DIQUAT RESISTANCE OF DIFFERENT PARAQUAT-RESISTANT CONYZA-CANADENSIS (L) CRONQ BIOTYPES

Photosynthetic responses of paraquat/atrazine coresistant (PqAR) and only paraquat resistant (PqR) biotypes of horseweed [Conyza canadensis (L.) Cronq.] on diquat (Reglone) treatment were investigated. Both resistant biotypes showed a transitory inhibition of photosynthetic activity characterized by in vivo CO2 fixation and F-v in the first 2 hours after spraying of the intact plants with 0.5 mM diquat. Similar to the kinetics observed after paraquat spraying of the resistant plants this transitory inhibition was also followed by a gradual recovery. Comparison of paraquat and diquat treatment of paraquat/atrazine coresistant plants showed that recovery after diquat spraying was slower. Ultrastructure of chloroplasts from resistant plants treated by floating the leaves on 10 mu M diquat solution showed only moderate changes, while chloroplasts from sensitive Conyza plants seemed to be severely damaged. Paraquat/atrazine coresistant Conyza biotypes of different origin exhibited different, but relatively high resistance against diquat (resistance factor of PqAR(1) = 50, PqAR(2) = 102). Resistance factors for paraquat were higher (PqAR(1) = 210, PqAR(2) = 650).