Abstract
Legumes play a critical role in natural ecosystems, agriculture, and agroforestry, where their ability to fix nitrogen (N) in symbiosis makes them excellent colonizers of low-N environments, and hence an economic and environment-friendly crop, pasture, and herb species. Few reports exist on natural nodulation of wild legumes. The natural nodulation resource of five pastoral legumes, among which three Loteae (Lotus ssp.), one Psoraleae (Psoralea ssp.) and one Trifoleae (Ononis ssp.), prospected from different sites in an arid area of Tunisia, was investigated. The occurrence of nodulation and the morphology of the nodules were observed. Nodulation is reported here for the first time for two taxa. Three taxa presented globular nodules and two spindle-shaped ones. Nodule shape is characteristic of legume species but independent of Rhizobium strains. The colour of the nodules is not related to that of the roots, but depends on soil quality and colour, and on the presence of leg-haemoglobin in the nodules. Nitrogen fixation seems to be intense for the majority of these species. This characteristic enables them to benefit from the symbiosis with rhizobia in order to resist the edaphic aridity of soils.