Understanding cross- and multiple-herbicide resistance in Setaria adhaerens from olive orchards with two decades of multiple herbicides use

Two populations of Setaria adhaerens (resistant [R] and susceptible [S]) have been identified in Spanish olive orchards. The R population shows multiple resistance to chlorotoluron (PSII inhibitor), diclofop-methyl (ACCase inhibitor), tribenuron-methyl (ALS inhibitor), glyphosate (EPSPS inhibitor), and oxyfluorfen (PPO inhibitor), along with potential natural tolerance to diflufenican (PDS inhibitor). This study evaluated herbicide efficacy at field rates, enzyme activity (ALS, ACCase, EPSPS, PPO, PSII), and the role of cytochrome P450 and glutathione-S-transferases using malathion and NBD-Cl, respectively. The S population was fully controlled by all herbicides except diflufenican, which showed only ∼45 % control in both populations, indicating possible innate tolerance. In the R population, chlorotoluron, diclofop-methyl, tribenuron-methyl, and glyphosate had no effect on enzyme activity, suggesting non-target site resistance (NTSR). In contrast, oxyfluorfen inhibited PPO activity only in the S population, indicating target-site resistance (TSR). NBD-Cl showed no activity, ruling out glutathione-S-transferases-mediated metabolism. However, malathion restored over 50 % sensitivity to chlorotoluron, diclofop-methyl, and tribenuron-methyl in the R population, suggesting enhanced metabolism likely mediated by cytochrome P450 monooxygenases. Metabolic profiling further confirmed enhanced herbicide detoxification in the R population as a key resistance mechanism. These findings highlight the complexity of resistance in S. adhaerens and underline the urgent need to incorporate metabolic resistance monitoring into weed management programs. Future studies will focus on unraveling the molecular basis of these resistance mechanisms.