Symbiotic nitrogen fixation efficiency of native rhizobia in selected South African legume crops

This Master’s dissertation reports about the screening and characterization of selected alpha and beta rhizobial isolates from wild legumes in South Africa for their nodulation and nitrogen fixation properties on the cultivated legumes lucern (Medicago sativa L), cowpea (Vigna unguiculata L) and siratro (Microptilium atropurpeum D.C) under glasshouse and field conditions. The rhizobia were initially in-vitro characterized for their tolerance to various abiotic stresses and most of the strains were found to be tolerant to extremes of environmental factors such as acidity, aluminium toxicity, salinity and temperature. They were then screened for nodulation and nitrogen fixation efficacy under glasshouse and field conditions. Additional in-vitro screening for essential plant growth promoting traits including the production of siderophores, indole acetic acid, ACC-deaminase and phosphate solubilization was conducted. Most of the isolates from the wild legumes, i.e., 7 strains (6 Bradyrhizobium and 1 Paraburkholderia nodulated cowpea, 1 Bradyrhizobium strain nodulated lucerne and 13 strains (3 Paraburkholderia and 10 Bradyrhizobium strains) nodulated siratro, in the glasshouse experiment with a statistically significant number of nodules (p > 0.05). Plant biomass, including fresh weight and dry weight, were significantly improved by Bradyrhizobium strains 10BB and Arg68 in cowpea and siratro compared to un-inoculated controls. Five strains for cowpea, six strains for siratro and one strain for lucerne were selected as the best strains for field trial. After harvest, cowpea plant biomass were significantly increased when inoculated with Bradyrhizobium sp. Arg68 followed by Paraburkholderia sp. KB15 with significant increase in the amount of fixed nitrogen. There was significant difference in the amount of nitrogen fixed when inoculated with different strains of rhizobia. In siratro, plant biomass was increased after inoculation with Bradyrhizobium sp. Fp1c strain followed by Bradyrhizobium sp. 10BB although the amount of nitrogen fixed had significant different and same applies to lucerne with no nodules formed on control plant. All of the Bradyrhizobium strains tested positive for the presence of nifH gene while Bradyrhizobium strains Arg68 and Arg62 strains contained the nodC genes. The study has generated important baseline data, which can be used for further development of the rhizobial strains as legume inoculants for cowpea, siratro and lucerne, but warrants further nodulation screening study in these and other legumes of similar cross inoculation groups with cowpea, lucerne and siratro.