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The efficiency and balance of nitrogen from one year's application was studied in a long-term fertigation experiment. Enriched nitrogen fertilizer, K~SNO3, was applied to a 22-year-old Shamouti orange tree with a history of high N applications (N3) and to an N-starved tree (N I ). The distribution of N in the different parts of the trees and in the soil was determined after the experimental trees were excavated.Similar total recovery of the labeled fertilizer N was found in the trees and soil in both treatments (N~ -61.7% N 3 -56%). However, the distribution between tree and soil was different. The amount of recovered residual fertilizer in the soil was much larger in the N 3 treatment than in N~.The highest percentage of fertilizer N was found in the new organs, i.e. fruits, twigs and leaves. The roots and branches took up only 6-14% from the labeled fertilizer.Only 20.9% of the leaf N and 23.4% of the fruit N in the N 3 tree originated in the labeled fertilizer, indicating translocation of N from older parts of the tree to new growth.Evidence was found of storage of N in the wooded branches, while the roots contained a surprisingly small part of labeled fertilizer.

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Ammonium influx into roots and N translocation to the shoots were measured in 3-week-old hydroponically grown rice seedlings (Oryza sativa L., cv. IR72) under conditions of N deprivation and NH 4 + resupply, using I3 NH 4 + as a tracer. Root NH^ influx was repressed in plants continuously supplied with NH 4 + (at 0.1 mM), but a high proportion of absorbed N (20 to 30%) was translocated to the shoot in the form of N assimilates during the 13-min loading and desorption periods. Interruption of exogenous NH4" supply for periods of 1 to 3 d caused NH 4 + influx to be de-repressed. This same treatment caused N translocation to the shoot to decline rapidly, until, by 24 h, less than 5% of the absorbed 13 N was translocated to the shoot, illustrating a clear priority of root over shoot N demand under conditions of N deprivation. Upon resupplying 1 mM NH 4 + , root NH 4 + influx responded in a distinct four-phase pattern, exhibiting periods in which N H / influx was first enhanced and subsequently reduced. Notably, a 25 to 40% increase in root influx, peaking at -2 h following re-exposure was correlated with a 4-to 5-fold enhancement in shoot translocation and a repression of root GS activity. The transient increase of NH 4 + influx was also observed in seedlings continuously supplied with NO 3 ~ and subsequently transferred to NH 4 + . Extended exposure to NH 4 + caused root NH 4 + influx to decrease progressively, while shoot translocation was restored to -30% of incoming NH 4 + . The nature of the feedback control of NH 4 + influx as well as the question of its inducibility are discussed.