Variation in the effectiveness of symbiotic associations between native rhizobia and temperate Australian Acacia: within-species interactions

1. The ability of different rhizobial isolates collected from any one site to establish effective nitrogen-fixing associations with host-plants from t hat 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 interactio ns showed marked host population and rhizobial-isolate effects in a study o f eight, four and nine populations of A. dealbata, A. implexa and A. mearns ii. A more complete trial involved three populations of each of A. dealbata , A. implexa, A. irrorata, A. mearnsii and A. melanoxylon. These were inocu lated with a range of rhizobial isolates previously shown to be highly, mod erately 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 sym biotically with that species in other populations. This will make their pra ctical application as inoculants in revegetation and forestry situations mu ch easier. 4. Significant host-based variability in the ability to form effective symb iotic interactions was detected in comparisons of half-sib families of A. d ealbata, A. mearnsii and A. melanoxylon. In the case of A. dealbata, the in teraction between half-sib family lines and rhizobial isolates was complex, with 'locally' derived isolates performing better than 'foreign' ones. The re were also significant interaction effects, In A. mearnsii, on the other hand, the only significant differences were detected between the response o f different half-sib families to the same rhizobial isolate. The occurrence of host-based variability indicates that in Acacia breeding programmes att ention should be given to the possibility of inadvertent selection affectin g these relationships.