Biochemical aspects of assimilate transfers along the phloem path: N-solutes in lupins

Lupins (Lupinus angustifolius L., L. albus L.) are prolific 'phloem bleeder s', allowing collection of exudates en route to or at phloem-fed sinks such as shoot apices, developing fruits and the root system, as well as at or e xiting the sources of phloem-borne solutes, leaves, petioles and stems. Con sequently, it has been possible to quantify the relative contributions of x ylem and phloem solutes to the nutrition of each developing organ of a lupi n plant. Studies that identify the sites and, especially for solutes contai ning N, the nature of transfers between the two long distance translocation channels are reviewed. These transfers are solute-specific and can be acco unted for largely by transfer of asparagine. In leaves, direct transfer of asparagine from xylem to phloem in minor veins, such that metabolism is pre cluded, is the most significant feature of N redistribution in the shoot. C urrent research is aimed at identifying and isolating genes encoding amino acid transporters expressed in strategically placed cells (possibly transfe r cells) at nodes, at the inner epidermis of the seed coat and the epidermi s of the developing cotyledons. Special emphasis is placed on transporters for selective transfer of asparagine.