Abstract

The rates of nitrogen fixation by symbiotic rhizobia are often two to three times higher than the ones exhibited by free-living nitrogen-fixing bacteria in soil. In this study, Rhizobium leguminosarum, Rhizobium meliloti, Rhizobium loti and Rhizobium ciceri were isolated and identified from leguminous plants grown at the north of Jordan. The aim of this study was initially to determine the most rhizobial species resistant to salt and heat stress. Nitrogenase activity and ammonium production were then used to select the most active bacterial species. Nitrogenase activity, using acetylene reduction technique, was 0.763 and 0.475 µmol C₂H₄ / mg protein/ h for R. leguminosarum and R. meliloti respectively. However, those two species growing on a nitrogen-free media produced 2 to 3.4 and 2.8 to 5.0 µg ml^-1 of ammonium with glutamate or histidine, as nitrogen sources respectively. The specific association between rhizobia and plant species was revealed under the transmission electron microscopy. The effect of salt on the growth of Rhizobium appeared to depend on the strains themselves, as the responses of the strains were disparate in the same soil and region.

Key words: Jordan, nitrogen fixation, nitrogenase, Rhizobium