Assessing host range, symbiotic effectiveness, and photosynthetic rates induced by native soybean rhizobia isolated from Mozambican and south African soils

Article purchasedHost range and cross-infectivity studies are important for identifying rhizobial strains with potential for use as inoculants. In this study, 10 native soybean rhizobia isolated from Mozambican and South African soils were evaluated for host range, symbiotic effectiveness and ability to induce high rates of photosynthesis leading to enhanced plant growth in cowpea (Vigna unguiculata L. Walp.), Bambara groundnut (Vigna subterranean L. Verdc.), Kersting’s groundnut (Macrotyloma geocarpum Harm) and soybean (Glycine max L. Merr). The test isolates had different growth rates and colony sizes. Molecular analysis based on enterobacterial repetitive intergenic consensus (ERIC)-PCR revealed high genetic diversity among the test isolates. The results further showed that isolate TUTLBC2B failed to elicit nodulation in all test plants, just as TUTNSN2A and TUTDAIAP3B were also unable to nodulate cowpea, Kersting’s bean and Bambara groundnut. Although the remaining strains formed ineffective nodules on cowpea and Kersting’s bean, they induced effective nodules on Bambara groundnut and the two soybean genotypes. Bacterial stimulation of nodule numbers, nodule dry weights and photosynthetic rates was generally greater with isolates TUTRSRH3A, TUTM19373A, TUTMCJ7B, TUTRLR3B and TUTRJN5A. As a result, these isolates elicited significantly increased accumulation of biomass in shoots and whole plants of Bambara groundnut and the two soybean genotypes. Whole-plant symbiotic nitrogen (N) of soybean and Bambara groundnut was highest for the commercial strains CB756 and WB74, as well as for TUTRLR3B, TUTMCJ7B and TUTRSRH3A, suggesting that the three native rhizobial isolates have potential for use as inoculants

Rhizosphere acid and alkaline phosphatase activity as a marker of P nutrition in nodulated Cyclopia and Aspalathus species in the Cape fynbos of South Africa

AbstractCyclopia and Aspalathus species are important economic legumes in the Cape fynbos of South Africa, as they are used for making Honeybush and Rooibos tea, and for trade in the cut wild flower industry. The aim of this study was to assess acid and alkaline phosphatase activity in the rhizosphere of Cyclopia genistoides, Cyclopia subternata, Aspalathus caledonensis and Aspalathus aspalathoides as an indicator of P supply and P nutrition in the nutrient-poor soils of the Cape fynbos. Whether at Kokrivier or Kanetberg, the P enzyme activities were much higher in the rhizospheres of the legumes C. genistoides, C. subternata, A. caledonensis, and A. aspalathoides compared to those of the non-legumes Leucadendron strictum, Elegia thyrsoidea and Mimetes cucullatus, or bulk soil. As a result, plant-available P concentration in the rhizosphere, as well as shoot P levels closely mirrored acid and alkaline phosphatase activity in the rhizosphere of each plant species. Relative to younger plants, older Cyclopia species exhibited, much greater acid and alkaline phosphatase activity in the rhizosphere and this again resulted in much higher plant-available rhizosphere P. C. subternata plants developed from cuttings at Kanetberg showed greater rhizosphere acid and alkaline phosphatase activity than seedlings and bulk soil. As a result, the concentration of plant available-P and organic P were much higher in the rhizosphere of cuttings than seedlings, leading to greater shoot P in cuttings than seedlings. Taken together, these data suggest that rhizosphere P enzyme activity can be used as a good indicator of P supply and P nutrition in Cyclopia cuttings and seedlings, but less so in Aspalathus species in the Cape fynbos. The enhanced P nutrition in plants from cuttings probably accounts for the higher tea yields obtained by farmers when they use cuttings instead of seedlings in their plantations

Identification and distribution of microsymbionts associated with soybean nodulation in Mozambican soils

Article purchasedIndigenous soybean rhizobial strains were isolated from root nodules sampled from farmers’ fields in Mozambique to determine their identity, distribution and symbiotic relationships. Plant infection assays revealed variable nodulation and symbiotic effectiveness among the 43 bacterial isolates tested. Strains from Ruace generally promoted greater whole-plant growth than the others. 16S rRNA-RFLP analysis of genomic DNA extracted from the rhizobial isolates produced different banding patterns, a clear indication of high bacterial diversity. However, the multilocus sequence analysis (MLSA) data showed alignment of the isolates with B. elkanii species. The 16S rRNA sequences of representative soybean isolates selected from each 16S rRNA-RFLP cluster showed their relatedness to B. elkanii, as well as to other Bradyrhizobium species. But a concatenated phylogeny of two housekeeping genes (glnII and gyrB) identified the soybean nodulating isolates as Bradyrhizobium, with very close relatedness to B. elkanii. The nifH and nodC sequences also showed that the majority of the test soybean isolates were closely related to B. elkanii, albeit the inconsistency with some isolates. Taken together, these findings suggest that the B. elkanii group are the preferred dominant microsymbiont of soybean grown in Mozambican soils. Furthermore, the distribution of soybean rhizobia in the agricultural soils of Mozambique was found to be markedly influenced by soil pH, followed by the concentrations of plant-available P and Mn. This study suggested that the identified isolates TUTMJM5, TUTMIITA5A and TUTLBC2B can be used as inoculants for increased soybean production in Mozambique

The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems

Abstract Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30-40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineralN (intercropping legumes with cereals) can also be critical. Much of the N2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability ofN in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogen emitted from nodules as a by-product of'N, fixation