References

404 L.G. Perry et al.
properties (Sudha and Ravishankar 2002; Bais et al. 2004, Bais et al. 2005),
leading to inhibition of competitors, pathogens, herbivores, and parasites.
Still other root exudates elicit herbivore defense responses in neighboring
plants (Dicke and Dijkman 2001). In addition, root exudates can alter nutrient
cycling, facilitate root movement, increase nutrient acquisition, and
reduce metal toxicity (Dakora and Phillips 2002; Hawes et al. 2003).
The role of phytotoxic secondary metabolites in plant–plant interactions
(i.e., allelopathy) has been the subject of considerable research and debate
(Nilsen 2002; Bertin et al. 2003; Weir et al. 2004). Plants that produce and accumulate
phytotoxins in the soil are thought to limit establishment, growth,
and survival of neighboring plants, thus reducing local resource competition
and increasing their own success. Potent phytotoxins have been found
in plant leaf and root tissue, leaf leachates, leaf volatiles, and root exudates.
However, because effects of phytotoxins on plant interactions are difficult
to separate from effects of resource competition, many ecologists are not
convinced that allelopathy plays an important role in plant communities
(Fitter 2003).
In addition, the evolution of phytotoxic secondary metabolites in plants
is not well understood, in contrast to the evolution of plant secondary
metabolitesthatrepelorattractinsectsandmicrobes(EhrlichandRaven
1964; Whittaker and Feeny 1971; Scribner 2002). The selection pressures
that could account for the development of phytotoxins in plants are relatively
easy to imagine (e.g., reduced competition from neighbors). However,
the selection pressures that might maintain the production of phytotoxic
secondary metabolites over time are unclear (Fitter 2003). The argument
that natural selection should operate against continued phytotoxin productionisasfollows.Allelopathicplantsmustbeexposedtorelativelyhigh
concentrations of their own phytotoxin. Consequently, to benefit from
being allelopathic, a plant must be at least partly resistant to its own allelochemical.
Further, the metabolic cost of resistance together with the
metaboliccostofproductionmustbelowrelativetotheresourcebenefitof
competitor inhibition. If an allelopathic plant is able to develop relatively
cheap resistance to its own allelochemical, then other plants should also be
able to evolve resistance at relatively little cost. Once other plants develop
resistance, the competitive benefits of producing the phytotoxin should
disappear.Thecostofproducingthephytotoxinonceitsbenefitsarelost
should result in evolution of reduced production.
Examples of allelopathic plants that have become invasive when transported
to new continents suggest that allelopathy may be particularly effective
in novel habitats where native species have not had the opportunity to
evolve resistance to the invaders’ allelochemicals (Rabotnov 1982; Callaway
and Aschehoug 2000; Bais et al. 2003; Vivanco et al. 2004). These studies emphasize
the potential importance of allelopathy in novel plant interactions,