Nontarget-site Resistance by Altered Herbicide Distribution

Cases of nontarget-site resistance by altered herbicide distribution have been reported for two important herbicides, paraquat and glyphosate. 

Intensive use of the herbicide paraquat has resulted in the evolution of resistance in various weed species. Intensive research on the resistance mechanisms was mainly carried out with resistant biotypes from Hordeum spp. and Conyza spp., and altered distribution of the herbicide in the resistant weeds was suggested as the cause - or at least the partial cause - of resistance. In resistant Conyza canadensis it was supposed that a paraquat inducible protein may function by carrying paraquat to a metabolically inactive compartment, either the cell wall or the vacuole. This sequestration process would prevent the herbicide from getting in sufficient amounts into the chloroplasts as the cellular site of paraquat action. Inhibitors of membrane transport systems, e.g., N,N-dicyclohexylcarbodii-mide (DCCD), caused a delay in the recovery of photosynthetic functions of the paraquat-resistant biotype, when given after the herbicide. These transport inhibitor experiments supported the involvement of a membrane transporter in paraquat resistance [75]. 

Translocation studies with two paraquat-resistant biotypes of Hordeum lepori-num revealed that the basipetal transport of paraquat in resistant H. leporinum was much reduced compared with susceptible plants. It was concluded that the resistance to paraquat was the result of the reduced herbicide translocation out of the treated leaves [76]. One can suppose that also in this species herbicide sequestration may have been the primary cause for the altered long-distance transport. 

Selection of multiple resistance after sequential use of different herbicides has been described for a biotype of Kochia scoparia from North America. Many years 

Obviously, multiple resistance leads to complex patterns of broad herbicide resistance, particularly in cross-pollinating weed species. This seriously restricts the remaining options of chemical weed control in agricultural practice. 

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