Paraquat resistance and hormetic response observed in Conyza sumatrensis (Retz.) E. Walker (tall fleabane) in Australian cotton cropping systems

Herbicide resistance in weeds necessitates the continued monitoring of herbicide resistance evolution in weed species. This study investigated the level of paraquat resistance in two species of Conyza including Conyza sumatrensis (Retz.) E. Walker (tall fleabane) and Conyza bonariensis (L.) Cronquist (flaxleaf fleabane). Twenty-two biotypes of C. bonariensis and three biotypes of C. sumatrensis were screened for paraquat resistance evaluation. All tested biotypes of C. bonariensis were susceptible to paraquat at the recommended rate of 500 g a.i./ha. Whole plant dose–response assays showed that the two biotypes (TF-COLEA and TF-NANDI) of C. sumatrensis are resistant to the herbicide paraquat, with a ED50 more than fourfold that of the susceptible biotype (TF-TALWD). The two resistant biotypes also showed a hormetic growth increase at two lower doses (62.5 and 125 g a.i./ha) of paraquat, resulting in 31% and 17% higher biomass production at 62.5 g a.i./ha of paraquat, and 19% and 11% at 125 g a.i./ha of paraquat for the resistant biotypes TF-NANDI and TF-COLEA, respectively, than those of the untreated control. The two resistant biotypes at the two lower doses of paraquat generated 33% to 65% higher numbers of seed buds/plant in TF-COLEA and 40% to 68% in TF-NANDI compared to the untreated control. Such stimulatory responses to lower rates of paraquat can enhance the competitiveness of these resistant plants against neighboring weed and crop plants, potentially increasing the seedbank size of resistant biotypes.