Target-site resistance and an altered glyphosate metabolite profile in invasive Cenchrus setaceus populations from southern Spain.

Cenchrus setaceus is an invasive perennial grass spreading along Mediterranean and southern Atlantic transport corridors in the Iberian Peninsula. Recurrent glyphosate use in roadside vegetation management may impose sustained selection for resistance. We characterized whole-plant glyphosate response and candidate resistance mechanisms in 10 populations collected in southern Spain. Three populations (S, CO3, and SE2) were susceptible, whereas seven populations had GR₅₀ values of 804.0-1108.6 g ae ha-1 and resistance factors of 20.3-28.0. Foliar spray retention and 14C-glyphosate absorption and distribution did not differ among populations at 96 h after treatment. In metabolism studies, the amount of glyphosate at 0 HAT was similar among populations. At 96 HAT, CO1, MA1, MA2, and SE1 contained less glyphosate and more aminomethylphosphonic acid (AMPA) than the susceptible populations. Analyte identity was confirmed against authentic standards by reversed-polarity capillary electrophoresis. Because only one post-treatment time was evaluated, these data describe an endpoint profile and do not estimate a metabolic rate. CA1, CA2, CO2, and MA1 carried a Pro-106-Ser substitution and showed markedly reduced EPSPS sensitivity. Similar basal EPSPS activity among populations provided no biochemical evidence of constitutive EPSPS overexpression. Overall, resistance-associated traits differed among populations, indicating that no single resistance pattern explains all cases and underscoring the need to diversify roadside management beyond repeated glyphosate use.