title: Recycling of nitrogen in the xylem of Prunus avium trees starts when spring remobilization of internal reserves declines.
abstract: Nitrogen (N) storage capacity of cherry (Prunus avium L.) trees grown in sand culture was preconditioned by applying contrasting N supplies for one year. During the spring of the following year, a constant amount of 15N was supplied and the dynamics of N remobilization and root uptake were characterized as a function of internal N status of the trees. To calculate the flux of N through xylem, both xylem sap N concentration and whole-tree transpiration rates were measured. By comparing the cumulative flux of N through the xylem with the amount of N recovered in the new above ground growth, we indirectly evaluated the recycling of N in the xylem, i.e., the amount of N derived from shoot-root translocation that was subsequently reloaded into the xylem. The contrasting N storage capacities imposed during the first year affected both N remobilization and uptake from roots in the following year. Recycling of N in the xylem apparently did not occur during the remobilization of internal reserves (i.e., during the first 6-8 weeks after bud burst). However, when remobilization declined, measurement of the cumulative flux of N through the xylem overestimated the amount of N recovered in the new biomass, allowing the extent of N recycling to be evaluated. The amount of N recycling in the xylem was greater in high-N trees, which also took up less N through their roots than trees preconditioned to have a lower internal N status. This suggests that recycling of N in the xylem is a mechanism by which plants regulate N uptake by roots.
authors.affiliation: Dipartimento di Colture Arboree, Univ. of Bologna, via Fanin 46, 40127 Bologna, Italy. grassi@agrsci.unibo.it
authors.forename: Giacomo
authors.initials: G
authors.lastname: Grassi
authors.forename: Peter
authors.initials: P
authors.lastname: Millard
authors.forename: Paola
authors.initials: P
authors.lastname: Gioacchini
authors.forename: Massimo
authors.initials: M
authors.lastname: Tagliavini
mesh_terms: D009584:Nitrogen / Q000378:metabolism / Q000502:physiology; D018547:Plant Stems / Q000502:physiology; D027861:Prunus / Q000254:growth & development / Q000502:physiology*; D012621:Seasons; D014197:Trees / Q000254:growth & development / Q000502:physiology*
chemical_list: D009584:Nitrogen