Seasonal Variation in Nitrate Uptake Efficiency and Distribution of Absorbed Nitrogen in Non-bearing Prune Trees1

Non-bearing prune ( Prunus domestica L. cv. Agen) trees were fertilized with 15 N-KNO 3 for 10 days during 9 phenological periods. Nitrate uptake efficienty (NUE) and the distribution of absorbed 15 N in the trees were determined for each of these application periods. Nitrate uptake was dependent on presence of leaves, and NUE was low from the period of natural leaf fall until shoot growth had commenced the following spring. NUE increased dramatically during the rapid phase of shoot elongation, and remained high until leaf fall. Nitrogen absorbed from fertilizer was rapidly mobilized by swelling buds and rapidly elongating shoots. The spring flush of vegetative growth utilized both the currently available fertilizer ( 15 N) nitrogen and tree nitrogen reserves. Rapid shoot elongation was primarily dependent, however, on the redistribution of storage N.