Characterization of cross-resistance patterns in acetyl-CoA

The purpose of this study was to determine cross-resistance patterns among wild oat lines resistant to acetyl-CoA carboxylase (ACCase) inhibitors and to determine which, if any, cross-resistant type was more common than another. Discriminatory concentrations of two aryloxyphenoxy-propionates (APP) and three cyclohexanedi- ones (CHD) were determined using a petri-dish bioassay. These concentrations were then applied to 82 resistant wild oat lines identified in previous studies. In addition, two resistant standards (UM1 and UM33) and a susceptible standard (UM5) were included in the experiments. Coleoptile lengths expressed as percentages of untreated controls were used to assess the level of resistance to each herbicide. Large variations were observed among wild oat lines and herbicides. However, cluster analysis sum- marized the relationship between the five herbicides (variables) and the wild oat lines into three main cross-resistance types. Type A included wild oat lines with high resistance to APP herbicides and no or low resistance to CHD herbicides. Types B and C included those with low to moderate resistant and high levels of resistance to all five herbicides, respectively. Type C was the most common cross-resistance type. Relationships among herbicides were determined using pairwise correlation and principal component analysis (PCA). All correlations were high between APP her- bicides and between CHD herbicides but not between APP and CHD herbicides. The first two axes of the PCA accounted for 88.4% of the total variance, with the first axis correlated to the CHD herbicides and the second axis correlated to the APP herbicides. In the PCA, wild oat lines were segregated into the three types identified in the cluster analysis. Although CHD and APP herbicides bind at the same region on the ACCase, resistant wild oat lines respond differently to them. Nomendature: Clethodim, clodinafop, Diclofop methyl, fenoxaprop-P, sethoxy- dim, tralkoxydim, blackgrass, Alopecurus myosuroides Huds., ALOMY; giant foxtail, Setaria faberi Herrm., SETFA; goosegrass, Eleusine indica L., ELEIN; green foxtail Setaria viridis L., SETVI; Italian ryegrass, Lolium multiflorum Lam., LOLMU; large crabgrass, Digitaria sanguinalis L., DIGSA; wild oat, Avena fatua L. AVEFA.