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Articles

Paraquat Resistance in Conyza¹

MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824, School of Biological Science, University of Bath, Bath BA2-7AY, United Kingdom, Department of Crop and Soil Science, Michigan State University, East Lansing, Michigan 48824

A biotype of Conyza bonariensis (L.) Cronq. (identical to Conyza linefolia in other publications) originating in Egypt is resistant to the herbicide 1,1'-dimethyl-4,4'-bipyridinium ion (paraquat). Penetration of the cuticle by [¹⁴C]paraquat was greater in the resistant biotype than the susceptible (wild) biotype; therefore, resistance was not due to differences in uptake. The resistant and susceptible biotypes were indistinguishable by measuring in vitro photosystem I partial reactions using paraquat, 6,7-dihydrodipyrido [1,2-:2',1'-c] pyrazinediium ion (diquat), or 7,8-dihydro-6H-dipyrido [1,2-:2',1'-c] [1,4] diazepinediium ion (triquat) as electron acceptors. Therefore, alteration at the electron acceptor level of photosystem I is not the basis for resistance. Chlorophyll fluorescence measured in vivo was quenched in the susceptible biotype by leaf treatment with the bipyridinium herbicides. Resistance to quenching of in vivo chlorophyll fluorescence was observed in the resistant biotype, indicating that the herbicide was excluded from the chloroplasts. Movement of [¹⁴C] paraquat was restricted in the resistant biotype when excised leaves were supplied [¹⁴C]paraquat through the petiole. We propose that the mechanism of resistance to paraquat is exclusion of paraquat from its site of action in the chloroplast by a rapid sequestration mechanism. No differential binding of paraquat to cell walls isolated from susceptible and resistant biotypes could be detected. The exact site and mechanism of paraquat binding to sequester the herbicide remains to be determined.

³ Current address: Philip Morris Research Center USA, P.O. Box 26583, Richmond, VA 23261.

⁴ Current address: Central Research and Development Department, Life Sciences, Experiment Station, The DuPont Company, Wilmington, DE 19898.

¹ Supported in part by United States Department of Energy contract No. DE-AC02-76ERO-1338.

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