Phloem Loading and Metabolism of Xylem-Borne Amino Compounds in Fruiting Shoots of a Legume

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Abstract
Cut, fruiting shoots of Lupinus albus L. supplied with 14C- and 15N-labelled L-asparagine, L-glutamine, L-aspartic acid, or L-glutamic acid through the transpiration stream readily transferred the labelled carbon and nitrogen of each compound to pods and seeds of fruits. A time course of labelling of phloem sap collected from petioles and fruit tips following feeding of labelled asparagine indicated that xylem to phloem exchange in leaflets was an immediate and effective route of transfer of the amide to fruits and that this and the loading onto phloem of additional asparagine from unlabelled pools of the amide in stems furnished a major source of the nitrogen for fruit filling. Xylem to phloem exchange of nitrogen was accomplished in different ways for each amino acid. The amide nitrogen of asparagine was transferred mainly in the unmetabolized compound, the nitrogen of aspartate and glutamate largely in a wide range of amino acids synthesized in the leaf, and the amide nitrogen of glutamine was transferred in a manner intermediate between these extremes. Glutamine and asparagine were the principal phloem solutes labelled with nitrogen from any of the supplied compounds, but the photosynthetically produced amino acids, glutamate, aspartate, serine, alanine, and valine also became 15N-labelled in phloem. The main pathway for glutamine synthesis in vegetative parts of the shoot appeared to be by amidation of glutamate, but asparagine was not considered to be derived similarly from aspartate. Leaflets metabolized glutamine more readily than asparagine, but in each case the amide nitrogen was used for synthesis of a variety of amino acids and the carbon was recovered largely in non-amino compounds.