THE ACTIVITY OF OXYRADICAL-DETOXIFYING ENZYMES IS NOT CORRELATED WITHPARAQUAT RESISTANCE IN CONYZA-CANADENSIS (L.) CRONQ

The activities of ascorbate peroxidase (APX), glutathione reductase (G R), and catalase (CAT) were investigated in the rosette stage of paraq uat-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. T he APX activity in the total leaf extracts of the untreated paraquat-r esistant biotype was higher by 25% relative to the paraquat-susceptibl e 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 in duced by paraquat treatment in leaves of sprayed intact plants, wherea s the GR activity was slightly increased (by 15-25%). In a series of e xperiments, the paraquat RF and superoxide dismutase (SOD), APX, and G R activities were compared in the rosette and flowering stages of C. c anadensis. An increased (RF similar to 1000) level of paraquat resista nce was observed in the flowering stage, but only the SOD activity lev el was enhanced (by 50%) in the paraquat-resistant biotype. Foliar app lication of diethyldithiocarbamate (DDC, an in vivo inhibitor of Cu/Zn -SOD) or amitrole (AM, an in vivo inhibitor of CAT) caused a marked (5 0-70%) reversible loss of SOD activity in the chloroplast extract and an 80-90% irreversible decrease in CAT activity in the total cell extr acts in all biotypes. However, the paraquat-resistant plants survived the combined treatment with DDC+paraquat or AM+paraquat, and the trans ient 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 d oes not explain the very high level of paraquat resistance of the inve stigated C. canadensis plants. (C) 1998 Academic Press.