Fig. 6.

Schematic presentation of the roles of transport and the possible metabolic role of PEPCK in nitrogen recycling from the xylem. Transport involves unloading of amino acids and amides (N) from the xylem, passage through the vascular parenchyma between the xylem and phloem, and loading into the phloem. Metabolism involves similar transport processes in conjunction with metabolism in the vascular parenchyma. Specifically, the role of PEPCK would involve the metabolism of OAA (oxaloacetate) deriving from Asp and, ultimately, Asn. N released by Asn and Asp metabolism could then be also transported into the phloem.