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2 To find an alternative tool to chemical analysis for assessing the nitrogen (N) nutritional status, the SPAD-Minolta-502 portable chlorophyll meter was tested in one-to three-years-old 'Rocha' pear trees fertilized with 0, 10, 20 and 40 kg N ha −1 year −1 . SPAD readings in the third and fourth fully expanded leaves were as responsive to N fertilization as leaves sampled for foliar chemical analysis (medium third of one-year-old shoots), but individual leaves showed heterogeneous chlorophyll patterns. SPAD readings in leaves sampled at 60 and 110 days after full bloom (DAFB) related linearly with leaf N concentration but significant relationships could also be obtained later. Assuming an optimum leaf N concentration of ≥20 g kg −1 dry weight, SPAD readings ≥33 in leaves sampled at 60{110 DAFB corresponded to a good nutritional N status of young pear trees, making this diagnostic technique more practical, non-destructive and inexpensive, compared with chemical leaf analysis.

Synchronizing the availability of N in the rhizosphere with its uptake pattern by the trees throughout the vegetative cycle enhances the efficiency of its use. This study aimed to: i) quantify the N use efficiency in the non-bearing pear trees, fertigated with 15 N and ii) identify the pear tree N storage organs and access the distribution of N derived from the fertilizer (NDFF) in the organs. One-year old pear trees (Pyrus communis cv. Rocha on quince BA29 rootstock) planted in a loamy sand soil at 4 m x 1.5 m spacing, were supplied from April to October with 6 g N tree-1 of ammonium nitrate, doubled labeled with 5 atom% of 15 N. Leaf samples were monthly collected to determine %NDFF. At the beginning of leaf fall five trees were wrapped with nets and all abscised leaves were collected. At the end of leaf fall trees were removed from the soil and divided into fine and coarse roots, trunk and shoots. Each plant sample was weighed and analyzed for total N and 15 N abundance. The trunk was the main storage organ of the tree (44.5% of the total N), followed by the coarse roots (24.8%). 15 N enrichment was greater in the trunk (0.18 g tree-1), which represented 47.3% of the total 15 N recovered in the tree tissues. Senescent leaves showed the lowest %NDFF, retaining 6.2% of the absorbed N, which represented 9.6% of the tree 15 N enrichment. After the first year of N fertigation, newly-planted 'Rocha' pear trees showed a low N-fertilizer use efficiency (6.3%) possibly due to the lower N uptake during the initial three months after planting.

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