The Significance of Transpirationally Derived Nitrogen in Protein Synthesis in Fruiting Plants of Pea (Pisum sativum L.)

Feeding of ¹⁵N-nitrate, ¹⁵N(amide)-L-glutamine, or ¹⁵N-L-glutamicacid to detached shoots of pea through the transpiration streamresults in the soluble and insoluble nitrogen of stem, leaves,and fruits becoming extensively enriched with isotopic nitrogen.The time course of labelling suggests that non-reproductiveparts are the principal centres of uptake and assimilation andthat from them translocation takes place to the developing seeds. Distribution patterns for ¹⁵N in free and protein-bound aminoacids of leaf and seed indicate that each labelled source donatesnitrogen to a wide range of amino compounds, with no evidenceof consistent differences in the manner in which each is assimilated.Alanine, glutamic acid, homoserine, and -aminobutyric acid,are the main recipients of ¹⁵N in the soluble fraction of theleaves, whilst in the insoluble fraction nitrogen of the aminoacids serine, glycine, alanine, threonine, glutamic acid + glutamine,and aspartic acid + asparagine achieves high specific labelling.Amino acids of the seeds are labelled more uniformly with ¹⁵N. A complementary ¹⁴C-labelling experiment on the translocationof photosynthetically fixed carbon from leaf to seed is describedand the labelling patterns obtained for amino acids in leaf,seed, and phloem exudate are discussed in relation to thosefor ¹⁵N.