Respiration in support of N2 fixation by rhizobia in legume root nodules depends on an adequate supply of O2, but excessive O2 can damage nitrogenase, the key enzyme. The movement of O2 into and within the nodule is driven by gradients in the concentration of O2 or in the oxygenation of the O2-carrier, leghaemoglobin. Steeper gradients may increase flux to the sites of respiration, but gradients also raise the possibility of inadequate O2 in some nodule zones and excessive O2 in others. No detailed study of O2 gradients in the interior of nodules has been published previously. Spectral changes in leghaemoglobin with oxygenation, previously used to measure the average O2 status of the nodule interior, were used to map longitudinal gradients in O2 and in respiratory capacity in the elongated nodules of alfalfa (Medicago sativa L.) and sweetclover (Melilotus officinalis L.). Variability among nodules under air in the magnitude and direction of internal O2 gradients was seen in both species. Despite consistently higher respiratory capacity near the meristematic tip, a majority of nodules had higher O2 towards the tip than towards the base. These results contrast with a previous report, apparently based on limited data, but they are consistent with anatomical and tracer studies showing higher gas permeability near the tip.
Bibliographical noteFunding Information:
Supported by the National Science Foundation grant 0077903 and USDA grant 99-35305-8646. The grazing study was conducted at USDA's Appalachian Soil and Water Conservation Research Laboratory. We thank Bob Rousseau for assistance with the experiments and Karin Jacobsen for comments on the manuscript.