Nodule phosphorus requirement and O 2 uptake in common bean genotypes under phosphorus deficiency

The effect of P deficiency on nodulation, nodule P content, nodule O-2 permeability and N fixation rates in Phaseolus vulgaris-rhizobia symbiosis was studied under glasshouse conditions. Four recombinant inbred lines (L34, L83, L115 and L147) and one variety cultivated in Morocco (Concesa) were inoculated with Rhizobium tropici CIAT 899 in hydroaeroponic culture. Two P levels i.e. 75 (deficient level) and 250 mu mol plant(-1) week(-1) P (sufficient level) were applied and the trial was assessed 42 days after transplanting that coincide with plant flowering stage. Under P-deficiency, decrease of plant growth (18%) and nodule biomass (19%) was detected and significantly pronounced in the sensitive line L147 compared with the remaining genotypes. Additionally, under P-deficiency, the efficiency in use of rhizobial symbiosis, estimated by the slope of the regression model of shoot biomass as a function of nodule biomass, was significantly increased in the four lines. This constraint did not significantly influence nodule P content in Concesa, but it was 24 and 41% lower in the tolerant and in the sensitive lines, respectively. Nodule P content was positively correlated to nodule biomass, r = 0.75, and shoot N, r = 0.92. These genotypic variations were associated with variability in nodule O-2 permeability that was significantly affected by the P level-bean genotype interaction. Under P-deficiency, nodule O-2 permeability was significantly reduced in the tested genotypes and accompanied with a decrease in shoot N content, especially in the sensitive lines (35%). Moreover, the ratios plant N fixed: nodule P content and plant N fixed: nodule dry weight were affected under P-deficiency in four lines with an exception observed in Concesa. Depending on the observed data we concluded that N-2 fixation efficiency could be influenced by nodulation and level of nodule P requirement which depend on both bean genotypes and P level.