Two non-target mechanisms are involved in glyphosate-resistant horseweed (Conyza canadensis L. Cronq.) biotypes

The physiological and biochemical bases for glyphosate resistance and susceptibility in horseweed (Conyza canadensis L. Cronq.) populations collected from Córdoba, Huelva, Málaga, Jaén and Seville in southern Spain were investigated. Screening 25 populations treated with glyphosate (238 g acid equivalent ha−1) at the rosette stage (BBCH 14–15) revealed reductions in fresh weight (fw) of 9–99%. The resistant biotype (R C004) was 6.1 times more resistant than the susceptible biotype (S). Shikimate accumulation in both biotypes increased until 72 h after treatment (HAT), and then continued to increase (to 61.2%) in the S biotype, but decreased by 40% in the R (C004) biotype. Differential glyphosate spray retention and foliar uptake of applied 14C-glyphosate between the R (C004) and S biotype had no effect on resistance to this herbicide. Quantitative and qualitative tests showed greater 14C-glyphosate mobility in the S biotype than in the R (C004) biotype. Glyphosate was metabolized faster in the R (C004) biotype than in the S biotype. The herbicide disappeared completely from the R (C004) biotype by conversion into glyoxylate, sarcosine and aminomethylphosphonic acid within 96 HAT. On the other hand, 41.43 nmol g−1 fw of all glyphosate applied remained in the S biotype and glyoxylate was its only non-toxic metabolite. These results suggest that glyphosate resistance in horseweed is due to two different non-target mechanisms, namely: (a) impaired glyphosate translocation and (b) glyphosate metabolism to other compounds.