Screening of grassland plants for restoration after spotted knapweed invasion

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.