Dual role for an allelochemical: (±)-catechin from Centaurea maculosa root exudates regulates conspecific seedling establishment

1 Plant allelopathic compounds may have other roles than interspecific interference. We investigated whether (±)-catechin, a phytotoxin exuded from the roots of the exotic invader Centaurea maculosa (spotted knapweed), is also one of the factors that regulates C. maculosa recruitment. 2 Adding activated carbon, which adsorbs organic compounds, to soil around C. maculosa adults in the field increased seedling density by 78% 25 days after sowing, and by 34% 32 days after sowing, suggesting that soil-borne compounds inhibited or delayed recruitment. 3 Analysis of field soils near mature C. maculosa revealed that they can contain exceptionally high (±)-catechin concentrations (mean = 1.55 mg g ^-1 dry soil, with 60% of samples containing ≥ 1.0 mg (±)-catechin g ^-1 ). 4 In laboratory experiments, treatment with ≥ 1.0 mg (±)-catechin mL ^-1 reduced seedling root elongation by > 50%, indicating that field concentrations are sufficient to inhibit C. maculosa recruitment. Provided that 10% methanol was used to maintain (±)-catechin in solution for > 1 day, treatment with ≥ 1.0 mg mL ^-1 also reduced C. maculosa germination by > 70%. 5 (±)-Catechin maintained in solution with methanol did not significantly reduce C. maculosa seed survival, suggesting that inhibition of germination was due, at least in part, to delayed germination rather than to seed mortality. 6 Depending on the solubility of (±)-catechin in soil and on the duration of its effects on recruitment, C. maculosa may avoid intraspecific competition or regulate the timing of seedling establishment by reducing seedling growth or postponing germination in response to its own phytotoxin. 7 Chemical regulation of C. maculosa recruitment, as demonstrated here, suggests a dual role of (±)-catechin as an allelochemical and an autoinhibitor. The potential for a single plant root exudate to influence both inter- and intraspecific interactions emphasizes the complex effects that plant secondary metabolites may have on plant population and community structure.