Paraquat resistance of horseweed (Erigeron canadensis L.) is not caused bypolyamines

The correlation between polyamine content and paraquat resistance in horsew eed (Erigeron canadensis L.) was investigated. Untreated paraquat-resistant and paraquat/atrazine-coresistant plants contained about two to three time s higher amounts of polyamines, especially putrescine, compared to sensitiv e ones. A marked increase of polyamine content was detected after paraquat treatment in each biotype which decreased to the initial level in 6 h. Func tional activity (characterized by variable fluorescence) of sensitive plant s continuously decreased after paraquat treatment, while resistant biotypes recovered their activities after a transitory inhibition. Changes in polya mine content did not correlate with the changes in functional activity. Exo genously added 10(-4) mol L-1 putrescine combined with 10(-5) mol L-1 paraq uat in detached sensitive leaves showed a slight protective effect, while p araquat-resistant and paraquat/atrazine-coresistant leaves remained unaffec ted. The polyamine level of the cycloheximide-treated or long-term cold-tre ated plants also changed independently from their functional activity. It c an be concluded that the higher polyamine level of the treated resistant le aves and the increase of the polyamine content after paraquat treatment see m to be a general stress response rather than the specific reason For paraq uat resistance. (C) 2000 Academic Press.