Résumé
Phosphorus deficiency is a major environmental factor limiting plant growth in more than 60%of tropical soils. This affects particularly symbiotic legumes which usually have higher P requirements than non-symbiotic plants (Israel 1987). As partners in a multidisciplinary project to improve legume nitrogen fixation and yield in low-P soils, we investigated the effects of P deficiency on nodule respiratory (CO2 evolution and O2 uptake) and nitrogenase activities (C2H2 reduction). Nodulated soybean (Glycine max L. Merr.) and common bean (Phaseolus vulgaris) were grown in intensely aerated liquid nutrient solutions. This hydroponic culture optimized (i) the control of P supply (ii) the nodulation and nitrogen fixation - dependent growth, and (iii) the in situ measurement of the nodule gas-exchanges. During measurements of the nitrogenase acetylene reduction activity (ARA), there was an acetylene-induced decline (C2H2-ID) in nitrogenase activity which was significantly increased by P deficiency. By plotting CO2 evolution as a function of ARA during the decline, we found that nodules of P-deficient soybean plants evolved significantly more CO2 per C2H4 than nodules of P-sufficient plants. However, both types of nodules had similar nitrogenase activity, measured as pre-decline ARA. P deficiency also increased the nitrogenase-linked O2 uptake, which was the variable component of the response of nodulated-root O2 uptake to changes in rhizosphere pO2 between 15 and 21 kPa O2. These effects of P deficiency were generally associated with an increase in nodule permeability to O2 diffusion, except for the more tolerant common bean genotype, G19839. It is concluded on P deficiency effects on nodule alternative oxidase activity, cell shape variation and gene expression in the nodule cortex where the variable nodule permeability is thought to operate.