© 2003 Heron Publishing—Victoria, Canada
Recycling of nitrogen in the xylem of Prunus avium trees starts when spring remobilization of internal reserves declines
Giacomo Grassi (1, 2), Peter Millard (3), Paola Gioacchini (4) and Massimo Tagliavini (1)
1. Dipartimento di Colture Arboree, Univ. of Bologna, via Fanin 46, 40127 Bologna, Italy / 2. Author to whom correspondence should be addressed (grassi@agrsci.unibo.it) / 3. Macaulay Institute, Craigiebuckler, Aberdeen AB16 8QH, U.K. / 4. DISTA, Univ. of Bologna, via Fanin 46, 40127 Bologna, Italy / Received November 12, 2002; accepted April 5, 2003; published online September 15, 2003
Summary
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 aboveground
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.
Keywords:
amino acid translocation, cherry, nitrogen storage, nitrogen uptake, root-shoot-root cycling.
