References

27 Root exudation and rhizosphere biology 405
and the potential for the benefits of allelopathy to disappear over time as
the competitors develop resistance. However, the selection pressures that
lead to continued production of phytotoxins by allelopathic species in their
native range, where their competitors have developed resistance, remain
unclear. Plant–plant chemical interactions may be more complex than traditionally
thought. For example, indirect effects of allelochemicals on soil
communities may sometimes be more important to plant community composition
than direct effects on plant growth or survival (Wardle et al. 1990;
Inderjit and Weiner 2001). Such indirect modes of chemical interaction
between plants have rarely been examined explicitly.
Recently, our research on phytotoxic plant root exudates has highlighted
how single plant secondary metabolites can perform many functions
within plant and soil communities, depending on concentration, the
species that are present, and perhaps environmental conditions. The potential
for single secondary metabolites to have numerous effects in the
rhizosphere provides insights into the complex nature of the selection
pressures that may act on secondary metabolite production and secretion.
A secondary metabolite that can serve multiple functions may be produced
initially for one purpose, and later for a different purpose. Further, the
metabolic costs, and associated selection pressures, of producing a secondary
metabolite may be substantially reduced relative to the benefits if
the metabolite serves multiple functions. In this chapter, we will focus on
one secondary metabolite, catechin, which is exuded as a racemic mixture
of (+)-catechin and (–)-catechin from the roots of Centaurea maculosa
(Lam.) (spotted knapweed), an exotic invasive weed in North American
grasslands. Recent research indicates that (±)-catechin has the potential to
act as an allelochemical, autoinhibitor, and antimicrobial and nematicidal
agent, and to increase soil nutrient availability in C. maculosa communities
(Fig. 27.1).
27.2
C. maculosa Invasion Ecology
C. maculosa is a tap-rooted, short-lived perennial, native to grassland
steppes of central and eastern Europe. C. maculosa was accidentally introduced
to northwestern North America in the late 1800s (Sheley et al.
1998). To date, C. maculosa hasinfestedover2.8millionhectaresofNorth
American grassland, mainly in the midwestern and western regions of
North America (Müller-Scharer and Schroeder 1993). While C. maculosa
is not particularly abundant in European grasslands, it is often the dominant
plant in invaded North American grasslands (Ridenour and Callaway
2001). In North American infestations, C. maculosa densities can exceed