Nodulation and nitrogen fixation of Lupinus species with Bradyrhizobium (lupin) strains in iron-deficient soil

The effect of six Bradyrhizobium sp. (lupin) strains (WPBS 3201D, WPBS 3211 D, USDA 3040, USDA 3041, USDA 3042 and CB 2272) and Fe supply on nodulation , Np-fixation and growth of three lupin species (Lupinus termis, L. albus a nd L. triticale) grown under Fe deficiency in an alkaline soil, were examin ed in sterilized and non-sterilized pot experiments. When inoculated with U SDA 3040, 3041, 3042 and CB2272 without Fe addition, the three lupin specie s had a very low nodule number and mass, low shoot and root dry matter accu mulation and lower N yield. However, inoculation with WPBS 3201D and 3211D without Fe treatments increased all these parameters substantially. The abi lity of WPBS 3201D and 3211D to form nodules on the three lupin species und er conditions of Fe stress could be attributed to their ability to scavenge Fe from Fe-deficient environments through their siderophore production. Ad dition of Fe to the other four strains significantly increased nodulation a nd N-2-fixation of the three lupin species, indicating that the poorer nodu lation and N-2-fixation of these strains in the absence of Fe, resulted fro m a low ability to obtain Fe from alkaline soils. Bradyrhizobium strains WP BS 3201D and 3211D were superior to the other four strains in terms of prom oting greater nodulation, N-2-fixation, plant growth and N accumulation of L. termis and L. albus. However, the other four strains were more efficient in symbiotic association with L. triticale. The greater variations in nodu le efficiencies (specific nitrogenase activity) under different levels of F e supply could be attributed to the quantities of bacteroid protein and leg haemoglobin in the nodules. The results suggested that Bradyrhizobium (lupi n) strains-differ greatly in their ability to obtain Fe from alkaline soils , and that the selection of bradyrhizobial strains which are tolerant of Fe deficient soils could complement plant breeding for the selection of legum e crops for Fe-deficient soils.