Selectivity in xylem to phloem transfer of amino acids in fruiting shoots of white lupin (Lupinus albus L.).

(14)C-labelled amino compounds were fed singly to fruiting shoots of white lupin as a 15 min pulse through the transpiration stream. The time course of labelling of phloem was followed over a 6 h period by making successive collections of phloem sap from the cut tips of attached fruits. The distribution of (14)C amongst solutes of phloem sap was examined. Seeds harvested at 6 h were analyzed for (14)C content and (14)C distribution amongst amino acid residues of protein. Certain amino compounds (valine, asparagine, threonine, serine, citrulline, glutamine) were transferred rapidly in unchanged form from xylem to phloem. Each labelled phloem sap intensely, and its (14)C was transfrrred to fruit and seed with high efficiency. Other amino compounds (glycine, methionine, aspartic acid, homoserine, glutamic acid and γ-amino butyric acid) passed unchanged to phloem only in small amounts. The bulk of their (14)C appeared in phloem sap attached to other substances, especially non-amino compounds and amino acids metabolically related to the one applied. The (14)C from each of this second group of amino acids labelled phloem less intensely and was donated to fruit and seed with low efficiency. Selectivity in transfer of amino acids from xylem to phloem appeared to be an important element in determining the overall balance of nitrogenous solutes in the translocation stream. The significance of this and coupled loading of photosynthate was examined in relation to the seed's requirements for amino acids in protein synthesis.