Department of Plant Sciences

We evaluated the effect of the time of nitrogen (N) application on N uptake efficiency (NUE), partitioning and remobilization in walnut trees of cv Chandler grafted on seedlings. In February 2007, thirty, 2-year-old, trees were planted in 40-L pots and divided in 3 groups of 10 each, according to the time of N application: April 21 (bud burst), May 25 (pistillate flower maturity), September 12 (late summer). Each tree was addressed with 1 g of N (dissolved in water) as ammonium nitrate in which both ammonium and nitrate N were 15N enriched (5% atm.). One week after fertilization, 5 trees for each set were harvested and divided in: current growing shoots, leaves, stem and roots. All organs were dried, weighed milled and analyzed for 15N. At the end of the season the other 5 trees, for each set, were removed from the soil and two trees each group were harvested as above, while the other 3 trees were planted in fresh, 15N-uncontaminated soil until harvest (May 2008), when trees were divided into roots, stem, twigs, shoots, leaves and developing fruits. One week after fertilization the percentage of N derived from fertilizer (NDFF) was higher in trees fertilized in late summer followed by pistillate flower maturity and bud burst. At the end of the trial, the percentage of NDFF and NUE were unaffected by time of fertilization; shoot, leaves and developing fruits presented a higher percentage of NDFF than stem and roots, with no difference induced by timing of N application.