NITROGEN UPTAKE BY CITRUS LEAVES

We studied whether foliar-applied N uptake from a single application of low-biuret N-15-urea or (KNO3)-N-15, to citrus leaves was affected by N source, leaf age, or whole-shoot N content, In a glasshouse experiment using potted 18-month-old Citrus paradisi (L.) 'Redblush' graprefruit trees grown in full sun, 2- and 6-month-old leaves on single shoots were dipped into a 11.2 g N/liter (1.776% atom excess N-15-urea) solution with 0.1% (v/v) Triton X-77, Two entire trees, were harvested 1.5, 6, 24, and 48 hours after N-15 application. Uptake of N-15 per unit leaf area was 1.6- to 6-fold greater for 2-month-old leaves than for older leaves. The largest proportion of N-15 remained in the treated leaf, although there was some acropetal movement to shoot tips. In a second experiment, 11.2 g N/liter (3.78% atom excess) urea-N-15 and 3.4 g N/liter (49.2% atom excess) (KNO3)-K-15 solutions of comparable osmotic potential were applied to 8-week-old leaves on 5-year-old 'Redblush' grapefruit field-grown trees of differing N status. Twenty-four percent of the applied N-15-urea was taken up after 1 hour and 54%, after 48 hours. On average, only 3% and 8% of the (KNO3)-N-15 was taken up after 1 and 48 hours, respectively. Urea increased leaf N concentration by 2.2 mg N/g or 7.5% of total leaf N after 48 hours compared to a 0.5 mg N/g increase (1.8% of total leaf N) for KNO3. Foliar uptake of N-15 from urea, however, decreased (P < 0.05) with increasing total shoot iv content after 38 hours (r(2) = 0.57).