Abstract
In a ®eld experiment on `Golden Delicious' apple trees, different combinations of labelled (10 at. % 15N) urea concentrations (4, 20 and 40 g l±1) and spray volumes (1500, 300 and 150.l ha±1) were used in mid summer to foliar apply 920.mg N per tree by the following treatments: 1) low concentration-high volume, 2) medium concentration-low volume, 3) high concentration-ultra low volume. The fate of urea-N sprayed on leaves was followed during a 5.d period from spraying by determining 15N concentration on leaf samples collected at different timings and on entire branches, previously girdled. Basal and apical leaves were sampled 1, 5, 24, 48 and 120.h after spraying. Leaves intercepted on average 24.mg labelled N per m2 of leaf area, regardless of treatment and leaf age. Leaf N uptake within 48.h was highest when urea was sprayed at low concentration-high volume. At the end of the experiment (120.h), however, no signi®cant effect of treatments on leaf uptake was recorded. Urea-N penetrated at higher rates on apical than basal leaves. While about 50% of the intercepted urea-N was recovered in the leaves sampled 120.h after application, the sum of fertilizer N recovered in the different organs of the girdled branches accounted for 84% of the estimated amount of N intercepted. The discrepancy between these two estimations can be explained considering that part of the N taken up from the leaves was rapidly exported and recovered in the fruits. Application of low volume with a high concentration of urea should be considered as a tool to decrease costs while maintaining a high N use ef®ciency.