Altered Translocation Pattern as Potential Glyphosate Resistance Mechanism in Blackgrass (Alopecurus myosuroides) Populations from Lower Saxony

Glyphosate is a broad-spectrum herbicide widely used. After years of extensive usage, many weed species have developed resistance due to both target-site (TSR) and non-target-site resistance mechanisms (NTSRs). Alopecurus myosuroides is a competitive weed species. Greenhouse monitoring trials in Germany have revealed reduced glyphosate efficacy against some populations of Alopecurus myosuroides. In a foregoing dose–response study, individual plants from four out of six tested populations survived full (1800 g a.i. ha−1) or double (3600 g a.i. ha−1) glyphosate dose rates permitted, suggesting the presence of tolerant biotypes with yet unknown resistance mechanisms. Our aim was to investigate the absorption and translocation patterns of glyphosate in these biotypes. The plants were first treated with 14C-glyphosate, and 14C-glyphosate absorption and translocation were subsequently visualized by phosphorimaging and finally quantified by liquid scintillation counting. The results showed significant differences in the distribution of glyphosate in different plant organs, with significantly more being translocated out of the treated leaf in glyphosate-resistant compared to sensitive (S-) biotypes. The study’s findings are partly in contrast to previous studies that have found reduced translocation. Our study demonstrates the complex nature of glyphosate resistance and suggests further experiments to finally elucidate the underlying resistance mechanisms in the biotypes of the Alopecurus myosuroides studied.