Evaluating variation, or “conditionality”, in plant interactions is crucial to understanding their ecological importance.  Much is known about conditionality in competition, facilitation, and herbivory, but not allelopathy, which likely contributes to the equivocal nature of reports on this topic.  Centaurea maculosa (spotted knapweed) is a highly invasive species in North America, whose success has been attributed in part to the allelochemical root exudate, (±)-catechin.  Among other things, understanding the ecological relevance of an allelopathic root exudate necessitates measurements of toxicity in reasonably relevant ecological conditions and determining how it interacts with various soil components.  We established low concentrations of catechin in nine different natural soils and then planted seeds of the native grass Koeleria macrantha.  We also applied low doses of catechin, resulting in soil concentrations below our detectable limit of 5 μg g^-1, applied to the rhizospheres of three native species at nine different field sites in western Montana.  Finally, we tested the effects of adding metals to catechin solutions on catechin degradation and the phytotoxic effects of different metal-catechin solutions.

Results/Conclusions

Catechin treatments to a range of different soils demonstrated phytotoxicity.  Additionally, low doses of catechin applied to native species in the field demonstrated phytotoxicity.  Furthermore, in other experiments we found that some metals caused rapid declines in measurable catechin in solution, while calcium slowed its auto-oxidation, which maintained catechin concentrations at higher levels for longer periods if time than was found for solutions of pure catechin.  Using these solutions, we found that catechin was highly phytotoxic at low experimental soil concentrations. Interestingly, certain catechin-metal complexes were more phytotoxic than catechin alone while other solutions showed decreased toxicity.  

The variable toxicity of these complexes indicates that catechin effects can be enhanced or diminished by pleiotropic interactions in soils, resulting from catechin forming complexes with different metals or other soil components.