The aim of the present study was to test the hypothesis that the N con
tent or the composition of the phloem sap that supplies nodulated root
s may play a role in the feedback regulation of nitrogenase activity b
y increasing nodule resistance to O-2 diffusion. Treating shoots of lu
pin (Lupinus albus cv Manitoba) or soybean (Glycine mar L. Merr. cv Ma
ple Arrow) with 100 mu L L(-1) NH3 caused a 1.3-fold (lupin) and 2.6-f
old (soybean) increase in the total N content of phloem sap without al
tering 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 s
tart of the NH3 treatment. However, the potential nitrogenase activity
values in the treated plants were not significantly different from th
ose of the control plants. These results provide evidence that changes
in the N composition of the phloem sap, particularly the glutamine co
ntent, may increase nodule resistance to O-2 diffusion and, thereby, d
own-regulate nodule metabolism and nitrogenase activity by controlling
the supply of O-2 to the bacteria-infected cells.