Variation in the effectiveness of symbiotic associations between native rhizobia and temperate Australian Acacia: within-species interactions.
A study was conducted during 1993-95 to assess the ability of different rhizobial isolates collected from 67 populations of 22 species of Acacia across south-eastern Australia to establish effective nitrogen-fixing associations with host-plants from each site. Significant variation was observed between the 22 host Acacia species and between sites. The average Acacia-Rhizobium combination was ∼70% effective. Many combinations were far less successful, resulting in plants less than one-tenth the size of the best combinations. The ability of rhizobial isolates to form effective symbiotic interactions exhibited marked host population and rhizobial-isolate effects in a study of 8, 4 and 9 populations of A. dealbata, A. implexa and A. mearnsii, respectively. A more complete trial involved 3 populations each of A. dealbata, A. implexa, A. irrorata, A. mearnsii and A. melanoxylon, which were inoculated with a range of rhizobial isolates previously shown to be highly, moderately or weakly successful in forming an effective association. Evidence of marked host population and rhizobial origin effects was observed but there was little evidence of isolate-host population interaction effects, indicating that rhizobial strains selected as highly effective for an Acacia species growing in a particular population would generally perform well symbiotically with that species in other populations thus making their practical application as inoculants in revegetation and forestry situations much easier. Significant host-based variability in the ability to form effective symbiotic interactions was detected in comparisons of half-sib families of A. dealbata, A. mearnsii and A. melanoxylon. In the case of A. dealbata, the interaction between half-sib family lines and rhizobial isolates was complex, with 'locally' derived isolates performing better than others. There were also significant interaction effects. The occurrence of host-based variability indicated that in Acacia breeding programmes the possibility of inadvertent selection affecting these relationships should be considered.