Waterlogging of soybean plants (Glycine max L.) led to impaired symbiotic N-2 fixation and a marked decline in glutamine (Gln) concentration in xylem bleeding sap. Xylem Gln concentration increased during the growth cycle of the plant and was correlated with nodule formation. Treatments known to impair N-2 fixation, such as exposing the root system to pure N-2 gas or a mixture of Ar and O-2 (80:20; v/v), led to specific declines in xylem sap Gln. The decrease in Gln observed during waterlogging was also seen on transfer of nodulated plants to aerated hydroponics, where the decline was highly correlated with ureide content in the xylem sap. Upon flooding the nodulated root system, the specific decline in xylem sap Gln could be detected within 10 min and reached a minimum within 60 min, indicating that waterlogging has an immediate effect on N-2 fixation. It is concluded that xylem Gln arises directly from N-2-fixation and is a useful indicator of N-2 fixation activity of symbiotic soybean plants.503405410