Rhizobial Diversity Associated with South African Legumes

N₂-fixing legumes play a major role in the natural ecosystems in areas of Mediterranean-type climates, accounting for 80% of biologically-fixed N (McInroy et al., 1999). However, information is needed particularly for shrubby legumes and for those that, like many acacias, succeed in new environments and out-compete the native flora. Although mutualisms, involving N₂-fixing bacteria, could constrain legume invasion success, they have not been investigated in this context (Yates et al., 2004). This work aims to identify rhizobial strains associated with six South African shrubby legumes and to investigate the effect of Bradyrhizobium on the growth of Acacia karooHayne and Acacia cyclops A.Cunn. ex G.Don. Bradyrhizobium strains were isolated from root nodules of all the selected species and were characterized morphologically and by PCR and 16S rDNA. Seeds from the two acacias were inoculated with 2 mL of the appropriate bacterial inocula, which consisted of bacterial strains from the six South African and six Spanish species. Ten plants per treatment were grown in a greenhouse for 20 weeks and watered with a sterile N-free nutrient solution. Morphological and molecular characterization show that all extracted strains are different bradyrhizobia. Both acacias are promiscuous hosts and form efficient nodules with Bradyrhizobium strains from the six South African and six Spanish shrubby legumes. The number of nodules and resulting biomass production was higher in Acacia cyclops than in Acacia karoo when inoculated with any of the strains. The better ability of A. cyclops to produce more effective nodules probably accounts for the better ability of A. cyclops to colonize new environments. Once Acia cyclops is established, it dominates other shrub vegetation and alters grassland and woodland under-storey habitats by modifying light and soil moisture regimes and nitrogen dynamics, thus preventing other plants establishing. The lack of specificity of A. cyclops suggests that isolates that establish early in the plant’s growth are more likely to contribute inoculum for other nodules as the roots growth.