Contribution of Biological Nitrogen Fixation in Lesser Yam (Dioscorea esculenta L.) Associated with Endophytic Diazotrophic Bacteria

Abstract

Three accessions of lesser yam (Dioscorea esculenta L.) were grown under different fertilizer regimes in pots for 220 days at Miyako Island, Okinawa, Japan. Biological nitrogen fixation (BNF) was evaluated in non-fertilized plants. Nitrogen-fixing endophytic bacteria were isolated on nitrogen-free Modified Rennie medium at 180 days after planting (DAP). Results show that growth of lesser yam was affected by fertilizer regimes. Plant nitrogen content was increased under chemical fertilizer and organic matter treatments, while plant dry matter was not different among treatments at 180 DAP. Leaf δ¹⁵N decreased over time in control plants. BNF ranged from 20 to 65%, depending on accessions. The lowest value of %Ndfa was found in accession E-1 and the highest value was found in E-2 at 220 days after planting. 52 bacteria strains belonging to 17 genera and 36 species were isolated from root, stem, tuber and leaf. Surprisingly, most endophytic diazotrophic bacteria were isolated from the leaves and the stems (76%) in all accessions. Proteobacteria including bacterial genera such as Acinetobacter, Pantoea, Pseudomonas, Pseudomonas, Serratia and Stenotrophomonas were identified as well as Actinobacteria and Firmicutes (Bacillus and Paenibacillus). We report here, for the first time, different nitrogen-fixing bacteria isolated from tuber of lesser yam. Our study suggests, for the first time that lesser yam can obtain substantial amount of nitrogen from air in association with diazotrophic endophytic bacteria. This is to our knowledge the first report on estimation of BNF in lesser yam.