The symbiotic effectiveness and geographic origin of morphological-agronomic groups of Stylosanthes accessions

Abstract

A collection of 241 accessions of 15 Stylosanthes species was tested against Rhizobium CB756, a standard wide spectrum cowpea type strain of African origin, for effectiveness in nitrogen fixation of the symbiosis. The response, which was described as ineffective, effective, or highly effective, was closely related to the morphological-agronomic (M-A) groupings of the accessions within species. Species and M-A groups that require a different or specific rhizobial strain were indicated. The relation of the effectiveness of the symbiosis with Rhizobium CB756 to geographic origin was examined for 184 accessions of known origin. The phytogeographic regions of Good (1964) were used as a basis for comparison. With S. guyanensis eight of the ten M-A groups that were ineffectively nodulated by CB756 largely originated in the South Brazilian region. The remaining two M-A groups were restricted either to the Pampas of Uruguay and Argentina or the Venezuela-Guiana region.