Effets du N minéral et du génotype de la plante sur la croissance et la nodulation de Faidherbia albida

Abstract

Sahelian soils are recognized for their low N content, but their mineral N concentrations can often be high enough to inhibit N2 fixation in cultivated legumes. However, the quest for symbiotic plant genotypes tolerant of mineral N is poorly documented for nitrogen-fixing trees. The behaviour of N2 fixing symbioses with regard to nitrogen fertilization was observed in different Faidherbia albida (Del) A. Chev. genotypes. Twelve distinct lineages of the species, obtained from four regions of western Africa, were cultivated for 75 days in the presence of 0–6 mol·m–3 N provided as NH4NO3. Plants were separately inoculated with two Bradyrhizobium strains, and cultivated on sandy soil in a nursery. Aerial and root parts lengths and masses, collar diameters, and ratios of height to length (H/P) of taproot, and aerial to root (MA/MR) parts masses increased with increased N level. Plants showed significant growth differences linked with seed origin, and with lineage for a same origin. Genotypical variations were observed for concentrations as well as C and N total uptakes, C/N ratio measured in aerial parts, and N2 fixation. Nodule number and mass were greatly reduced in the presence of 2 mol·m–3 N. The extent of this reduction was dependent, to a lesser degree, on seed origin and Bradyrhizobium strain. Data show that F. albida – Bradyrhizobium symbiosis is highly sensitive to mineral N. It is suggested that selection of symbiotic partners for tolerance to mineral N might greatly improve the agronomic behaviour of F. albida.Key words: Faidherbia albida, lineages, Bradyrhizobium, symbiosis, nodulation, growth, ammonium nitrate.