SummaryF2, monosomic analysis involving crosses between the monosomic series of a resistant wheat variety, Chinese Spring, and a susceptible variety, Sicco, has located a major gene locus, designated Dfql, on chromosome 2B of wheat which determines the differential response of these varieties to treatment with the wild oat herbicide, difenzoquat. The allele from Chinese Spring conferring resistance is dominant and studies of the responses of Chinese Spring single chromosome substitution lines and nullisomic–tetrasomic lines for chromosome 2B indicate that this allele actively promotes resistance to the herbicide. It is suggested that this gene may prevent inhibition of DNA synthesis in the apical meristem, which is the site of action of the herbicide (Pallett & Caseley, 1980).Other chromosomes were also implicated as carrying ‘modifier genes’ which affect the ratio of resistant: susceptible plants in F2 monosomic families, namely 1D, 2D, 3A, 3B, 5B and 5D. These chromosomes may affect the retention and translocation of the herbicide to the target site and hence the threshold of response.The simple inheritance of difenzoquat resistance indicates that it should be easy by conventional breeding techniques to transfer the resistance into susceptible varieties.
The responses of wild populations of emmer wheat (Triticum dicoccoides), from different ecogeographical areas of Israel, to three herbicides, difenzoquat, chiortoluron and metoxuron, commonly used on cultivated wheats, were studied. Although cultivated wheats are polymorphic for a response to difenzoquat, all families of all populations of the wild species were resistant. The species was, however, polymorphic for response to both chiortoluron and metoxuron. In addition, there appeared to be differentiation between populations in the frequencies of resistant and susceptible morphs for these herbicides. There was also a close correspondence between the responses of individual families to chiortoluron and metoxuron, which suggests a common genetic control. The implications of these findings for understanding the evolution of herbicide resistance, and for developing strategies for breeding for resistance in the cultivated species are discussed.
SUMMARYSingle chromosome substitution lines of hexaploid wheat were developed using a variety resistant to difenzoquat, Chinese Spring, as donor and a susceptible variety, Sicco, as recipient, and were used to identify chromosomes carrying genes which modify the responses of these varieties. It was found that chromosomes 3B and 5D from Chinese Spring might act to reduce the amount of damage caused by the herbicide in the presence of the allele for susceptibility at the Dfql locus. The intermediate response to the herbicide, which is shown by some commerical varieties, was also investigated using a backcross reciprocal monosomic analysis. In these varieties, the allele at the Dfql locus determining the reaction to the herbicide was shown to be similar to that of the susceptible variety Sicco. It is, therefore, probable that the responses of intermediate varieties are due to the effects of modifier genes increasing resistance.
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