Abstract
Identification and symbiotic characterization of indigenous rhizobial isolates are the basis for inoculant formulations needed for sustainable grain legume production. This study screened for morpho-genetic diversity of indigenous cowpea nodulating rhizobia in farmers’ fields across two contrasting agroecological zones of Northern Mozambique. The photosynthetic function induced by the isolates in their homologous cowpea was assessed. The results showed high genetic variability among the isolates based on morphology and ERIC-PCR fingerprinting. The trap cowpea genotype did not influence the diversity of isolates collected from the two different agroecologies, suggesting that the cowpea-rhizobia compatibility may be conserved at species level. Phylogenetic analysis of the 16S rRNA gene assigned representative rhizobial isolates to species in the Bradyrhizobium and Rhizobium genera, with some isolates showing high divergence from the known reference type strains. The isolates from both agroecologies highly varied in the number and biomass of nodules induced in the homologous cowpea, resulting in variable plant growth and photosynthetic activities. A total of 72% and 83% of the isolates collected from the agroecological zones 7 and 8 were respectively classified as highly effective candidates with > 80% relative effectiveness compared to plants fertilized with nitrate, indicating that elite native strains populated the studied soils. Moreover, the top 25% of high N2-fixing isolates from the two agroecologies recorded relative effectiveness ranging from 115 to 154%, values higher than the effectiveness induced by the commercial Bradyrhizobium sp. strain CB756. These strains are considered as having potential for use in inoculant formulations. However, future studies should be done to assess the ecologically adaptive traits and symbiotic performance under field conditions.
Highlights
Nitrogen is one of the essential macro nutrient elements required for plant growth, synthesis of macromolecules such as chlorophyll needed for photosynthesis[1,2] and other biomolecules such as Rubisco which reduces CO2 during photosynthesis[3]
Hundred and thirty-seven isolates (79 from Agroecological zone 7 (AEZ 7) and 58 from Agroecological zone 8 (AEZ 8)) were used for evaluation of the symbiotic effectiveness and gasexchange capacities
95% of isolates from AEZ 7 and 93% from AEZ 8 were gummy in texture while only 5% and 7%, respectively, exhibited dry colony texture
Summary
Nitrogen is one of the essential macro nutrient elements required for plant growth, synthesis of macromolecules such as chlorophyll needed for photosynthesis[1,2] and other biomolecules such as Rubisco which reduces CO2 during photosynthesis[3]. Indigenous rhizobia may be abundant in soils and infected root, but yet be inefficient or exhibit low N2-fixing efficiency, leading to retarded plant growth[30]. Under such conditions, inoculation of the legume with highly effective rhizobial strains can improve N2 fixation, plant growth and grain yield[31,32]. Earlier reports have revealed the presence of efficient N2-fixing rhizobia nodulating soybean in Mozambique[36,37] This suggests the need of screening for efficient symbionts for other grain legumes, including cowpea. Despite the presence of highly diverse cowpea-nodulating rhizobia in Africa and e lsewhere[38,39,40,41,42,43,44], little is known about their diversity in the Mozambican legume production fields
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