1. The ability of different rhizobial isolates collected from any one site to establish effective nitrogen-fixing associations with host-plants from that site showed significant variation in 22 host Acacia species and nearly all of 67 populations. The average Acacia host–Rhizobium strain combination was only about 70% effective. Many combinations were far poorer; in a few cases the worst combination resulted in plants less than one-tenth the size of the best combinations.
2. The ability of rhizobial isolates to form effective symbiotic interactions showed marked host population and rhizobial-isolate effects in a study of eight, four and nine populations of A. dealbata, A. implexa and A. mearnsii. A more complete trial involved three populations of each of A. dealbata, A. implexa, A. irrorata, A. mearnsii and A. melanoxylon. These 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 found but there was very little evidence of isolate–host population interaction effects.
3. The general lack of host population–rhizobial origin interaction effects suggests that rhizobial strains selected as highly effective for an Acacia species growing in a particular population will generally perform well symbiotically with that species in other populations. This will make their practical application as inoculants in revegetation and forestry situations much easier.
4. 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 ‘foreign’ ones. There were also significant interaction effects. In A. mearnsii, on the other hand, the only significant differences were detected between the response of different half-sib families to the same rhizobial isolate. The occurrence of host-based variability indicates that in Acacia breeding programmes attention should be given to the possibility of inadvertent selection affecting these relationships.