This work intended to measure the nodulated-roots oxygen consumption, proton efflux and phytase activity in 2 lines of common bean (Phaseolus vulgaris) (115, 147) at 2 levels of P supply. Rooted seedlings were inoculated with Rhizobium tropici CIAT 899 in hydroaeroponic cultivation under glasshouse. Phosphorus was supplied as KH(2)PO(4) at 15 and 250 mu mol pl(-1) week(-1) (15P and 250P, respectively). Our results showed that plant growth nodulation and symbiotic nitrogen fixation were significantly affected by P limiting (15P) for the both lines, but this adverse effect was more pronounced in 147 than in 115. For the both lines, the phytase activity, higher in roots than in nodules, was significantly increased by P limiting, but 115 maintained higher values as compared to 147 line. In cotyledons, the phytase activity was higher in 115 than in 147. Phosphorus shortage increased the cumulated proton release only in 115, whereas it was lowered for 147. In this line, the proton release was linked to symbiotic nitrogen fixation. Under 15P, the proton efflux per unit of nodulated-root biomass was 25% greater for 115 than 147, suggesting that under P limitation, proton efflux may constitute an efficient way to increase P uptake in the tolerant line (115). 15P increased significantly nodulated-root O(2) consumption per g nodule DW and nodule conductance, but to a higher extent in 147. As a whole, bean plants at P-deficient conditions increased the activity of phytases and proton efflux, thus maintaining the oxygen diffusion in nodules. This may represent an adaptive mechanism for N(2)-fixing legumes to respond to P deficiency, by increasing the utilisation and the uptake of phosphorus for symbiotic nitrogen fixation.