Variation in the effectiveness of symbiotic associations between native rhizobia and temperate Australian legumes: interactions within and between genera

1. Tests of the effectiveness of rhizobia isolated from several common Acacia species showed significant variation among host populations in plant growth but no indication that plant performance depended on which Acacia species the rhizobial isolate was derived from. Variation in effectiveness was primarily between rhizobial isolates within a host species rather than across rhizobial isolates from different hosts. No interactions were found between host population, host origin of rhizobial strain and rhizobial isolate. 2. A study using a range of less common Acacia species showed significant variation in performance of several species (A. silvestris, A. nano-dealbata, A. trachyphloia, A. filicifolia, A. leucoclada) as a function of the inoculating rhizobial strain; two other species (A. cangaiensis, A. glaucocarpa) were marginally non-significant. For A. filicifolia and A. silvestris, growth was significantly better when plants were inoculated with their own isolates vs. those from other host species. 3. Comparisons of the performance of a third set of Acacia species found that several common species (A. melanoxylon, A. mearnsii, A. irrorata, A. binervata) varied little in their response to particular rhizobial isolates; for these species, most host-rhizobial combinations resulted in similar growth responses. Several less common species showed significant variation in performance across the rhizobial isolates tested. 4. Preliminary investigations of host specificity among legume genera showed significant variation with respect to host performance as a function of the rhizobial isolate with which plants were inoculated. Some species (e.g. A. mearnsii and Indigofera australis) were variable in their responses, while others (e.g. Hardenbergia violacea and A. melanoxylon) performed well with most isolates. 5. A comparison of the effectiveness of several Acacia-derived strains on their own hosts vs. their effectiveness on the other legume genera in the study showed that, in general, the Acacia strains performed well on other genera. However, on some host species (e.g. I. australis) the Acacia strains were less effective than the Indigofera strains, suggesting some host specificity. 6. While there was a general correlation at the among-species level in Acacia-rhizobia interactions (i.e. isolates that performed well on one host species often performed well on others), growth performance with any particular host-rhizobial combination was more difficult to predict than within-species comparisons. Our data also suggest that, at least for the genus Acacia, rare species are likely to exhibit a greater degree of host specificity than hosts that are widely distributed, and re-establishment may require special attention with respect to rhizobial strains that are used. 7. Overall, these results have clear implications for applied projects focused on the restoration of degraded habitats or abandoned farmland. Simply replanting native species into such areas may be unsuccessful unless appropriate rhizobial strains are also reintroduced. Our results suggest that when no strains for a particular host species are available, strains from its closest relative will have the highest probability of success.