• allelochemical resistance;
  • allelopathy;
  • Centaurea maculosa;
  • chemical inhibition;
  • root elongation;
  • seed size


Invasions of North American grasslands by Spotted knapweed (Centaurea maculosa Lam.) are mediated in part by Spotted knapweed root exudation of (±)-catechin, a potent phytotoxin. Residual soil (±)-catechin may interfere with reestablishment of native grassland species even after Spotted knapweed populations are controlled. Grassland species that are resistant to (±)-catechin may be more successful for restoration of areas infested by Spotted knapweed. We evaluated the (±)-catechin resistance of 23 grassland species by measuring the effects of seven (±)-catechin concentrations (0–4.0 mg/mL) on seed germination, seedling root and shoot elongation, and seedling mortality. (±)-Catechin treatments were chosen to reflect the range of observed Spotted knapweed field soil (±)-catechin concentrations. Inhibition of root elongation was the strongest and most common effect of (±)-catechin treatment. High (±)-catechin concentrations reduced mean root lengths of 5 of the species by more than 75% and another 10 species by more than 55%. Experimentally derived concentrations needed to reduce root length by 50% (EC50), an indicator of (±)-catechin resistance, ranged from 0.43 mg/mL ± 0.30 SE to greater than 4.0 mg/mL among species. Eight species with EC50s greater than 3.0 mg/mL were identified as resistant to (±)-catechin and are likely suitable for revegetation of Spotted knapweed–infested areas. (±)-Catechin resistance was positively correlated with mean seed mass, suggesting that seed carbohydrate reserves may allow seedlings to detoxify (±)-catechin, develop barriers to (±)-catechin exposure, or sustain a positive growth rate, despite (±)-catechin-induced cell death. Future efforts to identify allelochemical-resistant grassland species should focus on large-seeded species.