Transport, Metabolism, and Redistribution of Xylem-Borne Amino Acids in Developing Pea Shoots

Abstract

Amino acid metabolism and transport was investigated in the leaves of 3-week-old nonnodulated seedlings of Pisum sativum L. Xylem sap entering the shoot contained nitrate (about 5 millimolar), and amino compounds (11 millimolar) of which 70% was asparagine plus glutamine; aspartate and homoserine were also present. Mature leaves showed stable nitrogen levels and incoming nitrogen was redistributed to growing leaves. Younger leaves, still enclosed in the stipules, showed negligible rates of transpiration, suggesting that most of their nitrogen must arrive in the phloem.(14)C-Labeled amides and amino acids were supplied to detached shoots through the xylem, and metabolism and redistribution were followed over 12 hours in light. Asparagine entering mature leaves was reexported directly to young leaves, with relatively little metabolic conversion. Substantial amounts of glutamine were converted to glutamate, which was exported (with unchanged amide) with little further conversion. The pattern of redistribution was confirmed when (14)C-labeled amino acids were applied directly to the under surface of mature leaves. Labeled compounds were found in the phloem exudate from treated leaves, and the composition resembled the pattern of labeling in the compounds arriving in the young developing leaves.