Phloem glutamine and the regulation of O-2 diffusion in legume nodules

The aim of the present study was to test the hypothesis that the N content or the composition of the phloem sap that supplies nodulated roots may play a role in the feedback regulation of nitrogenase activity by increasing nodule resistance to O-2 diffusion. Treating shoots of lupin (Lupinus albus cv Manitoba) or soybean (Glycine mar L. Merr. cv Maple Arrow) with 100 mu L L(-1) NH3 caused a 1.3-fold (lupin) and 2.6-fold (soybean) increase in the total N content of phloem sap without altering its C content. The increase in phloem N was due primarily to a 4.8-fold (lupin) and 10.5-told (soybean) increase in the concentration of glutamine N. In addition, there was a decline in both the apparent nitrogenase activity and total nitrogenase activity that began within 4 h and reached about 54% of its initial activity within 6 h of the start of the NH3 treatment. However, the potential nitrogenase activity values in the treated plants were not significantly different from those of the control plants. These results provide evidence that changes in the N composition of the phloem sap, particularly the glutamine content, may increase nodule resistance to O-2 diffusion and, thereby, down-regulate nodule metabolism and nitrogenase activity by controlling the supply of O-2 to the bacteria-infected cells.