ABffI~CT. Six Bradyrhizobium (lupin) strains were evaluated for their ability to produce siderophores using four chemical
assays. Two strains gave positive reactions with chrome azurol S assay (CAS) and produced hydroxamate-type siderophores.
The other four strains gave negative results for siderophore production using the four assays. Generation time, growth yield
and hydroxamate production of one strain (WPBS 3201 D) were affected by the iron concentration of the culture medium and
the previous culture history of the cells. Resuspension of washed ceils grown previously in media supplemented with 0 and
20 IJmol/L Fe into differing iron regimes (0, 0_5, 1, 2, 4, 8, 10, 15 and 20 lamol/L Fe) suggest that the extent of hydroxamate
production depended on the growth history of the cells. Cells pregrown in 20 lamol/L Fe produced a high amount of hydroxamares
compared with cells pregrown in iron-free medium when resuspended in medium containing up to 4 Izmol/L Fe. Cells
pregrown in 20 lamol/L Fe were more sensitive to iron repression than those pregrown in 0.5 lamoi/L Fe. Mannitol was the best
carbon source for siderophore production. Siderophore synthesis was inhibited by 4-chloromercuribenzenesuifonie acid, 2,4-dinitrophenol,
sodium azide and MgCI 2 suggesting that an energized membrane and a mercapto group are essential and required
for hydroxamate synthesis in strain WPB5 3201 D