Conyza, herbicide resistance

While most plants are susceptible to paraquat, some paraquat-resistant horseweed (Erigeron sp. and Conyza sp.) biotypes are apparently insensitive to the herbicide due either to elevated levels of superoxide dismutase and other enzymes in a pathway detoxifying oxygen radicals or to differential sequestration of paraquat in the weed (8, 9). Data on the mechanism of most other types of herbicide resistance in weeds are still not complete. 

Lior, E., Y. Benyamini, J. Kigel, and B. Rubin (1996). Triazine resistance in Plantago lagopus and Conyza canadensis populations. In R. DePrado, J. Jorrin, L. Garcia Torres, and G. Marshall, eds. International Symposium on Weed and Crop Resistance to Herbicides. April 3-6, 1995. Cordoba, Spain Graficas TYPO, S.L., pp. 19-22. 

Atrazine-resistant lambsquarters appeared in corn/sugarbeet atrazine-/pyrazon rotations (16) following this prediction. A case of decreased sensitivity to all oxidant generating herbicides has appeared in Conyza canadensis in orchards treated with paraquat after this weed evolved atrazine resistance (17). 

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]. 

Some genera seem more prone than others to rapidly evolve resistance, as they have evolved resistance to more than one herbicide in many areas. These include Lolium spp, Conyza = Erigeron spp among others. There is even one case of sequentially appearing resistance to herbicides with different sites of action. Paraquat (a photosystem I inhibitor) was used to eliminate atrazine-resistant Conyza in vineyards, but resistance to the paraquat used to control the triazine-resistant weed then evolved [32]. 

Resistance was not initially expected to evolve to paraquat, as this herbicide has no biological persistence it is only a contact herbicide, and is immediately bound to soil colloids. Farmer persistence made up for this resistant weed populations evolved where paraquat was applied 6-10 times annually in a variety of weeds. It was clearly shown that PSI was totally suppressed in isolated thylakoids [cf. 11] in paraquat resistant Conyza, yet extreme membrane damage appeared only in the paraquat treated, intact chloroplasts of the sensitive biotype [37]. Photosynthesis in intact leaves was immediately inhibited in both biotypes, but the resistant biotype recovered in a few hours [37]. 

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