Association between metabolic resistances to atrazine and mesotrione in a multiple-resistant waterhemp (Amaranthus tuberculatus) population

Metabolic resistances to atrazine (atz-R) and mesotrione (meso-R) occur in several waterhemp [Amaranthus tuberculatus(Moq.) Sauer] populations in the United States. Interestingly, although metabolic atz-R but mesotrione-sensitiveA. tuberculatuspopulations have been reported, anAmaranthuspopulation has not been confirmed as meso-R but atrazine-sensitive, implying an association between these traits. Experiments were designed to investigate whether the single gene conferring metabolic atz-R plays a role in meso-R. An F(2)population was generated from a multiple herbicide-resistantA. tuberculatuspopulation from McLean County, IL (MCR). A cross was made between a known meso-R male clone (MCR-6) and a herbicide-sensitive female clone from Wayne County, IL (WCS-2) to develop an F(1)population. Survival of MCR-6 plants following atrazine POST treatment (14.4 kg ha(-1)) indicated the male parent was homozygous atz-R. F(1)plants were intermated to obtain a segregating pseudo-F(2)population. Dose-response and metabolic studies conducted with mesotrione using F(1)plants indicated intermediate biomass reductions and metabolic rates compared with MCR-6 and WCS. F(2)plants were initially treated with either mesotrione (260 g ha(-1)) or atrazine (2 kg ha(-1)) POST, and after 21 d of recovery, vegetative clones from surviving resistant plants were subsequently treated with the other herbicide. When mesotrione was applied first, the meso-R frequency was 8.2%, and when atrazine was applied first, the atz-R frequency was 75%. However, the meso-R frequency increased to 16.5% following preselection for atz-R, and 100% of surviving meso-R plants were atz-R. Our findings indicate that the gene conferring metabolic atz-R is also involved with the meso-R trait within the population tested.