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 foliarly-applied urea depleted in N-15 on various dates. The treatments consisted 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 exception of FF trees, which received half the dose. Labelled N from the SG application was allocated preferentially to the fruit, leaves, shoots, and branches and to a lesser extent to the roots. Leaves on young shoots had higher concentrations of N-15 label than those on older shoots or spurs, indicating an increasing dependence of the tree on soil N from spring to summer. The amount of N allocated to above-ground tissues decreased as the season progressed. Very little N-15 from the PHG application reached the leaves, fruit, buds, or branches but the roots were heavily labelled and substantial amounts of labelled N were found in the following season's fruit, leaves, and shoots. Losses of fertilizer-N-15 from the PHG application to fruit removal, leaf fall, and pruning were minimal suggesting a higher utilization efficiency of absorbed N. The percentage of N derived from the FF spray was low in all 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 concentrations 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 management, and may be more subject to the adverse effects of over-fertilization. The subtle differences in N partitioning between tree types may cause spur-type trees to recycle N more efficiently.