Response of wild Brassica juncea populations to glyphosate

Wild Brassica juncea (L.) is a troublesome arable land weed and ruderal. It is critical to understand the responses of this weed to herbicides, because the assessment of its susceptibility profile has important ecological and evolutionary consequences for future cultivation of herbicide-tolerant oilseed rape. The response of 31 wild populations from different geographic origins in China to glyphosate was evaluated with two bioassay methods, and variable responses were found in initial studies. Dose-response assays were conducted to characterize the extreme populations further, and shikimate accumulation in vivo was determined using a spectrophotometric method.

On the basis of ID(50) values, the resistance ratios (R/S) were 5.85 and 4.19 for two glyphosate-resistant B. juncea populations in germination tests, whereas they were 4 times more resistant to glyphosate in spray tests. There were differences in shikimate accumulation patterns between the two biotypes. Shikimate concentrations in resistant populations began to decline from 6 days after treatment (DAT), while they increased continually in susceptible populations.

The results obtained suggest that the populations responded differentially to glyphosate, and this variability may provide the genetic basis for evolution of individuals with increased resistance to glyphosate, with important implications for herbicide resistance management, especially in the context of risk assessment of glyphosate-tolerant crops.