Nitrogen storage and translocation in walnut plant: free amino acids

In the seed, arginine, glutamate and alanine dominated cotyledonary free amino acid pool, whereas alanine was the main component of the embryo free amino acid pool. Seed germination resulted in marked changes in the metabolism of free amino acids present in cotyledons. In one month of seedlings subjected to nitrogen autotrophyc growth, cotyledons were characterized by high concentrations of free arginine and citrulline. The relative high percentage of arginine and citrulline characterized not only the cotyledonary tissue, but all the tissues of one-month-old seedlings, especially the embryonic axis, tap root and stem. In these tissues, which have a nutrient storage function, the concentrations of citrulline and arginine made up about 80% of the total free amino acid, whereas in fine roots and leaves, characterized more as sink tissues, the relative percentages of citrulline and arginine were less than the 30%. In young walnut trees (2 years old), at spring regrowth, the concentration of free amino acids and in particular the concentrations of citrulline, arginine and proline present in woody tissues were dependent on the availability of nitrogen in soil, while the protein content of these tissues did not show significantly dependence. The xylem transport of free amino acids during a nitrogen autotrophyc growth (germination) involved mainly citrulline. Its relative concentration in the xylem sap was greater than 50% of the total free amino acid translocated. Citrulline functions as a preferential compound by which nitrogen (amino acids) is translocated also during, spring re-growth or after stress recovery; i.e. at all physiological stages where mobilization of nitrogen stored compounds occurs. The role of ureids in nitrogen storage and translocation from root to shoot is discussed.