A diclofop-methyl-resistant biotype of Italian ryegrass was characterized to determine the expression and inheritance of herbicide resistance and whether this trait was due to the presence of a diclofop-insensitive form of acetyl-coenzyme A carboxylase (ACCase). At the whole plant level, the resistant biotype was > 93-fold more resistant to diclofop-methyl than the susceptible biotype. Crosses of diclofop-resistant and –susceptible plants were performed to produce F1 plants. No maternal effects were evident in responses of reciprocal F1 plants to diclofop. GR50 diclofop rates determined for resistant, F1, and susceptible plants were 15, 6.3, and 0.16 kg ha−1, respectively. F2 populations treated with a 7.5 kg ha−1 rate of diclofop exhibited three injury response phenotypes 3 wk after treatment: a susceptible (S) phenotype which was killed, an intermediate resistance (I) phenotype with severe injury, and a resistant (R) phenotype with little or no injury. Testcross progeny exhibited only I and S phenotypes. Observed segregation of phenotypes in F2 and testcross populations conformed to segregation ratios predicted for a trait with inheritance controlled by a single partially dominant nuclear gene. ACCase activity determined in crude cell-free extracts of resistant, F1, and susceptible biotypes exhibited I50 values of 50, 20, and 0.7 μM diclofop, respectively. A positive relationship between the injury response phenotype and site of action (ACCase) response to diclofop was evident in both F1 and F2 populations. In extracts from R, I, and S phenotype F2 plants, 20 μM diclofop acid inhibited ACCase-mediated incorporation of 14C by 27.1, 45.1, and 78.9%, respectively. The ACCase data are consistent with the hypothesis that diclofop resistance in Italian ryegrass is conferred by a diclofop-insensitive form of ACCase.

The objectives of this study were to determine the inheritance of aryloxyphenoxypropionate (APP) resistance in the wild oat population UM33 and to determine the genetic relationship between resistance in UM33 and another population, UM1, which has a different cross-resistance pattern. Reciprocal crosses were made between UM33 and a susceptible population UM5, and between UM33 and UM1. Initial screenings of F1 and F2 Is populations derived from crosses between UM33 and UM5 were conducted over a range of fenoxaprop-P rates to determine a discriminatory dosage. F2 populations and F2-derived F3 families were then screened at this dosage (1200 g ai ha−1) to determine segregation patterns. Results from reciprocal UM33 x UM5 F1 dose-response experiments, and F2 and F2-derived F3 segregation experiments indicated that UM33 resistance to fenoxaprop-P was governed by a single, partially dominant nuclear gene system. To determine if resistance in UM1 and UM33 results from alterations at the same gene locus, 584 F2 plants derived from reciprocal UM33 x UM1 crosses were screened with 150 g ha−1 fenoxaprop-P. This dosage was sufficient to kill susceptible plants (UM5), but was not sufficient to kill plants with a resistance allele from either parent. None of the treated F2 plants exhibited injury or death, indicating that UM1 and UM33 resistance genes did not segregate independently. From this it was concluded that resistance in both populations is encoded at the same gene locus.

Resistance to fenoxaprop-P and other aryloxyphenoxypropionate and cyclohexanedione herbicides in the wild oat population, UM1, is controlled by a single, partially dominant, nuclear gene. In arriving at this conclusion, parents, F1 hybrids, and F2 plants derived from reciprocal crosses between UM1 and a susceptible wild oat line, UM5, were treated with fenoxaprop-P over a wide range of dosages. Based on these experiments, a dosage of 400 g ai ha−1 fenoxaprop-P was selected to discriminate between three response types. At this dosage, susceptible plants were killed and resistant plants were unaffected, whereas plants characterized as intermediate in response were injured but recovered. Treated F2 plants segregated in a 1:2:1 (R, I, S) ratio, indicative of single nuclear gene inheritance. This was confirmed by selfing F2 plants and screening several F3 families. Families derived from intermediate F2 plants segregated for the three characteristic response types, whereas those derived from resistant F2 plants were uniformly resistant. Chisquare analysis indicated the F2 segregation ratios fit those expected for a single partially dominant nuclear gene system. In addition, F2 populations from both crosses were screened with a mixture of fenoxaprop-Pand sethoxydim. The dosages of both herbicides (150 g ai ha−1 fenoxaprop-P and 100 g ha−1 sethoxydim) were sufficient to control only susceptible plants. Treated F2 populations segregated in a 3:1 (R:S) pattern, thereby confirming that resistance to the two chemically unrelated herbicides results from the same gene alteration.

Two DNA molecular marker techniques were used to evaluate genetic diversity in 58 accessions of jointed goatgrass and 6 accessions of the related wild species barb goatgrass. Random amplified polymorphic DNA (RAPD) assays were performed on 8 U.S. and 50 Eurasian jointed goatgrass accessions using 30 random decamer primers. The frequency of scorable polymorphic bands within jointed goatgrass was 6 out of 90 (6.7%). Cluster analysis of RAPD data showed small genetic distances (values of 0.005 or less) among jointed goatgrass accessions. To validate the effectiveness of RAPD techniques to detect genetic diversity in tetraploid Aegilops species, six accessions of barb goatgrass were assayed using a subset of 20 decamer primers (from the original 30). RAPD data for barb goatgrass were pooled with jointed goatgrass data from the same primers. A total of 63 scorable bands were generated, of which 27 (43%) were polymorphic between two or more accessions. RAPD analysis readily distinguished between the two species and detected much greater levels of genetic diversity within barb goatgrass than between the jointed goatgrass accessions. Amplified fragment length polymorphism (AFLP) assays were performed on a subset of the 58 jointed goatgrass accessions, 3 U.S. and 13 Eurasian. These accessions were selected to represent a range in geographic diversity within our collection. Ten primer combinations generated 560 scorable bands of which 28 (5%) were polymorphic. Cluster analysis of AFLP data showed a slightly smaller range in genetic distance (0.0002 to 0.0022) among accessions compared with RAPD results; however, AFLPs distinguished among all but 2 of the 16 accessions surveyed. Although AFLP produced more scorable bands than RAPD did, both methods revealed limited genetic diversity in jointed goatgrass.