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; BCS)
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, BCS 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 BCS populations, whereas mesotrione
responses showed continuous distributions of phenotypes in F2 and
BCS 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.
