Auxin-mediated responses of kochia biotypes resistant to dicamba (HRd) or resistant to dicamba and fluroxypyr (HRdf) were compared with those of two susceptible biotypes. Rates of shoot and root gravitropic response and patterns of apical dominance, as determined by lateral bud sprouting after decapitation, were determined in the absence of herbicide treatment. Shoots of susceptible plants reoriented toward vertical at a rate of 23.4° h⁻¹, whereas the rates of HRd and HRdf shoot reorientation were significantly slower at 7.2° and 14.4° h⁻¹, respectively. Root gravitropic responses were not different between resistant and susceptible biotypes. In contrast to susceptible biotypes, both apical and basal lateral buds on HRd plants elongated after decapitation, although differences between HRd and susceptible biotypes became smaller during succeeding weeks. The elongation pattern of HRdf lateral buds was intermediate to that of susceptible and HRd plants. Inhibition assays of root growth by natural and synthetic auxins showed that HRd root growth was less sensitive to dicamba, 2,4-D, naphthalene-1-acetic acid, and indole-3-acetic acid than was root growth of HRdf or the susceptible biotypes. Collectively, results support the hypothesis that auxin binding or signal transduction pathways are impaired in resistant biotypes and that HRd may contain different lesions than does HRdf.

Nomenclature: Dicamba; fluroxypyr; 2,4-D; kochia, Kochia scoparia L. Schrad KCHSC.