Nitrogen partitioning and remobilization in field-grown apple trees

Graduation date: 1996Uptake and cycling of nitrogen (N) in mature trees are not well understood.\ud Fertilizer-N uptake, partitioning, and use-efficiency were studied in standard strain\ud 'Topred Delicious' and spur strain 'Redspur Delicious' mature apple trees {Malus\ud domestica Borck) on 'Malling 7A' (M.7A) rootstocks. The treatments consisted of a\ud ground application in spring, a preharvest ground application in August, a foliar spray\ud in fall, or a combination of each of the last two treatments with the first. When soil-applied\ud in spring, labeled N (¹⁵N) was allocated preferentially to above ground tissues\ud and to a lesser extent to roots of both strains. The amount of newly absorbed soil-N\ud allocated to above ground tissues decreased as the season progressed, little ¹⁵N from preharvest\ud ground applications reached the leaves, fruit, buds, or branches, while roots were\ud heavily labeled.\ud Total fertilizer-N recovery in the trees was similar regardless of the time of\ud fertilizer application. However, losses of ¹⁵N to fruit removal, leaf fall, and pruning\ud were most severe when N was applied in spring and minimal for the pre-harvest timing\ud compared to all others. About a third of the variability in recovery was due to variation\ud in tree size. When recoveries were adjusted to account for size differences, spur-type\ud trees tended to be more efficient at utilizing fertilizer-N.\ud When ¹⁵N-urea was applied in April to young apple leaves, the label was not\ud exported. Labeled N from fall urea sprays was exported from the leaves to the buds, but\ud was restricted to the treated spurs and branches. Foliar urea sprays immediately after\ud harvest contributed more N to the buds than later applications.\ud Mobilization of ¹⁵N from storage in various tree parts was assessed. In\ud moderately vigorous trees, N stored in aerial parts of the tree was mobilized first,\ud followed by simultaneous mobilization of root and soil N. Utilization of root reserves\ud depended on the N status of the tree. When the buds were low in N but the roots had\ud adequate N reserves, root to shoot N transport started early in winter