Abstract:
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.
