Experiments were conducted with `Hamlin' orange [Citrus sinensis (L.) Osb.] budded on Cleopatra mandarin (Citrus reticulata Blanco) or Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedling rootstocks to determine minimum container solution N concentrations required for optimum growth and fertilizer uptake efficiency at various growth stages. Plants were fertigated daily with 1 liter of N solution at either 0, 12.5, 25, 50, 100, or 200 mg·liter-1 from NH4NO3 or 0, 3.13, 6.25, 12.5, 25, or 50 mg·liter-1 from NH4NO3 dissolved in a complete nutrient solution, respectively. Percentage of N in the mature plant tissues increased as N concentration in the medium solution increased. Shoot length and leaf area increased as N concentrations increased up to a critical concentration of 15 to 19 mg·liter-1. The critical N concentration for root, shoot, and total plant dry weight was ≈18 mg·liter-1 for `Hamlin'-Cleopatra mandarin nursery plants and 15 mg·liter-1 for `Hamlin'-Carrizo nursery plants. The critical N concentration for relative total plant dry weight accumulation (percentage) for the two experiments was 16.8 mg·liter-1. In a separate experiment, plants were given labeled fertilizer N (FN) (15NH415NO3) at one of five growth stages: A) in the middle of rapid shoot extension of the third flush, B) immediately following the cessation of the third flush shoot extension but during leaf expansion, C) immediately following leaf expansion, D) before the fourth flush, or E) in the middle of rapid shoot extension of the fourth flush. Labeled FN recovery increased during rapid shoot extension of the fourth scion flush compared to the other labeling periods. FN uptake per gram of total plant dry weight was greatest during rapid shoot extension (A and E) and lowest during the intermediate labeling periods (B-D). FN supplied 21% to 22% of the N required for new growth during rapid shoot extension.
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