Remedial inoculation of Rhizobium meliloti strains and nodule occupancy on Medicago rigidula (L.) All. and M. truncatula Gaertn

Abstract

Because post-planting inoculation of soil with Rhizobium sp. is often practiced when seed inoculation fails, the potential of delayed (remedial) root inoculation as a feasible agricultural practice for two annual pasture legumes was investigated. Roots of Medicago rigidula International Center for Agricultural Research in the Dry Areas (ICARDA) sel.716, and Medicago truncatula cv. Jemalong, were pre-exposed to Rhizobium meliloti strains. Subsequently, roots were exposed to a secondary inoculum after 6, 48, and 168 h (8 days) to simulate remedial inoculation. Subsequent establishment of the secondary inoculum strains in the nodules was investigated. Combinations of highly effective (capacity for nitrogen fixation) and host compatible strains (ICARDA M15 and M28) for both hosts, and effective–ineffective strains (M28 and M34) for M. rigidula and M. truncatula, respectively, were used to determine the proportional nodule occupancy responses based on their differential resistance to spectinomycin and streptomycin, and in the case of M34, on nodule characteristics. When equal populations of M28 and M15 were applied immediately after inoculation, nodule population differences were not significant in M. rigidula but were significantly greater ( P≤0.01) for strain M28 in M. truncatula. M. trunculata plants, pre-exposed to M15 and receiving the ineffective M34 strain as a secondary inoculant, showed that predisposition of the plant to accept the primary inoculant was not a factor in nodule occupancy success. Nodule occupancy of the primary inoculant (M15) was suppressed significantly ( P≤0.01) by M34. This was not the case for all other combinations of effective strains in both legumes. Results indicate that the early events in the nodulation process of annual Medicago spp. coupled with host-specificity factors are perhaps the most critical for competition among R. meliloti strains for nodule occupancy. We conclude that remedial R. meliloti inoculation applied after the seed has been planted and colonized by soil bacteria may be of little benefit to legume growers.