Assessing the biological potential of N2-fixing Leguminosae in Botswana for increased crop yields and commercial exploitation

The Leguminosae are a major source of food, fodder, timber, phytochemicals, phytomedicine, nutriceuticals and N fertility in Botswana. Although the country is beset by drought as a result of low rainfall (150 – 650 mm per year) and high potential evapotranspiration, the Leguminosae appear adapted to this low-nutrient environment. Conducting field inoculation trials with infective rhizobia and selecting legume genotypes or host/strain combinations for increased plant growth would help identify superior symbiotic performers under the environmental stress conditions of Botswana. Thus, selecting high N2-fixing legume genotypes has the potential to overcome low soil fertility as a production constraint in Botswana, and increase fodder and grain production to support the growing local livestock and wildlife industries. Additionally, identifying native rhizobia in Botswana that alleviate water stress when in symbioses with legumes could be a first step to tapping the biological potential of the Leguminosae for increased yields in drier environments

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

Labile Soil Carbon Inputs Mediate the Soil Microbial Community Composition and Plant Residue Decomposition Rates

• Root carbon (C) inputs may regulate decomposition rates in soil, and in this study we ask: how do labile C inputs regulate decomposition of plant residues, and soil microbial communities? • In a 14 d laboratory incubation, we added C compounds often found in root exudates in seven different concentrations (0, 0.7, 1.4, 3.6, 7.2, 14.4 and 21.7 mg C g soil) to soils amended with and without 13C-labeled plant residue. We measured CO2 respiration and shifts in relative fungal and bacterial rRNA gene copy numbers using quantitative polymerase chain reaction (qPCR). • Increased labile C input enhanced total C respiration, but only addition of C at low concentrations (0.7 mg C g-1) stimulated plant residue decomposition (+2%). Intermediate concentrations (1.4, 3.6 mg C g-1) had no impact on plant residue decomposition, while greater concentrations of C (\u3e 7.2 mg C g-1) reduced decomposition -50%). Concurrently, high exudate concentrations (\u3e 3.6 mg C g-1) increased fungal and bacterial gene copy numbers, whereas low exudate concentrations (\u3c 3.6 mg C g-1) increased metabolic activity rather than gene copy numbers. • These results underscore that labile soil C inputs can regulate decomposition of more recalcitrant soil C by controlling the activity and relative abundance of fungi and bacteria