Symbiotic Potential and N Returns in Forage and Grain Legumes for Improving the N Economy of Tropical Cropping Systems

There were marked cultivar and species differences in the amounts of N fixed and potential N returns in five forage legumes and ten groundnut (Arachis hypogaea L.) cultivars evaluated for symbiotic performance in both Guinea and Sahel savanna environments. Of the forage legumes, the Crotalaria and Lablab species accumulated greater symbiotic N and therefore showed higher potential N returns for improving the N economy of soils in both Guinea and Sahel savanna in Africa. However, Lablab appeared to be better adapted to the drier Sahel environment compared to the other species. When nutrient imbalances in the Guinea savanna soil were resolved by supplying plants with a moderate dose of a complete fertilizer (minus N), two of ten groundnut cultivars outperformed the rest in dry matter yield, N fixed and potential N returns in both fertilized and unfertilized conditions. These findings suggest that the two cultivars have the genetic potential for dry matter accumulation and enhanced N2 fixation even under varying soil nutrient conditions. Another two groundnut cultivars showed evidence of soil N depletion with negative values of potential N returns, clearly indicating that their continued cultivation in a soil is likely to lead to mineral N exhaustion

Assessing the genetic diversity of 48 groundnut (Arachis hypogaea L.) genotypes in the Guinea savanna agro-ecology of Ghana, using microsatellite-based markers

Groundnut (Arachis hypogaea L.) is the most important grain legume in Ghana. However, its production is constrained by a myriad of biotic and abiotic stresses which necessitate the development and use of superior varieties for increased yield. Germplasm characterisation both at the phenotypic and molecular level is important in all plant breeding programs. The aim of this study was to characterise selected advanced breeding groundnut lines with different phenotypic attributes at the molecular level using simple sequence repeats (SSR) markers in Ghana. A total of 53 SSR markers were screened and 25 were found to be polymorphic with an average polymorphic information content (PIC) value of 0.57. Of the 48 groundnut genotypes studied, 67% showed very close relationship (~100% similarity) with one or more genotypes among themselves. In fact, there were 14 instances where two to three genotypes within the same sub-cluster exhibited 100% similarity even though they displayed different phenotypic attributes. The remaining 33% of the groundnut genotypes were distant from each other and could therefore serve as effective parental material for future work. In this study, the SSR-based markers were found to be quite discriminatory in discerning variations between and among groundnut lines even where the level of variation was low. Microsatellite-based markers therefore represent a useful tool for dissecting genetic variations in cultivated crops, especially groundnu

Competitiveness and communication for effective inoculation byRhizobium, Bradyrhizobium and vesicular-arbuscular mycorrhiza fungi

After a short summary on the ecology and rhizosphere biology of symbiotic bacteria and vesicular-arbuscular (VA) mycorrhiza fungi and their application as microbial inocula, results on competitiveness and communication are summarized. Stress factors such as high temperature, low soil pH, aluminium concentrations and phytoalexins produced by the host plants were studied withRhizobium leguminosarum bv.phaseoli andRhizobium tropici onPhaseolus beans. Quantitative data for competitiveness were obtained by usinggus + (glucoronidase) labelled strains, which produce blue-coloured nodules. ForPhaseolus-nodulating rhizobia, a group specific DNA probe was also developed, which did not hybridize with more than 20 other common soil and rhizosphere bacteria. Results from several laboratories contributing to knowledge of signal exchange and communication in theRhizobium/Bradyrhizobium legume system are summarized in a new scheme, including also defense reactions at the early stages of legume nodule initiation. Stimulating effects of flavonoids on germination and growth of VA mycorrhiza fungi were also found. A constitutive antifungal compound in pea roots, -isoxazolinonyl-alanine, was characterized