Dioumacor Fall

The rhizosphere of the halophytic grass Sporobolus robustus Kunth hosts rhizobium genospecies that are efficient on Prosopis juliflora (Sw.) DC and Vachellia seyal (Del.) seedlings

The aim of this study was to survey the abundance and genetic diversity of legume-nodulating rhizobia (LNR) in the rhizosphere of a salt-tolerant grass, Sporobolus robustus Kunth, in the dry and rainy seasons along a salinity gradient, and to test their effectiveness on Prosopis juliflora (SW.) DC and Vachellia seyal (Del.) P.J.H. Hurter seedlings. The results showed a significant decrease in LNR population density and diversity in response to salinity, particularly during the dry season. A phylogenetic analysis of the 16S-23S rRNA ITS region clustered the 232 rhizobium isolates into three genera and 12 distinct representative genotypes: Mesorhizobium (8 genotypes), Ensifer (2 genotypes) and Rhizobium (2 genotypes). Of these genotypes, 2 were only found in the dry season, 4 exclusively in the rainy season and 6 were found in both seasons. Isolates of the Mesorhizobium and Ensifer genera were more abundant than those of Rhizobium, with 55%, 44% and 1% of the total strains, respectively. The abundance of the Mesorhizobium isolates appeared to increase in the dry season, suggesting that they were more adapted to environmental aridity than Ensifer genospecies. Conversely, Ensifer genospecies were more tolerant of high salinity levels than the other genospecies. However, Ensifer genospeciesproved to be the most efficient strains on P. juliflora and V. seyal seedlings. We concluded that S. robustus hosts efficient rhizobium strains in its rhizosphere, suggesting its ability to act as a nurse plant to facilitate seedling recruitment of P. juliflora and V. seyal in saline soils.

Growth Response of Different Species and Provenances of Jujube Seedlings to Inoculation with Arbuscular Mycorrhizal Fungi

Many species of Ziziphus are underutilized crops despite their potential usage in agroforestry systems. The aim of our study was to evaluate the effects of arbuscular mycorrhizal fungi (AMF) on the growth and mineral nutrition of Ziziphus spp. and provenances of Z. mauritiana in greenhouse conditions. Three isolates of AMF were used: Rhizophagus irregularis IR27, Funneliformis mosseae DAOM227131, and Rhizophagus irregularis DAOM197198. They were inoculated on seven species of Ziziphus (Z. mauritiana, Z. lotus, Z. spina-christi, Z. mucronata, Z. amphibia, Z. abyssinica, and Z. sphaerocarpa) and six provenances of Z. mauritiana (Senegal, Mali, Mauritania, Burkina Faso, Niger, and India). Plants were grown in nursery receiving 20 g of crude inoculum of AMF. Two greenhouse experiments were set up: (1) a 4×6 factorial design consisting of three AMF, one control (disinfected soil without inoculum), and six provenances of Z. mauritiana and (2) a 4×7 factorial design consisting of three AMF, one control and seven Ziziphus spp. Inoculation by AMF significantly improved growth and mineral nutrition of Ziziphus spp., particularly P nutrition. All the Ziziphus spp. (except for Z. lotus) and provenances of jujube seedlings showed the highest mycorrhizal dependency (MD) values when inoculated with R. irregularis IR27. The differences of MD among the tested Ziziphus spp. seem to be due to the differences in the development of hyphal length in the soil and in P uptake by the external hyphae. Rhizophagus irregularis IR27 constitutes a promising biological tool for the production of higher-quality nursery stock with expected improved performance of Ziziphus spp. and Z. mauritiana provenances in agroforestry systems.

Selection of arbuscular mycorrhizal fungal strains to improve Casuarina equisetifolia L. and Casuarina glauca Sieb. tolerance to salinity

Key messageSelection of the best salt-tolerant combination ofCasuarinasp. and arbuscular mycorrhizal fungi (AMF) is one of the key criteria for successful setup of saline land rehabilitation program.ContextLand salinization is a serious problem worldwide that mainly leads to soil degradation and reduces crop productivity. These degraded areas could be rehabilitated by planting salt-tolerant species like Casuarina glauca Sieb. and Casuarina equisetifolia L. These are pioneer plants, able to form symbiotic associations with arbuscular mycorrhizal fungi (AMF), ectomycorrhizal fungi (EMF), and nitrogen-fixing bacteria.AimsThe aim of this study was to select the highest salt-tolerant combination of Casuarina/AMF that can be used for the rehabilitation of lands degraded by salinity.MethodsC. equisetifolia and C. glauca were grown in sandy sterile soil in the greenhouse and inoculated separately with Rhizophagus fasciculatus (Thaxt.) C. Walker & A. Schüßler, Rhizophagus aggregatus (N.C. Schenck & G.S. Sm.) C. Walker, and Rhizophagus intraradices (N.C. Schenck & G.S. Sm.) C. Walker & A. Schüßler. After confirming the establishment of a symbiosis, the plants were watered with gradually increasing concentrations of saline solution. After harvest, size and biomass of the seedlings, root colonization by AMF, and AMF metabolic activities were evaluated.ResultsA larger growth was obtained in the two species when the individuals were inoculated with R. fasciculatus. Root colonization rates did not differ among fungal species, but fungal metabolic activities were higher in mycorrhizal roots of C. glauca plants inoculated with R. fasciculatus.ConclusionAmong the three mycorrhizal fungi, R. fasciculatus was more efficient in association with Casuarinaceae species under salt stress. Our results suggest that selection of appropriate fungal strains is crucial to improve plant performance in saline soils.

Rhizobial Inoculation Increases Soil Microbial Functioning and Gum Arabic Production of 13-Year-Old Senegalia senegal (L.) Britton, Trees in the North Part of Senegal

Rhizobial inoculation has been widely used in controlled conditions as a substitute for chemical fertilizers to increase plants growth and productivity. However, very little is known about such effects on mature trees in natural habitats. In this study, we investigated the effect of rhizobial inoculation on soil total microbial biomass, mineral nitrogen content, potential CO2 respiration, fluorescein diacetate (FDA), acid phosphatase activities, and gum arabic production by 13-year-old Senegalia senegal (synonym: Acacia senegal) under natural conditions in the north part of Senegal during two consecutive years. Rhizobial inoculation was performed at the beginning of the rainy season (July) for both years with a cocktail of four strains (CIRADF 300, CIRADF 301, CIRADF 302, and CIRADF 303). Rhizospheric soils were collected in both dry and rainy seasons to a depth of 0–25 cm under uninoculated and inoculated trees. Trees were tapped in November (beginning of dry season) using traditional tools. Gum arabic was harvested every 15 days from December to March. The results obtained from both years demonstrated that rhizobial inoculation increased significantly the percentage of trees producing gum arabic, gum arabic production per tree, soil microbial biomass, FDA, and acid phosphatase activities. However, there was no significant effect on C mineralization and mineral nitrogen (N) content. Gum arabic production was positively correlated to rainfall, soil microbial biomass, and mineral nitrogen content. Our results showed a positive effect of rhizobial inoculation on soil microbial functioning and gum arabic production by mature S. senegal trees. These important findings deserve to be conducted in several contrasting sites in order to improve gum arabic production and contribute to increase rural population incomes.