangrasslandspecies,suchasF. idahoensis
(Idaho fescue) and Koeleria cristata (prairie junegrass), grown in
vitro in liquid media (Weir et al. 2003). Higher (±)-catechin concentrations
(125−500 µg ml −1 ) are required to significantly reduce the growth of
sensitive North American species germinated on filter paper (Perry et al.
2005a), but these concentrations are still well below reported C. maculosa
soil (±)-catechin concentrations. North American grassland species vary
considerably in sensitivity to (±)-catechin (Perry et al. 2005a), indicating
that (±)-catechin exudation probably gives C. maculosa an advantage
over some species and not others. In particular, seedlings of larger-seeded
species tend to be more resistant to (±)-catechin than seedlings of smallerseeded
species (Perry et al. 2005a). Some of the North American species
that are relatively resistant to (±)-catechin appear to persist in C. maculosa
populations, while others may be displaced by C. maculosa through
resource competition rather than allelopathy.
One hypothesis to explain C. maculosa invasiveness in North America
is that North American grassland species may be on average more
sensitive to (±)-catechin than the European species with which C. maculosa
naturally coexists, because the North American species have not had
an opportunity to evolve resistance to the allelochemical (i.e., the “novel
weapons” hypothesis; Callaway and Aschehoug 2000). To explore whether
North American grassland species are more sensitive to (±)-catechin than
European species, Bais et al. (2003) compared the effects of (±)-catechin on
three North American grasses that are commonly displaced by C. maculosa
and three European congeners that naturally coexist with C. maculosa.The
North American species were significantly more inhibited by (±)-catechin
treatment than the European species, suggesting that (±)-catechin may
give C. maculosa a greater advantage over its competitors in North America
than in Europe, probably contributing to C. maculosa invasiveness in
North American grasslands.