Abstract

Fertilizer isotope labelling (¹⁵N) of young Lane Late potted orange trees with differential N rates at three phenological periods (end of flowering, fruit set and fruit growth) was used to determine patterns of N uptake response and N efficiency (NUE) along the growing cycle. In a first experiment trees were fed with a saturating N solution in order to determine seasonal variations in N requirements. The second experiment tested four N rates which were equal to (N₁), two (N₂), three (N₃) and four-fold (N₄) the N requirements at each period. Increasing N rate diminished NUE being the decrease more acute at flowering as a consequence of the lower response in N uptake to increased N supply in this period. Theoretical NUE corresponding to a fertilizer rate that fulfills plant requirements in each period was calculated, being highest at the end of fruit set period (72%), which contrasted with the value obtained for flowering (28%), when potential nitrogen leaching reached therefore its maximum (71%).

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