A biotype of Conyza canadensis (L.) Cronq. from a vineyard subjected to repeated paraquat and triazine herbicide treatment and a wild type from an adjacent ruderal area were examined for resistance to paraquat and atrazine. CO2 fixation was reduced by paraquat for 3 hr, but thereafter it was strongly stimulated in the resistant biotype, while that of the susceptible biotype remained inhibited. The fluorescence quenching measurements show that paraquat inhibited both biotypes at the level of isolated chloroplasts. Paraquat quenched fluorescence in intact leaves during the first hour of treatment. Fluorescence quenching ceased in the resistant biotype within 24 hr of paraquat treatment. The I50 with paraquat in the resistant biotype was 170 times higher than that of the susceptible biotype in measurements of fluorescence induction quenching at 24 hr. There were no chlorophyll bleaching and chlorophyll/protein damage in the resistant biotype incubated with paraquat, in marked contrast to the susceptible biotype. The ultrastructure of the chloroplasts in the paraquat-treated leaves of resistant plants was similar to that of the untreated ones. Plastids of the susceptible biotype were disrupted, with considerable rearrangement of the lamellar system, after paraquat application. Superoxide dismutase, ascorbate peroxidase, catalase, and reduced glutathione levels in the total cell extracts were slightly lower in the resistant biotype, but there was a slightly higher level in the extracts of the susceptible biotype in the presence of paraquat. The paraquat-resistant biotype had co-resistance to atrazine as demonstrated through fluorescence induction measurements. The resistance ratio to atrazine was 300. From these results it was concluded that C. canadensis displays co-resistance to paraquat and atrazine; the data indicate that resistance to herbicides with different modes of action can be developed.
ASJC Scopus subject areas
- Agronomy and Crop Science
- Health, Toxicology and Mutagenesis