The Fate of Carbon and Nitrogen from Urea Applied to Foliage of Senescing Apple Trees1

Urea was applied to the foliage of 1-year-old apple Malus pumila Rehd., cv. Mailing Merton 106 (MM 106) trees during senescence and the fate of the urea C and N determined. The urea C was readily released as CO2, but abscised leaves still contained 18% of the total. Only 5% of the urea C was found in storage tissues (shoot bark and wood, stem bark and wood, and roots). By the time the leaves abscised, this C was found primarily in protein, sugars, and amino acids of storage tissues. Small amounts of urea were still present. The urea N was translocated from the leaves as amino acids or urea. At leaf abscission, 30% of the initial N was still present in the leaves. During senescence, storage tissues from untreated trees increased 1.5-fold in N while similar tissues from urea-treated trees increased 3-fold. This additional N was found primarily in stem and shoot bark and in the roots. Small amounts of N were stored as amino acids but the bulk of the urea N was stored as protein.