EFFECTS OF TIMING OF NITROGEN APPLICATION ON NITROGEN PARTITIONING BETWEEN VEGETATIVE, REPRODUCTIVE, AND STRUCTURAL COMPONENTS OF MATURE COMICE PEARS

Mature pear (Pyrus communis) trees were fertilized with ammonium nitrate depleted in N-15 at various dates. In an initial experiment, N was applied one month before bloom (BB), one month after bloom (AB), or at harvest (HT). Nitrogen applied BB partitioned almost equally among spur leaves, shoot leaves, and fruits, but N applied AB was partitioned preferentially to shoot leaves and fruits. Nitrogen applied at HT remained in the roots during the dormant season. Flower buds sampled in December were high in labelled N when it was applied BB and AB, but not at HT. Soil texture did not influence N partitioning between spur leaves, shoot leaves, and fruits. In subsequent trials, labelled N was applied at five dates from six weeks before harvest to six weeks after harvest. Allocation of N following later-season application was strikingly different from the spring applications. Much less N was partitioned to leaves, fruits, and shoots. The amount of N allocated to above-ground tissues decreased as the season progressed and became negligible at later dates. Roots always had the highest percentage of fertilizer-derived N. Nitrogen fertilizer applied 3-6 weeks before harvest increased N in above-ground storage tissues without producing excessive N in shoots and fruit.