Genetic studies with wild diploid Trifolium ambiguum M. Bieb. with respect to time of nodulation

Abstract

Population studies made with an alpine diploid form of the perennial rhizomatous Trifolium ambiguum, inoculated with moderately effective (best available) strain of Rhizohium trifolii from the margin of the restricted natural habitat of this plant, showed great variation within the host with respect to time of nodulation. When progenies derived from paired crossings between early-nodulating parents and similarly from late-nodulating parents were studied under standardized conditions, differences in time to initial nodulation between progenies of early-nodulating plants and progenies of late-nodulating plants were highly significant. Analysis revealed that parents within each group differed with respect to their general and specific combining abilities. Indications were that selection of parents following diallel crossing would permit faster selection responses to be obtained than were brought about following phenotypic selection and polycrossing. Nevertheless, at the beginning of the programme an effective selection response for early nodulation was achieved over four generations by polycrossing and single plant selection alone. This plant material and a combination of these phenotypic and genotypic plant selection techniques has resulted in the development of a registered cultivar (Summit) suitable for use at high altitudes.