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Généralement, les souches tropicales de rhizobia possèdent des propriétés physiologiques leur permettant de se développer dans les milieux arides et semi-arides. Les principaux facteurs limitant l'activité biologique dans les sols sont le déficit hydrique, la salinité, les températures élevées, les pH extrêmes et les carences en éléments nutritionnels. Les interactions fréquentes entre ces différentes contraintes affectent la croissance et la capacité de survie des micro-organismes dans les sols arides.Les résistances à certains de ces facteurs (hautes températures, salinité, stress hydrique) de nombreuses souches de rhizobia isolées de sols sahéliens sénégalais et tunisiens ont été étudiées. Nos résultats montrent que : (I) les souches sont capables de se multiplier à 40 °C et même à 45 °C pour cinq d'entre elles ; (2) les souches des sols sénégalais tolèrent des concentrations en sel supérieures à 2 %, alors que celles des sols de Tunisie sont inhibées à des concentrations de I % ; (3) toutes les souches tolèrent plus le stress osmotique que le stress salin.Une souche d'une espèce de rhizobium tolérante au sel a été retenue comme modèle pour évaluer les capacités de développement en conditions de déficit nutritionnel et salines élevées, conditions qui prévalent généralement en milieu aride. La souche étudiée s'est avérée résistante à une carence nutritionnelle pendant de longues périodes, surtout si elle a été préalablement adaptée à des conditions salines, tout en conservant des capacités de développement lorsque les nutriments devenaient disponibles. Cette souche peut être utilisée dans l'inoculation des sols arides dans les opérations de réhabilitation. It is generally recognized that tropical strains of rhizobia exhibit physiological characteristic that enable them to survive to typical arid and semi-arid soil environmental constraints. The most important factors limiting the growth and activity of rhizobia in soils are drought, salinity, high temperatures, pH and nutrient deprivation. In arid soils, some of these constraints often interact severely affecting the survival of rhizobia.Numerous strains of rhizobia isolated from different soils of Sahel in Senegal and Tunisia have been tested for their resistance to high temperatures, to salinity and to osmotic pressure. Our results have shown that: (I) the strains were able to grow at 40 °C and five of them at 45 °C ; (2) the strains from Senegalese soils tolerated salt concentrations up to 2 % while those from Tunisian soils were inhibited at 1%; (3) both Senegalese and Tunisian strains were more tolerant to osmotic stress than to salinity.A salt-tolerant Rhizobium sp. strain was chosen as a « model of study » for evaluating the capacity to survive under conditions of nutrient starvation and high salt concentrations. Dry and saline arid soils are generally low in nutrients so that inoculant strains of root-nodule bacteria have to cope with both high salt and low nutrient conditions. Therefore, a successful inoculation of selected rhizobia in soils requires that the microorganisms can adapt themselves to very slow or no growth at all and to sudden stressful change in the environmental conditions.The strain examined can withstand nutrient deprivations for long periods, especially when it is previously adapted to saline conditions, while retaining the capacity for active metabolism, if nutrients become available. This might indicate the potential for a long persistence in soil and for a rapid regrowth in the rhizosphere of an appropriate host and hence for its utilization as inoculant in revegetation strategies for arid and saline soils.

Three pre-selected poplar clones and two soil HCH degrader micro-organisms have been experimentally applied in a contaminated agricultural soil in the basin of Fiume Sacco near to Rome for its reclamation. The aim was to successfully associate soil cleaning by rhizoremediation with an economically sustainable biomass for energy production of large poplar plantations. Plants and micro-organisms were selected for the best association with bacteria to obtain 1) the maximum HCH concentration reduction in soil, 2) the minimum plant contamination with HCH, and 3) the maximum biomass production. Results showed that an association between all these traits is possible in a specific poplar clone inoculated with a selected HCH degrader bacterium. The need for a pre-remediation phase in situ to select best candidate plants and bacteria with lowest HCH accumulation in its organs is emphasized. Rhyzoremediation associated with the safe thermo-convertible biomass production is confirmed as a sustainable recovery of soils interdicted to food-agricultural activities.