The Effects of Flavonoid Allelochemicals from Knapweeds on Legume-Rhizobia Candidates for Restoration

Russian knapweed (Acroptilon repens) and Spotted knapweed (Centaurea maculosa) are allelopathic weeds invasive in North American grasslands. Both species contain at least one phytotoxic flavonoid root exudate with demonstrated negative influences on other plants. Previous findings indicated that Silky lupine (Lupinus sericeus), among other legumes, was relatively resistant to Spotted knapweed invasion and allelochemistry. We hypothesized that legume species may exhibit resistance to flavonoids in knapweed root exudates and may serve as candidate species for management efforts. Because legumes form symbiotic relationships with rhizobia, these bacteria must also be evaluated for allelochemical resistance before legumes can be recommended for restoration. In this study, we examined four legume species for effects of 7,8-benzoflavone (from Russian knapweed) and (+/-)-catechin (from Spotted knapweed) on rhizosphere interactions involving legume roots and associated rhizobia. Pure cultures of four rhizobia strains exhibited varied responses when grown with 7,8-benzoflavone or (+/-)-catechin. Alfalfa (Medicago sativa) and its bacterial symbiont, Sinorhizobium meliloti, exhibited allelochemical resistance that varied with (+/-)-catechin concentration when grown in vitro. Four legume species were grown under greenhouse conditions. Plants that were inoculated and nodulated generally exhibited no response to 7,8-benzoflavone or (+/-)-catechin treatments. Plants that were not inoculated exhibited stronger responses. Therefore, inoculation and nodulation may confer resistance to allelochemicals. These results, when coupled with previous research and field observations, suggest that legumes may not be susceptible to knapweed allelopathy and may be good choices in restoration of knapweed infestations when inoculated, particularly on sites with low soil nitrogen.