The Activity of Oxyradical-Detoxifying Enzymes Is Not Correlated with Paraquat Resistance inConyza canadensis(L.) Cronq.

Abstract The activities of ascorbate peroxidase (APX), glutathione reductase (GR), and catalase (CAT) were investigated in the rosette stage of paraquat-susceptible, paraquat-resistant, and paraquat/atrazine-coresistant biotypes of Conyza canadensis (L.) Cronq. plants of Hungarian origin, the latter with resistance factors (RF) of 160 and 650 to paraquat. The APX activity in the total leaf extracts of the untreated paraquat-resistant biotype was higher by 25% relative to the paraquat-susceptible and paraquat/atrazine-coresistant biotypes. The levels of GR in both paraquat-resistant biotypes were lower by about 20–25% compared with the susceptible biotype. The APX activity of these biotypes was not induced by paraquat treatment in leaves of sprayed intact plants, whereas the GR activity was slightly increased (by 15–25%). In a series of experiments, the paraquat RF and superoxide dismutase (SOD), APX, and GR activities were compared in the rosette and flowering stages of C. canadensis . An increased (RF ∼1000) level of paraquat resistance was observed in the flowering stage, but only the SOD activity level was enhanced (by 50%) in the paraquat-resistant biotype. Foliar application of diethyldithiocarbamate (DDC, an in vivo inhibitor of Cu/Zn-SOD) or amitrole (AM, an in vivo inhibitor of CAT) caused a marked (50–70%) reversible loss of SOD activity in the chloroplast extract and an 80–90% irreversible decrease in CAT activity in the total cell extracts in all biotypes. However, the paraquat-resistant plants survived the combined treatment with DDC + paraquat or AM + paraquat, and the transient character of paraquat inhibition remained unaffected. The results collectively suggest that the oxygen radical detoxifying pathway with SOD and the enzymes of the ascorbate-glutathione cycle or CAT alone does not explain the very high level of paraquat resistance of the investigated C. canadensis plants.