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The leguminous Acacia caven, Erythrina crista-galli, Enterolobium contortisiliquum, Prosopis affinis and P. nigra nodulated with rhizobial strains isolated from other legume trees; therefore they were considered to be promiscuous (poorly specific) for nodulation. Sesbania punicea and Parapiptadenia rigida did not nodulate with the different strains, indicating a significant specificity for nodulation. A. caven was promiscuous for nodulation and nodulated with both fast- and slow-growing strains of rhizobia and our data suggest that the most efficient strains would be fast-growing rhizobia. A. caven was specific for effectivity since the amount of N2 which was fixed markedly depended on the strain. The relative efficiency (RE) of 6 out of the 11 strains used was higher than 0.80, suggesting they had an active uptake hydrogenase. Ac5, a strain isolated from A. caven, exhibited the highest N2-fixing ability and highest NE (nodule efficiency) when inoculated on its host and also had the highest RE. Shoot dry weight and acetylene reducing activity (ARA) were significantly correlated with the amount of N2 fixed (determined by the total nitrogen difference method), indicating that any of these methods could be used for screening the strains according to their ability to fix N2. The relative efficiency was not correlated with any of these parameters. |
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