PHASEOLUS VULGARIS-RHIZOBIA SYMBIOSIS INCREASES THE PHOSPHORUS UPTAKE AND SYMBIOTIC N2 FIXATION UNDER INSOLUBLE PHOSPHORUS

Five north Tunisian native rhizobia were tested in symbiosis with Coco-blanc' variety under two phosphorus (P) supplies [P-sufficient and insoluble P (IP)] at 250mol P plant(-1). At flowering stage, responses against IP were assessed and indicated that inoculation with P.Ps.09 produced a high nodule biomass (0.12g). Similarly, P.Tb.09 produced 0.08g nodule biomass under both P and P.Bj.09 that significantly increased nodule number under IP. Shoot biomass exhibited high dry weight with P.Tb.09 and CIAT899 (1.6g plant(-1)). Inoculation with CIAT899 produced a high root biomass under both P. Analysis of a halo diameter of clear zone rounding each colony under IP indicated that P.Tb.09 has developed high halo O (1.7cm). These variations were associated under IP with a decrease of pH by P.Ar.09 and with increase in shoot phosphatase activity by P.Ar.09, P.Bj.09, P.OM.09 and P.Ps.09. The use of P-efficient rhizobia may constitute an adaptive mechanism against P-deficiency tolerance.