Kandioura Noba

Casuarina in Africa: Distribution, role and importance of arbuscular mycorrhizal, ectomycorrhizal fungi and Frankia on plant development

Exotic trees were introduced in Africa to rehabilitate degraded ecosystems. Introduced species included several Australian species belonging to the Casuarinaceae family. Casuarinas trees grow very fast and are resistant to drought and high salinity. They are particularly well adapted to poor and disturbed soils thanks to their capacity to establish symbiotic associations with mycorrhizal fungi -both arbuscular and ectomycorrhizal- and with the nitrogen-fixing bacteria Frankia. These trees are now widely distributed in more than 20 African countries. Casuarina are mainly used in forestation programs to rehabilitate degraded or polluted sites, to stabilise sand dunes and to provide fuelwood and charcoal and thus contribute considerably to improving livelihoods and local economies. In this paper, we describe the geographical distribution of Casuarina in Africa, their economic and ecological value and the role of the symbiotic interactions between Casuarina, mycorrhizal fungi and Frankia.

Changes in Land Use System and Environmental Factors Affect Arbuscular Mycorrhizal Fungal Density and Diversity, and Enzyme Activities in Rhizospheric Soils of Acacia senegal (L.) Willd.

The responses of the soil microbial community features associated to the legume tree Acacia senegal (L.) Willd. including both arbuscular mycorrhizal fungal (AMF) diversity and soil bacterial functions, were investigated under contrasting environmental conditions. Soil samples were collected during dry and rainy seasons in two contrasting rainfall sites of Senegal (Dahra and Goudiry, in arid and semiarid zone, resp.). Soils were taken from the rhizosphere of A. senegal both in plantation and natural stands in comparison to bulk soil. A multiple analysis revealed positive correlations between soil physicochemical properties, mycorrhizal potential and enzyme activities variables. The positive effects of A. senegal trees on soil mycorrhizal potential and enzyme activities indicates that in sahelian regions, AMF spore density and diversity as well as soil microbial functions can be influenced by land-use systems (plantation versus natural population of A. senegal) and environmental conditions such as moisture and soil nutrient contents. Our study underlines the importance of prior natural AMF screening for better combinations of A. senegal seedlings with AMF species to achieve optimum plant growth improvement, and for restoration and reforestation of degraded lands.

Chemotaxonomy in Nyctagynaceae family: Sterols and fatty acids from the leaves of three Boerhaavia species

Abstract The fatty acids and sterol compositions of the leaf lipid extracts of three senegalese Nyctaginaceae: Boerhaavia diffusa, B. erecta , and B. repens , not screened previously, were determined by capillary GC and GC-MS. Sixteen fatty acids were investigated with a predominance of linolenate, palmitate and linoleate in the three species. Ten sterols were identified by GCMS; in addition to the usually occurring plant sterols, three minor sterols, rarely reported in plant kingdom, wem detected: 5,23 stigmastadienol, 5,24(25) stigmastadienol and stigmastanol.

Phylogeny of Nodulation Genes and Symbiotic Diversity of "Acacia senegal" (L.) Willd. and "A. seyal" (Del.) "Mesorhizobium" Strains from Different Regions of Senegal

Acacia senegal and Acacia seyal are small, deciduous legume trees, most highly valued for nitrogen fixation and for the production of gum arabic, a commodity of international trade since ancient times. Symbiotic nitrogen fixation by legumes represents the main natural input of atmospheric N2 into ecosystems which may ultimately benefit all organisms. We analyzed the nod and nif symbiotic genes and symbiotic properties of root-nodulating bacteria isolated from A. senegal and A. seyal in Senegal. The symbiotic genes of rhizobial strains from the two Acacia species were closed to those of Mesorhizobium plurifarium and grouped separately in the phylogenetic trees. Phylogeny of rhizobial nitrogen fixation gene nifH was similar to those of nodulation genes (nodA and nodC). All A. senegal rhizobial strains showed identical nodA, nodC, and nifH gene sequences. By contrast, A. seyal rhizobial strains exhibited different symbiotic gene sequences. Efficiency tests demonstrated that inoculation of both Acacia species significantly affected nodulation, total dry weight, acetylene reduction activity (ARA), and specific acetylene reduction activity (SARA) of plants. However, these cross-inoculation tests did not show any specificity of Mesorhizobium strains toward a given Acacia host species in terms of infectivity and efficiency as stated by principal component analysis (PCA). This study demonstrates that large-scale inoculation of A. senegal and A. seyal in the framework of reafforestation programs requires a preliminary step of rhizobial strain selection for both Acacia species.

Arbuscular mycorrhizal fungi (AMF) communities associated with cowpea in two ecological site conditions in Senegal

The objective of this study was to characterize the diversity of arbuscular mycorrhizal fungal (AMF) communities colonizing the roots of Vigna unguiculata (L.) plants cultivated in two different sites in Senegal. Roots of cowpea plants and soil samples were collected from two fields (Ngothie and Diokoul) in the rural community of Dya (Senegal). Microscopic observations of the stained roots indicated a high colonization rate in roots from Ngothie site as compared to those from Diokoul site. The partial small subunit of ribosomal DNA genes was amplified from the genomic DNA extracted from these roots by polymerase chain reaction (PCR) with the universal primer NS31 and a fungal-specific primer AML2. Nucleotide sequence analysis revealed that 22 sequences from Ngothie site and only four sequences from Diokoul site were close to those of known arbuscular mycorrhizal fungi. Also, 47.6% of the clones from Ngothie site and 89.47% from Diokoul site were not close to known AMF. A total of 15 operational taxonomic units (OUT) were identified. Phylogenetic analyses showed that these clones belonged to the genera Glomus, Sclerocystis, Rhizophagus, Scutellospora, Gigaspora, Racocetra, Acaulospora and Redeckera. The genus Glomus is the most represented with six OTU, representing 40% of all OTU. Key words: Agriculture, Glomeromycota, Vigna unguiculata, diversity, soil origin.

Bryophytes of Kédougou (Eastern Senegal), with a key to the Fissidens of Senegal

The bryophyte flora of the Sahelian region of sub-Saharan Africa is characterized by a low species richness, but is also extremely poorly documented. We present here the results of a floristic survey in Kedougou (eastern Senegal), which yielded 22 species. Two liverworts and 15 moss species are new to the country, so that the bryophyte flora of Senegal now includes a total of 4 liverwort and 34 moss species. An identification key is provided for Fissidens, which, with eight species, is the richest moss genus of the country.

Effect of native and allochthonous arbuscular mycorrhizal fungi on Casuarina equisetifolia growth and its root bacterial community

ABSTRACT Exotic trees are often planted to recover degraded lands. Inoculation with mycorrhizal fungi can improve their survival. Plant growth is partly dependent on the strain used, but little attention has been paid to the selection of mycorrhizal fungi. The aim of this study was to determine whether the growth of Casuarina equisetifolia L. (Johnson) is affected by two different mycorrhizal inocula generated using fungal spores retrieved from an Australian site (allochthonous soil) and a Senegalese site (native soil) under C. equisetifolia trees. Comparative experiments were conducted with plants in a Senegalese soil, previously sterilized or not, and grown in a greenhouse. At harvest, parameters related to plant growth and mycorrhization were evaluated and soil bacterial communities were compared. Tree growth was significantly influenced by both types of inoculants. In unsterilized soil, plants inoculated with the native inoculant were taller than plants inoculated with the allochthonous inoculant and control plants. The frequency of mycorrhization with both inoculants was higher in unsterilized soil. The strongest effects of the mycorhizosphere on the soil microbiome were obtained with the allochthonous inoculum, and analysis of the taxonomic composition revealed mycorrhizal communities specific to each inoculum. These results suggest that the development of C. equisetifolia and its root bacterial community are dependent on the composition of the mycorrhizal inoculum. The functional consequences of this rhizosphere effect in terms of soil fertility should be further studied to better guide reforestation operations.

Environmental conditions and primary production in a Sahelian shallow lake (Lake Guiers, North Senegal)

This is a study of the environmental conditions and primary phytoplankton production in a Sahelian shallow lake of Senegal, West Africa. Environmental descriptors (nutrient, water transparency, temperature and hydrochemistry) and their effects on primary production were studied. Samples were collected monthly from March 2002 to February 2003 in 3 stations of the central part of water body. Climatic variables show a marked annual pattern following two dominant wind directions: NNE (North-North-East) and NNW (North- North-West). This cyclic evolution affects hydrobiological variables. In this central part of the lake, water body was not stratified with same measures between surface and bottom. Lake water was turbid with lower values during the maritime trade winds period characterized by high water temperatures and conductivity. Water pH was relatively higher than seven (07) associated with high contents of oxygen (approximately 90%) in all stations. The station Typha is marked by a low pH and high fluctuations in the rates of oxygen (20 to 120%). Nutrient concentrations were low, with high variability (from 0 to 30 μg.l -1 for DIN and from 0 to 18 μg.l -1 for PO 4 ). The primary production in Guiers Lake was high; it showed a vertical gradient with production of 2400 mgC.m -3 .days -1 in surface and a low of 200 mgC.m -3 days -1 at the bottom. In this shallow system, environmental conditions regulate the primary production. Keywords : Environmental descriptors, primary production, shallow lake, Senegal.