Phosphorus uptake by bean nodules

Phosphate (P) uptake experiments were conducted to determine the routes of P entry into nitrogen-fixing common bean root nodules and the availability of this P to the bacteroids within the nodules. Nodulated common bean plants were cultured for 30 days in a hydroponic system that contained either a complete nutrient solution (+P) or a nutrient solution where P was not added (-P). The plant roots were then exposed to [P-32]-phosphate-labeled +P nutrient solution and P accumulation in bacteroids and nodules was measured. For labeling, plants were suspended in the nutrient solution so that the root system was either fully or partially submerged. Because bean nodules are heavily clustered in the root crown, most of the nodules in the partial submergence treatments were not in contact with the nutrient solution and we could study nodules that had not been directly exposed to P-32. Regardless of P treatment of the host plant, P accumulation in bacteroids was significantly greater and more rapid when the nodule surface was in contact with the labeled nutrient solution, suggesting that nodules can import P from the solution. P entry through the nodule surface was verified in experiments where P-32-labeled nutrient solution was applied to the nodule surface only. In full submergence treatments, P-32 accumulation in bacteroids of -P plants was significantly greater than in bacteroids of +P plants. As measured by total leaf label, uptake and translocation of P to the shoot was greater in -P plants and still greater in partial submergence treatments where most of the nodules were not in contact with the labeled nutrient solution. Results of these experiments suggest that, in addition to P acquisition via the vascular system, P flow to symbiotic tissues and bacteroids can originate from the soil solution immediately surrounding the nodule. This was particularly evident in nodules of plants that had been limited for P.