Effects of endophytic Streptomyces and mineral nitrogen on Lucerne (Medicago sativa L.) growth and its symbiosis with rhizobia

The effects of three endophytic Streptomyces on plant growth and the symbiosis of Lucerne and its rhizobial partner were examined in the presence of three levels of soil nitrogen. Three Streptomyces strains, LuP30 and LuP47B isolated from the roots of Lucerne (Medicago sativa L.) and EN23 isolated from roots of wheat (Triticum aestivum L.) were added as spores to Lucerne seeds (with and without Sinorhizobium meliloti RRI 128) at three levels of applied NH4NO3: 3, 25 and 50 mg/kg of soil. Plant growth increased with the addition of the actinobacteria strains alone from 19 % to 33 %. Co-inoculation of LuP30 with rhizobia strain RRI 128 produced the largest increase in shoot weight (46 %) of Lucerne plants growing in soil with 25 mg/kg NH4NO3. Co-inoculation with each of the actinobacteria with the rhizobia increased the number of nodules by more than 100 % compared with RRI128 alone, 4 weeks after rhizobial inoculation. A labelled N-15 experiment showed co-inoculation with rhizobia and LuP30 or LuP47B enhanced N-2-fixation 47 % and 72 %, respectively. The actinobacteria significantly improved plant growth and N-2-fixation when applied with the rhizobia strain RRI 128 to Lucerne plants growing in soil supplied with 25 mg/kg NH4NO3.