Physiological and molecular insight on the mechanisms of resistance to glyphosate in Conyza canadensis (L.) Cronq. biotypes

The physiological and molecular basis of glyphosate resistance in susceptible (S) and resistant (R) horseweed (Conyza canadensis (L.) Cronq.) populations collected from regions across the USA (Arkansas, Delaware, Ohio, Virginia, Washington) was investigated. At two-leaf stage approximately the same ED50 values were observed for the S and R populations, while at the rosette stage the R biotypes were approximately three times more resistant than the S biotypes. After treatment with severe glyphosate doses (more than 1x the recommended field rate), different morphological responses in R and S biotypes were observed. In S biotypes, the first phytotoxic effects were found in the meristematic tissues, while in the R biotypes the first phytotoxic effects were observed in leaves. At 2 to 4 weeks after the treatment, R plants recovered by emitting new leaves and/or new branches from the center of the rosette. A significant increase of the mean number of branches per surviving R plants as a function of glyphosate-applied dose was observed. As regards the physiological mechanism of resistance, the main difference between R and S biotypes was the dissimilar mobility of glyphosate in the whole plant. In the R biotypes the herbicide was less translocated in the downward direction (from leaves to roots) and more translocated in the upward direction (from culm. to leaves) with respect to the S biotypes. Finally, in R biotypes the relative level of EPSPS mRNA was from 1.8 to 3.1 times higher than that found in S biotypes. On the basis of obtained results three factors may concur to glyphosate resistance in the investigated R biotypes: impaired translocation of the herbicide, increase in EPSP synthase transcript levels, and enhanced ramification. (c) 2006 Elsevier Inc. All rights reserved.