Symbiotic characteristics of a Rhizobium-specific annual medic, Medicago rigidula (L.) All

Abstract

Medicago rigidula (L.) All. is a potentially useful legume for ley-farming systems in West Asia. It is an omni-Mediterranean species, divided into four subspecies or varieties. Some soils within the habitat of M. rigidula contain no strains of Rhizobium meliloti effective for it. It is reputed to require specific rhizobia in order to fix nitrogen. Experiments were conducted with the four varieties of M. rigidula (and with two lines of one variety) to define the degree of specificity for strains of R. meliloti in the species and to locate effective strains. M. rigidula as a species was highly strain specific for effective symbiosis. Australian soils contained no suitable rhizobia but effective rhizobia were isolated from nodulated plants in Syria and from Syrian soil where the plant is native. Rhizobia effective for M. rigidula also fixed nitrogen with Medicago noëana Boiss. but not with 15 other species of Medicago. In general, strain specificity did not exist between varieties within M. rigidula but the evidence was equivocal. Strains of R. meliloti effective for all four varieties were identified. Provided that these strains prove satisfactory under field conditions, they would be suitable inoculants for M. rigidula in situations where inoculation is deemed necessary.