N-stress alters aspartate and asparagine levels of xylem sap in soybean

The influence of N-stress on the transport of amino acids in the xylem sap was studied for nodulated and non-nodulated soybean plants. The transfer of non-nodulated plants cultivated with nitrate or of nodulated plants dependent exclusively on N-2 fixation to hydroponics with N-free nutrient solution (N-stress), led to a pronounced increase in aspartate and fall in asparagine (Asn) in the xylem sap. Such a change was also observed during the 'N-hunger' phase of the growth cycle of nodulated plants, characterized by the transient yellowing of leaves and coinciding with the initial stages of nodule development. In this case, aspartate increased to around 68% of the xylem amino acids while Asn, normally the most abundant amino acid, fell to 10%. The changes were reversible on recovery of plants from N-stress. In the case of nodulated plants, the fall in xylem Asn during N-stress and its subsequent rise during recovery closely followed Asn synthetase activity in the nodule, while aspartate produced an inverse relationship. Aspartate was prominent in the phloem, independent of the treatment. The possibility that the phenomenon is related to the metabolism of aspartate recycled through the root system is discussed. (C) 2003 Elsevier Ireland Ltd. All rights reserved.