Potential for inoculation of common bean by effective rhizobia in Tunisian soils

Phaseolus vulgaris (L) is poorly nodulated in all the regions of Tunisia where this crop is grown. Selection of effective strains needs to be developed. An effectiveness test trial was carried out on Royalnel and Coco cultivars in hydroponic aseptic conditions for seven reference strains and three local isolates of common bean rhizobia. A significant interaction strain x cultivar was observed and, unlike local isolates, most reference strains were very effective. The ELISA technique using polyclonal antibody was used to evaluate nodule occupancy of inoculated common bean rhizobia in Tunisian soils. Rabbit antisera were raised against one local isolate (HB) and five reference strains (CIAT899, USDA2667, USDA2669, USDA2676 and IRATYH112). Analysis of relatedness between 44 strains and isolates showed that 77% of the local isolates did not react with any of the sera used, whereas the remaining isolates were distributed into three serogroups. An inoculation experiment with the selected reference strains CIAT899, USDA2667 and IRATYH112, which are serologically distinguishable from native rhizobia, was performed in a green house for Royalnel and Coco cultivars grown in soil cores from two sites (Boucharrai and Chrifet). Nodule occupancy of the inoculated strains was assessed and concomitant effect of inoculation on nodule number and shoot nitrogen content was analysed. A highly significant treatment x cultivar x site interaction was observed for the parameters studied. While in Boucharrai soil inoculation leads to a significant improvement of shoot nitrogen content for Royalnel cultivar, inoculation of the same cultivar in Chrifet soil leads to an increase in nodulation by native rhizobia without improvement of plant growth. For the two cultivars, leads for the two cultivars to a decrease in nodule number, particularly in Chrifet soil. Thus, to improve establishment of symbiotic process, the use of N-fertilizer should be rationalized.