Evaluation of Paraquat Resistance Mechanisms in Conyza

Experiments were conducted to determine the mechanism(s) of paraquat (1,1′-dimethyl-4, 4′-bipyridinium ion) resistance in a biotype of hairy fleabane ( Conyza bonariensis (L.) Cronq.). Thin-layer chromatographic analysis of leaf extracts indicated that paraquat is not metabolized in either the resistant (R) or the sensitive (S) biotype. Three in vitro studies demonstrated that electron transfer from the photosystem I (PSI) donor ( F A/ F B) to paraquat is similar in both biotypes as was the amount and character of the two F A/ F B iron-sulfur clusters. The relative activities of the stromal enzymes superoxide dismutase, ascorbate peroxidase, and glutathione reductase, which can detoxify paraquat-generated noxious oxygen species, were determined following separation by polyacrylamide gel electrophoresis. Of these enzymes, only an increase in ascorbate peroxidase activity (28%) was observed in stromal extracts of the R (relative to S) biotype. These data indicate that the 100-fold level of paraquat resistance observed in leaves of the R biotype of Conyza is not due to metabolic detoxification, an altered (insensitive) site of action, and/or enhanced activities of stromal enzymes. Paraquat-induced chlorosis (an indicator of sensitivity) was similar in illuminated chloroplast preparations of both biotypes indicating that the resistance factor is located outside of the chloroplast′s envelope. Similar rates of paraquat-induced chlorosis were also observed in illuminated protoplast preparations of both biotypes; however, leaf sections (1 mm width) of the R biotype exhibited a degree of paraquat resistance (81-fold) very similar to that exhibited by whole leaves. These data suggest that resistance is due to a sequestration mechanism that prevents paraquat from diffusing to PSI, the site of paraquat action. The sequestration mechanism appears to require a structurally intact cell wall to be functional.