Plant Physiol, October 2002, Vol. 130, pp. 1043-1053

Coupling Sap Flow Velocity and Amino Acid Concentrations as an Alternative Method to ¹⁵N Labeling for Quantifying Nitrogen Remobilization by Walnut Trees¹

Unité Mixte de Recherche (Institut National de la Recherche Agronomique-University Blaise Pascal), 234 avenue du Brézet, 63039 Clermont-Ferrand cedex 02, France (E.F., X.L.R., S.G.); and Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland, United Kingdom (E.F., P.M., R.W.)

The temporal dynamics of N remobilization was studied in walnut (Juglans nigra × regia) trees growing in sand culture. Trees were fed with labeled N (¹⁵N) during 1999 and unlabeled N in 2000. Total N and ¹⁵N contents in different tree compartments were measured during 80 d after bud burst and were used to estimate N remobilization for spring growth. The seasonal (and occasionally diurnal) dynamics of the concentration and ¹⁵N enrichment of the major amino acids in xylem sap were determined concurrently. Sap flow velocity was also measured for sample trees. A new approach coupling amino acid concentrations to sap flow velocity for quantifying N remobilization was tested. A decrease of the labeled N contents of medium roots, tap roots, and trunk was observed concurrently to the increase in the labeled N content of new shoots. Remobilized N represented from previous year storage 54% of N recovered in new shoots. Arginine, citruline, -amino butyric acid, glutamic acid, and aspartic acid always represented around 80% of total amino acid and amide N in xylem sap and exhibited specific seasonal trends and significant diurnal trends. N translocation was mainly insured by arginine during the first 15 d after bud burst, and then by glutamic acid and citruline. The pattern of N remobilization estimated by the new approach was consistent with that measured by the classical labeling technique. Implications for quantifying N remobilization for large, field-growing trees are discussed.