Fig. 1. Schematic representation of amino acid transport between the sites of primary nitrogen assimilation and the import-dependent tissues. Amino acids synthesized in the root from in organic forms of nitrogen or symbiotic relationships are transported to mature leaf tissue in the xylem. Amino acids synthe-sized in the leaf after primary assimilation, or arriving from the root in the xylem, are transported out of the leaf in the phloem to satisfy the needs of heterotrophic sinks. Import-dependent tissues include developing leaves roots, cortical cells in the stem, seed and fruits, and apical meristems(from(l) with permission)