Mechanism of resistance to quinclorac in smooth crabgrass (Digitaria ischaemum)

The mechanism of resistance to quinclorac was investigated in a smooth crabgrass biotype [Digitaria ischaemum (Schreb. ex Schweig) Schreb. ex Muhl] from Tulare County, California. Quinclorac (8.96 kg a.i. ha−1) had no effect (P = 0.18) on the resistant (R) biotype, but reduced fresh weight of a susceptible (S) biotype by 93%. After treatment with 4.48 kg a.i. quinclorac ha−1, the S biotype produced about three times more ethylene than the R biotype and accumulated cyanide in tissues. Similar amounts of endogenous cyanide resulting from treatment with KCN reproduced quinclorac phytotoxicity. Pre-treatment with the ACC synthase inhibitor AVG reduced quinclorac phytotoxicity by 37% and ethylene production by 89%. These data suggest a target site-based mechanism of resistance involving stimulation of ACC synthesis and accumulation of cyanide. Also, the R biotype had four times more β-cyanoalanine synthase activity than the S biotype, suggesting a higher ability to detoxify cyanide.