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In short-term water culture experiments with different ~SN labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH 4 at a higher rate than NO~. Maximum NO 3 uptake by the whole plant occurred at 120 mg L 1 NO3_N, whereas NH 4 absorption was saturated at 240 mg L -1 NH4_N. 15NI-1 + l,ii 4 accumulated in roots and to a lesser degree in both leaves and stems. However, ~5NO-3 was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH 4 (15-60 mg L ~ N) to nutrient solutions containing 120 mg L-~ N as 15N labeled nitrate reduced ~5NO3 uptake. Maximum inhibition of ~sNO 3 uptake was about 55% at 2.14 mm NH 4 (30mg L -~ NH4-N ) and it did not increase any further at higher NH 4 proportions.In a long-term experiment, the effects of concentration and source of added N (NO~ or NH4) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH 4 versus NO 3 nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO3-N:NH4-N ratios (100:0, 75:25, 50:50, 25:75 and 0" 100) at 120 mg L -~ total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO 3 or NH 4 as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO3-N/NH4-N ratio. With increasing proportions of N + H 4 in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn N + decreased. Yield in number of fruits per tree was increased significantly by supplying all N as H4, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NOf-N:NH4-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH 4 in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO3"NH ~ ratio.

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The mobilization of N from reserve organs (leaves, roots, branches and trunk) to developing new organs was studied at different moments of the growth cycle. Three-year-old Valencia Late orange trees (Citrus sinensis (L.) Osbeck)were grown individually outdoors in 150 L containers filled with siliceous sand. Trees were irrigated with a nutrient solution labelled with potassium nitrate with an enrichment of 4 atom 15N % excess during a complete growth cycle. At the following year, plants were irrigated with unlabelled nutrient solution, and harvested throughout the growth cycle (flowering, fruit set, second flush, third flush, and dormancy). Total N and 15N analyses were carried out in the different organs of the plants.The highest amounts of N were found in leaves and roots (33-42% and 30-38%, respectively). Distribution of 15N was similar to that obtained for total N (42 and 39% of total lSN in leaves and roots, respectively) as is expected after a long period of labelling. Old leaves were the main reserve organs, contributing a 40-50% of the total N exported. Roots and aerial woody tissues exported between 30-35% and 15-25% of total reserve N, respectively. N exported by old leaves was approximately 57% of the N accumulated during the preceding year, while roots translocated 40% and trunk plus branches 35%. More than 70% of N accumulated in new organs during spring came from N stored in old organs.

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