Effects of salinity on the nodulation in lentil (Lens Culinaris Medik.) and the diversity of its compatible nitrogen-fixing Rhizobium

Abstract

Salinity in soil causes risk for growth, nodulation and biological nitrogen fixation in legumes. Biological nitrogen fixation occurs through the symbiotic association between legumes and Rhizobium. Legume-rhizobia symbiosis and the efficiency of biological nitrogen fixation are influenced by a number of environmental factors including salinity. This study was carried out to evaluate the effects of salinity on the growth parameters and nodulation of lentil plants (Lens culinaris Medik.) at various NaCl concentrations. Seeds of lentil were sown in the experimental plots at the Botanical Garden of University of Dhaka. Each of the four salt treatments (0 mM, 50 mM, 100 mM and 200 mM NaCl) was applied in three replicate plots. Treatments were applied once in every seven days. The size of each plot was 0.0625 m2. Plants were grown for 2 months in the field. Shoot and root growth parameters including length, fresh and dry weight of shoot and root, leaf chlorophyll content, root to shoot ratio, root nodule number and proline content of leaf were compared among the different salt treatments. This study also examined the effects of soil salinity on the number of bacterial colonies obtained from root nodules, rhizosphere soil and bulk soil of lentil plants. The suspensions of root nodules were cultured in the nutrient agar medium and then bacterial colonies were isolated and counted. Bacterial DNA were isolated from pure isolates of bacterial colonies obtained from soils collected from different areas of Bangladesh. PCR amplification using bacterial DNA samples as template was done by targeting 16s rRNA gene. After purified PCR product was achieved, the sequences of different strains of rhizobia were identified by using NCBI blast. Phylogenetic tree was also constructed based on the 16S rRNA sequencing of isolated bacteria. In comparison to control (0 mM), shoot height, root fresh weight, number of nodules and chlorophyll content of leaf decreased with the increase of salt treatment although no significant difference appeared. Proline concentration of leaf of lentil increased significantly with the increase of salt concentrations. Number of bacterial colony increased with the increase of salt concentration. Compared to bulk soil, rhizosphere soil showed higher number of bacterial colonies. The result thus indicates that soil salinity can influence bacteria associated with the root nodule and rhizosphere of lentil plants. The clusters of phylogenetic tree indicated the genetic diversity of rhizobia associated with root nodules collected from different sites. Result of this study is expected to provide valuable information and may be useful to the researchers as well as the policy makers of both Government organizations and Non-government organizations for formulating appropriate policy for widespread cultivation of lentil in Bangladesh.