Differential Movement of Paraquat in Resistant and Sensitive Biotypes of Conyza

Previous indirect evidence indicates that paraquat resistance in hairy fleabane ( Conyza bonariensis [L.] Cronq.) is correlated with restricted movement in the resistant (R) biotype. Experiments were conducted to further characterize the mobility of paraquat (1,1′-dimethyl-4-4′bipyridinium ion) in R and sensitive (S) biotypes of Conyza. The lateral movement of [ 14C]paraquat from the point of application on he adaxial surface of intact leaves incubated in darkness was significantly restricted in the R relative to the S biotype. This restricted movement of paraquat may result from a mechanism of sequestration that conveys resistance to the R biotype. Autoradiographs of treated, intact leaves as well as leaf disks floated in an aqueous solution containing [ 14C]paraquat (dark incubation) revealed a distinctive pattern of black spots very similar to the density and distribution of unicellular and multicellular leaf hairs (trichomes). Paraquat accumulated in trichomes on the abaxial surface of leaf disks of both biotypes even though this surface was not in direct contact with the incubation solution. Microautoradiography and paradermal sectioning studies were conducted following the treatment of the adaxial surface of intact leaves of both biotypes with [ 14C]paraquat for 4 hr in darkness. Analyses of the data indicated a 10- to 100-fold increase in the amount of paraquat associated with trichomes (relative to other leaf cells) on the adaxial and abaxial leaf surfaces of both biotypes. It is unlikely that the accumulation of paraquat in abaxial trichomes within 4 hr after treatment can occur solely by diffusion. A 20% increase in the density of multicellular trichomes was observed in leaves of the R biotype. This small increase may contribute slightly to the 100-fold level of resistance typically observed in intact leaves of the R biotype. These data further support the hypothesis that paraquat resistance in Conyza is correlated with restricted movement (sequestration) in the R biotype.