A waterhemp population (McLean County resistant, MCR) from McLean County, Illinois is resistant to both mesotrione and atrazine by elevated rates of herbicide metabolism. Research was conducted to investigate the inheritance of these resistance traits. Resistant and sensitive plants were crossed to obtain reciprocal F₁ populations, which were then used to create pseudo-F₂ and backcross (to sensitive parent; BC_S) populations. The various populations were evaluated with whole-plant herbicide efficacy studies in a greenhouse. The responses of the F₁ populations to both mesotrione and atrazine were intermediate when compared with parental populations. In the case of atrazine, BC_S and F₂ populations segregated 1 : 1 and 1 : 3, respectively, for susceptibility (S) : resistance (R), at a dose that controlled the sensitive parent but not the F₁ or resistant parent. For mesotrione, variability was observed within the F₁ populations, suggesting that mesotrione resistance is multigenic and the resistant parents used in the cross were not homozygous at the resistance loci. Furthermore, at low mesotrione doses, more F₂ plants survived than expected on the basis of a single-gene trait, whereas at high doses, fewer F₂ plants survived than expected. Dry weight data confirmed the conclusions obtained from survival data. Specifically, atrazine responses segregated into two discrete classes (R and S) in both the F₂ and BC_S populations, whereas mesotrione responses showed continuous distributions of phenotypes in F₂ and BC_S populations. We conclude that metabolism-based atrazine resistance in MCR is conferred by a single major gene, whereas inheritance of mesotrione resistance in this population is complex.

Nomenclature: Atrazine; mesotrione; common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU.