Catechin-metal interactions as a mechanism for conditional allelopathy by the invasive plant Centaurea maculosa

1. Evaluating variation, or 'conditionality', in plant interactions is crucial to understanding their ecological importance and predicting where they might be at play. Much is known about conditionality for competition, facilitation and herbivory, but not for allelopathy, which likely contributes to the equivocal nature of reports on this topic. Centaurea maculosa (spotted knapweed) is an invasive species in North America, whose success has been attributed, at least in part, to the allelochemical root exudate (±)-catechin. 2. Understanding the ecological relevance of (±)-catechin necessitates determining how it interacts with various soil components. We found that some metals caused rapid declines in measurable (±)-catechin, while calcium impeded its auto-oxidation, maintaining concentrations higher than for (±)-catechin alone. Certain (±)-catechin-metal complexes were more phytotoxic than (±)-catechin alone, while others showed lower toxicity. 3. The variable phytotoxicity of these complexes suggests that (±)-catechin effects are enhanced, mitigated or otherwise affected by complexation with different metals and perhaps other soil components. 4. Synthesis. These findings serve to illustrate that the precise chemical forms, interactions and effects of catechin in the environment are highly variable and that further examination is warranted to increase our understanding of its role in invasion and allelopathy. The conditional effects observed for catechin detection and phytotoxicity likely extend to related allelopathic compounds, other root exudates and potentially other systems involving chemically complex and spatially heterogeneous environments.