Cycling of amino compounds in symbiotic lupin

Abstract

The composition of amino acids was determined in the xylem and phloem sap of symbiotic lupins grown under a variety of treatments designed to alter the rate of nitrogen fixation. Asparagine was the major amino acid in both xylem and phloem with glutamine, glutamate and aspartate also major components. GABA had a high concentration in the xylem while valine was a major component in the phloem. Exposure to combined nitrogen in the form of either ammonium or nitrate caused a reduction in specific nitrogenase activity and was associated with subsequent changes in both of the translocated saps. Inhibiting nitrogen fixation by exposing nodules to oxygen produced a lower amide to amine ratio in the xylem sap (1.3:1) compared with control and nitrate ratios (2.6:1) and ammonium ratios (7.1:1). Similar ratios for amide:amine were also observed in the phloem sap. Labelling studies using 15N2 to follow nitrogen fixation, ammonium assimilation and amino acid transport have shown rapid accumulation of label into glutamine with subsequent enrichment in glutamate, aspartate, alanine, and GABA. Asparagine was found in high concentrations in nodules and became slowly enriched. Labelled nitrogen fixed and assimilated in nodules was detected 40 min later in stem xylem extracts, largely as the amides glutamine and asparagine. These experiments provide evidence that large amounts of nitrogenous compounds are cycled through the root nodules of symbiotic plants (contributing approximately 50% of xylem N) and that differences in the composition of the phloem sap may influence nodule growth and activity.