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B.R. Bondada, J.P. Syvertsen and L.G. Albrigo

Foliar-applied urea nitrogen (N) has potential to become an important component in fertilizer programs for citrus in Florida and other citrus growing areas as it can reduce nitrate leaching into ground water. We evaluated seasonal absorption characteristics of three urea formulations, Triazone-urea, liquid urea, and spray grade urea by citrus leaves that were from 2 weeks to 6 months old. The effect of leaf age on 15N absorption by N-deficient and N-sufficient leaves, together with urea absorption over an eight-week period were studied using greenhouse-grown and field-grown plants. All foliar N applications were based on a recommended rate of 34 kg N/ha in 469 L of water. In the field studies, leaf N was increased similarly by the three urea formulations one week after three weekly applications. Young leaves (0.25 month and 1 month old) absorbed a greater percentage of N than the older leaves (3 month and 6 month old). Epicuticular wax concentration increased and 15N absorption declined with leaf age. Nitrogen deficient leaves (1.80% N) had greater wax concentration and lower N absorption than N sufficient leaves (2.60% N). Four to 8 weeks after urea applications, Triazone-urea sprayed leaves had significantly greater leaf N concentration than leaves sprayed with liquid urea or nonsprayed control leaves. The greenhouse studies revealed that the 15N absorption was greater through abaxial leaf surfaces than through adaxial surfaces regardless of leaf N level and application time. Applying foliar 15N-urea during night (2000 hr to 2200 hr) resulted in greater absorption of 15N than in the morning (0800 hr to 1000 hr) or afternoon (1200 hr to 1400 hr). It is clear that maximum N absorption from foliar urea sprays occurred at night through the abaxial surfaces of young leaves with sufficient N. Triazone-urea acted as a slow-release N source that could be exploited in supplying N over an extended period of time.

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John D. Lea-Cox and James P. Syvertsen

We studied whether foliar-applied N uptake from a single application of low-biuret N-urea or K NO 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' grapefruit 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-urea) solution with 0.1% (v/v) Triton X-77. Two entire trees were harvested 1.5,6,24, and 48 hours after N application. Uptake of N 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 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-15N and 3.4 g N/titer (4.92% atom excess) KNO 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-urea was taken up after 1 hour and 54% after 48 hours. On average, only 3% and 8% of the K NO 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 KNO. Foliar uptake of N from urea, however, decreased (P < 0.05) with increasing total shoot N content after 48 hours (r = 0.57).

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Guihong Bi and Carolyn F. Scagel

in plants derived from foliar 15 N–urea sprays increased with increasing N fertigation rate 15 d after plants were sprayed with urea and plants fertigated at 10 m m and below had less 15 N than plants fertigated at 15 and 20 m m N ( Fig. 2B

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Marlene Ayala, Lorena Mora and Joaquín Torreblanca

leaves of spurs and ES ( Lang, 2001 , 2005 ; Rivera et al., 2016 ). Ayala et al. (2014) reported that foliar application of 15 N-urea after harvest influenced N storage reserves in floral buds for the subsequent spring. Similarly, Ouzounis and Lang

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Lailiang Cheng, Shufu Dong and Leslie H. Fuchigami

Bench-grafted Fuji/M26 trees were fertigated with seven nitrogen concentrations (0, 2.5, 5.0, 7.5, 10, 15, and 20 mm) by using a modified Hoagland solution from 30 June to 1 Sept. In Mid-October, plants in each N treatment were divided into three groups. One group was destructively sampled to determine background tree N status before foliar urea application. The second group was painted with 3% 15N-urea solution twice at weekly interval on both sides of all leaves while the third group was left as controls. All the fallen leaves from both the 15N-treated and control trees were collected during the leaf senescence process and the trees were harvested after natural leaf fall. Nitrogen fertigation resulted in a wide range of tree N status in the fall. The percentage of whole tree N partitioned into the foliage in the fall increased linearly with increasing leaf N content up to 2.2 g·m–2, reaching a plateau of 50% to 55% with further rise in leaf N. 15N uptake and mobilization per unit leaf area and the percentage of 15N mobilized from leaves decreased with increasing leaf N content. Of the 15N mobilized back to the tree, the percentage of 15N partitioned into the root system decreased with increasing tree N status. Foliar 15N-urea application reduced the mobilization of existing N in the leaves regardless of leaf N status. More 15N was mobilized on a leaf area basis than that from existing N in the leaves with the low N trees showing the largest difference. On a whole-tree basis, the increase in the amount of reserve N caused by foliar urea treatment was similar. We conclude that low N trees are more effective in utilizing N from foliar urea than high N trees in the fall.

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Richard C. Rosecrance, Scott Johnson and Steven A. Weinbaum

The ability of peach leaves to absorbed and translocated foliarly applied 15N-urea in mature peach (Prunus persica) trees was determined. Urea uptake experiments were conducted in June, October, and November 1995. Peach leaves absorbed ≈80% of the urea within 48 hr of application in all three experiments based on urea rinsed from leaf surfaces. Similarly, leaf 15N content reached a peak 48 hr after application. Translocation of 15N out of leaves, however, was more rapid in October then November. In October, 24% of the 15N remained in the leaves 2 weeks after application, while, in November, 80% stayed in the leaves and fell to the orchard floor. Thus, applying urea in mid November did not allow enough time for the N to be transported out of the leaves before leaf abscission. Timing of foliar urea application is critical to maximize N transport into perennial tissues of peach trees. 15Nurea resorption out of leaves and into perennial tree parts (roots, trunk, current year wood, etc.) is discussed.

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Regina L. Reickenberg and Marvin P. Pritts

The dynamics of nutrient uptake from foliar applied 15N-urea and Rb (a K analog) were quantified in red raspberries. Both N and Rb in an aqueous solution were absorbed rapidly into the leaf and transported throughout the plant. In the greenhouse, about half of the urea and a third of the Rb were absorbed within 32 hours of application. The addition of a surfactant to the foliar solution reduced uptake, while solution pH, time of application and leaf age had little effect. The lower leaf surface exhibited a faster rate of absorption than the upper surface, but the difference was not large. In the field, some foliar N appeared to have been washed off leaves and taken up by the root system; however, none of the foliar applications affected plant growth. We conclude that significant uptake of foliar applied N and K occurs in raspberry, but the absolute amount delivered through a single foliar application is small. The percentage of total plant nutrient supplied through a foliar application is reduced to < 5% over time as the plant grows, so multiple applications would be required to maintain levels significantly higher than would exist through root uptake alone.

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Wei-Ling Yuan, Shang-yong Yuan, Xiao-hui Deng, Cai-xia Gan, Lei Cui and Qing-fang Wang

microplot was surrounded by 50 cm metal sheets inserted 30 cm deep in the soil. In 2012, all the microplots were only applied with labeled 15 N urea (Shanghai Chem-Industry Institute, China). In 2013, each microplot was divided into two parts evenly. Half

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

Funct. Ecol. 11 754 765 Han, Z. Zeng, X. Wang, F. 1989 Effects of autumn foliar application of 15N-urea on nitrogen storage and resuse in apple J. Plant Nutr. 12 675 685 Henry, P