Labeling of the Plasma Membrane of Pea Cells by a Surface-localized Glucan Synthetase

Abstract

Changes in total N and in free amino compounds were followed during growth of nodulated white lupin. Leaflets contained the greatest fraction of plant N but had lower proportions (1 to 4%) of their N in soluble amino form than stem + petioles (10 to 27%) and reproductive parts (15 to 33%). Mobilization of free amino compounds from plant parts to fruits contributed at most only 7% of the total N intake of fruits, compared with 50% in mobilization of other forms of N and 43% from fixation during fruiting. Asparagine was usually the most abundant free amino compound in plant parts, followed by glutamine and alanine. Valine, glycine, isoleucine, aspartic acid and γ-aminobutyric acid comprised the bulk of the remaining soluble amino N. Composition of tissue pools of amino-N closely resembled that of xylem and phloem exudates. Data on N flow and utilization were combined with information on composition of transport fluids to quantify syntheses, exchanges, and consumptions of asparagine, glutamine, aspartic acid, and valine by organs of the 51- to 58-day plant. These amino compounds carried 56, 29, 5, and 2%, respectively, of the N exported from nodules and contributed in roughly commensurate proportions to transport exchanges and N increments of plant parts. There were, however, more than expected involvements of glutamine and valine in mobilization of N from lower leaves, of asparagine in xylem to phloem transfer, and of aspartic acid in cycling of N through the root, and there was a less than expected participation of aspartic acid in xylem to phloem transfer and in phloem translocation to the shoot apex. The significance of these differences is discussed.