Nitrogen fixation (C2H2 reduction) by Medicago nodules and bacteroids under sodium chloride stress

Medicago ciliaris (L.) All., a salt-tolerant legume, was not nodulated by Rhizobium meliloti (2011), a strain commonly used for field inoculation of alfalfas. A strain of Rhizobium meliloti (ABS7) was isolated from saline Algerian soils. It is generally more salt-resistant than strain 2011, exhibits a higher rate of growth and induces the formation of nodules on M. ciliaris . C₂H₂ reduction activity of M. ciliaris nodules was inhibited by 50% in the presence of 200 m M NaCl in the culture medium. whereas 100 m M NaCl was sufficient to inhibit the activity of nodules of M. sativa (L. cv. Europe). C₂H₂ reduction by bacteroids, isolated from nodules of the two species of alfalfa, was directly inhibited by the presence of NaCl in the incubation medium. In both cases, glucose could support bacteroid nitrogen fixation, but only in a narrow range of O₂ tensions. Bacteriods from M. ciliaris were more tolerant to salt than M. sativa ones. The salt resistance of bacteroids from nodules of plants watered with NaCl solutions was not improved in either species. Salt directly added to the incubation mixture of bacteroids or to the culture medium of plants inhibited O₂ uptake of bacteroids isolated from nodules of both M. ciliaris and M. sativa . The depressive effect of NaCl on bacteroid C₂H₂ reduction could be directly related to the drop in bacteroid respiration. The nitrogen fixation capacity of the M. ciliaris-Rhizobium meliloti (ABS7) symbiosis under saline conditions leads us to recommend the introduction of this association in salt-troubled areas.