NITROGEN PARTITIONING IN APPLE AS AFFECTED BY TIMING AND TREE GROWTH HABIT

Nine year old standard-(Std) 'Topred Delicious' and spur-type 'Redspur Delicious' apple trees (Malus domestica Borkh.) on Malling 7A (M.7A) rootstocks were fertilized with ground-applied ammonium nitrate or fol iarly-applied urea depleted in N-15 on various dates. The treatments c onsisted of a ground application in spring (March) (SG), a preharvest ground application in August (PHG), a foliar spray in fall (September) (FF), or a combination of each of the last two treatments with SG (SG /PHG and SG/FF). All trees received a total of 120 g N each with the e xception of FF trees, which received half the dose. Labelled N from th e SG application was allocated preferentially to the fruit, leaves, sh oots, and branches and to a lesser extent to the roots. Leaves on youn g shoots had higher concentrations of N-15 label than those on older s hoots or spurs, indicating an increasing dependence of the tree on soi l N from spring to summer. The amount of N allocated to above-ground t issues decreased as the season progressed. Very little N-15 from the P HG application reached the leaves, fruit, buds, or branches but the ro ots were heavily labelled and substantial amounts of labelled N were f ound in the following season's fruit, leaves, and shoots. Losses of fe rtilizer-N-15 from the PHG application to fruit removal, leaf fall, an d pruning were minimal suggesting a higher utilization efficiency of a bsorbed N. The percentage of N derived from the FF spray was low in al l tissues. However, this treatment resulted in N-15 reaching the buds and shoots. Spur-type tree tissues tended to have higher N and N-15 co ncentrations compared with those of Std-type trees. These differences appear to be related to different tree size. These differences suggest that the smaller spur-type trees are more manipulable with N manageme nt, and may be more subject to the adverse effects of over-fertilizati on. The subtle differences in N partitioning between tree types may ca use spur-type trees to recycle N more efficiently.