Phosphorus use efficiency for symbiotic nitrogen fixation in voandzou ([i]Vigna subterranea[/i]) using isotopic exchange method in Rhizotron
Résumé
Low bioavailability of nitrogen and phosphorus is one of the main constraints in the acid soils with high P-fixing capacity. Plants adapt to low nutrient availability through various biological and physico-chemical mechanisms. Since genetic variation of N2 fixation exists in numerous legume species, optimization of symbiotic nitrogen fixation (SNF) under P deficiency could be a way to the replenishment of soil fertility in tropical soils. As the genetic potential of crops like Vigna subterranea (Bambara groundnut or voandzou) is little studied, although its agronomic potential is interesting for the farmers of Africa, a physiological study through legume screening for N2 fixation was performed with 54 cultivars from Madagascar, Niger and Mali, inoculated with the reference strain of Bradyrhizobium sp. Vigna CB756 in hydroponic culture under P deficiency and sufficiency (30 and 75 μmol KH2PO4 plant-1 week-1, respectively), corresponding respectively to 28 and 70 mg P kg-1 of soil. Large variability of nodulation and plant biomass was found among cultivars. These two parameters were generally correlated and the slope of the plant biomass regression as a function of nodulation was considered as an indicator of the efficiency in use of the rhizobial symbiosis. For the two cultivars most tolerant to P deficiency, V1 and V4 from Madagascar, the increase in use efficiency of the rhizobial symbiosis under P deficiency was linked with an increase in nodulated root O2 consumption linked to N2 fixation, and in phytase gene expression observed on the nodule sections by in situ RTPCR. As the complexity of P compartments makes it difficult to assess the P bioavailability in the plant rhizosphere, an isotopic 32P exchange method was carried out in a rhizotron in order to assess the direct effect of the roots on P mobilization in rhizosphere soil, comparing V1 and V4 with 28 or 70 mg P kg-1 of soil. Throughout this study, the various rhizospheric mechanisms involved in the P mobilization for the plant nutrition were assessed by diffusive phosphate ion (Pd), soil acidification by pH decrease, organic anion complexation inducing a low Al and Fe content, and mineralization of organic P through phosphatase. The gross amount of diffusive Pi (Pd) was determined as a function of Cp and time (t) by coupling sorption-desorption experiments with subsequent isotopic dilution kinetics in soil suspensions at steady-state. The Pd vs. (Cp, t) relationships varied significantly between treatments, indicating that roots modify soil properties and consequently re-distribute diffusive Pi between the soil solution and constituents. The Pd values were greater for the rhizosphere soil obtained with V1 after applying 28 mg P kg-1.This could be attributed to a strong re-supplying capacity of the soil solution in Pd along the exchange time leading to a large P nutrition of voandzou. It is concluded that genotypic variability exists among voandzou cultivars for internal adaptation to P deficiency.