POTENTIAL FOR INOCULATION OF COMMON BEAN BY EFFECTIVE RHIZOBIA IN TUNISIAN SOILS

Phaseolus vulgaris (L) is poorly nodulated in all the regions of Tunis ia where this crop is grown. Selection of effective strains needs to b e developed. An effectiveness test trial was carried out on Royalnel a nd Coco cultivars in hydroponic aseptic conditions for seven reference strains and three local isolates of common bean rhizobia. A significa nt interaction strain x cultivar was observed and, unlike local isolat es, most reference strains were very effective. The ELISA technique us ing polyclonal antibody was used to evaluate nodule occupancy of inocu lated common bean rhizobia in Tunisian soils. Rabbit antisera were rai sed 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 isolat es were distributed into three serogroups. An inoculation experiment w ith the selected reference strains CIAT899, USDA2667 and IRATYH112, wh ich are serologically distinguishable from native rhizobia, was perfor med in a green house for Royalnel and Coco cultivars grown in soil cor es from two sites (Boucharrai and Chrifet). Nodule occupancy of the in oculated strains was assessed and concomitant effect of inoculation on nodule number and shoot nitrogen content was analysed. A highly signi ficant treatment x cultivar x site interaction was observed for the pa rameters studied. While in Boucharrai soil inoculation leads to a sign ificant improvement of shoot nitrogen content for Royalnel cultivar, i noculation of the same cultivar in Chrifet soil leads to an increase i n nodulation by native rhizobia without improvement of plant growth. F or the two cultivars, N-fertilization leads for the two cultivars to a decrease in nodule number, particularly in Chrifet soil. Thus, to imp rove establishment of symbiotic process, the use of N-fertilizer shoul d be rationalized.