SIDEROPHORE PRODUCTION BY BRADYRHIZOBIUM SPP STRAINS NODULATING GROUNDNUT

Eighteen Bradyrhizobium spp. strains, four Rhizobium spp. strains and one Azorhizobium caulinodans strain were grown under Fe limitation and assayed for siderophore production. It was further assessed if Fe acc umulation in two groundnut cultivars was influenced by inoculant strai n or nitrate fertilisation. Growth of Bradyrhizobium spp. strains nodu lating groundnut was slow with mean generation times from 11-24 h. All strains, except MAR 967, showed a reduced growth rate when deprived o f Fe; none of the strains showed starvation at 1 mu M Fe. In the CAS ( chrome azurol S)-agar assay, all strains, which formed colonies, produ ced siderophores as visualised by orange halos around the colonies on blue plates. Bradyrhizobium strains produced much smaller halos than t he reference Rhizobium meliloti strain. In the CAS-supernatant assay, ail strains, except MAR 967, gave positive responses (measured as abso rbance at 630 nm) when supernatants of Fe-depleted cultures were assay ed with CAS-indicator complex in comparison with Fe-supplemented cultu res. Responses of all four Rhizobium spp. strains were large, while re sponses of all Bradyrhizobium strains, except B. japonicum MAR 1491 (U SDA 110), were small and mostly insignificant. A small response, i.e. a low Fe-scavenging ability, implies either the production of small qu antities of siderophores or the production of low affinity siderophore s. Among the Bradyrhizobium strains, MAR 1574 and MAR 1587 gave the la rgest responses taken over the two assays. Fe accumulation in groundnu t cultivar Falcon was seven times larger than in cultivar Natal Common . No correlation was found between the quantity of nodule tissue and F e accumulation, making it unlikely that bacteroids are involved in Fe acquisition by groundnuts. Nitrate-fertilised plants accumulated signi ficantly more Fe, suggesting involvement of nitrate reductase in Fe as similation in groundnut. The two most successful Fe-scavenging Bradyrh izobium spp. strains were also the most effective in nodulating ground nut, the reverse also being true. Strain MAR 967, with the lowest Fe r equirement, produced the largest nodule dry weight. These data indicat e that improved Fe scavenging properties and/or reduced Fe requirement improve rhizospheric growth and with that nodulation effectiveness.