REMOBILIZED NITROGEN AND ROOT UPTAKE OF NITRATE FOR SPRING LEAF GROWTH, FLOWERS AND DEVELOPING FRUITS OF PEAR (PYRUS-COMMUNIS L.) TREES

Both uptake of fertiliser N and remobilisation of stored N were quanti fied for the early growth of spur and shoot leaves, flowers and fruit development of pear trees. One-year old Abbe F. trees grafted on quinc e C rootstocks were fertilised with a generous N supply for one year a nd while dormant during the winter, transferred to sand cultures. Each tree received 3 g of labelled nitrate-N at the end of winter and in e arly spring. Leaves, flowers and fruit were sampled on 5 separate occa sions and the recovery of labelled N used to distinguish the remobilis ation of N and the root uptake of nitrate. Remobilisation of stored N accounted for most of the N present in leaves and flowers during bloss oming. Remobilisation of nitrogen stopped between petal fall and the b eginning of fruit development. Root uptake of nitrate linearly increas ed over time and at the last sampling, 55 days after bud burst, fertil iser N contributed approximately half of the total N recovered in both spur and shoot leaves, the remainder coming from remobilisation. Flow ers and fruits based their N metabolism more on remobilisation as comp ared to the leaves. This pattern of internal cycling of N is discussed in relation to fertilisation strategies for pear trees.