References

27 Root exudation and rhizosphere biology 413
Plant autoinhibition mediated by secondary metabolites has been the
subject of much research (Singh et al. 1999). However, (±)-catechin is one
of the first secondary metabolites determined to act as both an allelochemical
and an autoinhibitor. Autoinhibition of seedling establishment may
influence plant populations in several ways. First, autoinhibition may be
a mechanism through which adults avoid establishment of intraspecific
competitors in dense populations (Schenk et al. 1999). Second, seedlings
that produce autoinhibitors may reduce intraspecific competition by preventing
the establishment of their siblings and unrelated seedlings (Dyer
2004). Third, autoinhibitors that delay germination, rather than killing
seeds or seedlings, may postpone germination in areas where intraspecific
competition from adults would prevent seedling survival (Picman
and Picman 1984). Determining the specific effects of autoinhibition on
C. maculosa populations will require further research to determine whether
(±)-catechin reduces C. maculosa seedling root elongation or delays germination
under field conditions.
Autoinhibition by C. maculosa is probably most important in wellestablished
stands, where soil (±)-catechin is likely to be concentrated
and widespread. (±)-Catechin concentrations lower than 0.5 mg ml −1 have
little effect on C. maculosa root elongation, and no effect on germination
(Perry et al. 2005b). However, much lower (±)-catechin concentrations have
large effects on other, more (±)-catechin-sensitive species (Bais et al. 2003;
Weir et al. 2003; Perry et al. 2005a). Thus, C. maculosa seedlings should
have an advantage over more (±)-catechin-sensitive species in soils with
moderate (±)-catechin concentrations, while very high (±)-catechin concentrations
may prevent establishment of C. maculosa seedlings as well
as other species. In well-established C. maculosa populations with high
soil (±)-catechin concentrations, autoinhibition, water stress (Jacobs and
Sheley 1998), and seed predation (Muller-Schärer and Schroeder 1993)
probably operate together to limit C. maculosa seedling establishment.
27.5
(±)-Catechin Effects on Soil Communities
In addition to the inhibitory effects of (±)-catechin on plants, (+)-catechin
and (–)-catechin both reduce survival of some soil organisms. However,
the two enantiomers appear to affect different organisms. Bais et al. (2002)
found that (+)-catechin, but not (–)-catechin, inhibited growth of several
soil bacteria. Six bacterial strains were exposed to each of the enantiomers
or the racemic mixture, applied to filter discs. Xanthomonas campestris
ssp. vesicatoria, Erwinia amylovora, Pseudomonas fluorescens,andE. carotovora
were each susceptible to 100 µg ml −1 (+)-catechin or to 200 µg ml −1