RECYCLING OF NITROGEN IN FIELD-GROWN COMICE PEARS

Five-year-old 'Comice'/'Provence' quince BA29 pear trees growing in a Central Point sandy loam soil near Medford, Oregon, were fertilized with ammonium nitrate depleted in N-15 to study the capacity of the tree to use reserve N. Eight dormant trees were removed each year for three years and partitioning of biomass, total N, and depleted N-15 derived from the fertilizer were evaluated for various tree components. Losses of N in harvested fruits, leaf litter, and pruning were measured. New shoots and fruits were dependent on newly absorbed N. About 45% of the reserve N was partitioned to new growth in 1989. Reserve N accounted for 48% of the N in new growth. Leaves, fruits, and one-year-old wood consumed 28% and 34% of the total N reserve pool in 1988 and 1989, respectively. The main tissues exporting reserve N were branches, trunk, and roots. Partitioning of reserve N to new tissues was affected by a heavy crop load in 1989 with more N diverted into fruits at the expense of other tree components. The data show that pear trees recycle a substantial portion of their N content. Even though new tissues obtain a portion of their N from fertilizer additions, only small amounts are required.