Genetic control of strain-specific ineffective nodulation in Trifolium subterraneum L.

The nodulation response of the Trifolium subterraneum variety Northam First Early with a range of strains of root nodule bacteria has been investigated. An ineffective response was found with the normally effective bacterial strain NA30. Within F2 populations of crosses between Northam First Early and each of six other varieties of T. subterraneum, the response to inoculation with strain NA30 varied from fully ineffective through intermediate effectiveness to fully effective. The proportion of ineffectively noddated plants in these F, populations was also influenced by the non- Northam First Early parent variety. Differences in response among individual plants within the F, populations were probably due to the segregation of a single major gene and a number of modifying genes. Of 10 other strains examined, one strain showed a similar pattern of response to that obtained with NA30, while a second strain exhibited host x strain specific ineffectiveness of a different type.