THE EFFECT OF TIMING OF POSTHARVEST FOLIAR UREA SPRAYS ON NITROGEN ABSORPTION AND PARTITIONING IN PEACH AND NECTARINE TREES

The effects of timing of autumn foliar urea-N sprays on nitrogen absor ption and partitioning were studied in mature peach and nectarine tree s. A 10% N-15 enriched urea solution was applied by either dipping ind ividual shoots in 1995 or spraying whole tree canopies in 1996. Trees whose canopies were sprayed during the post-harvest period with a 10% w:v urea solution in 1996 were excavated in the dormant season, and N- 15 contents and distribution were determined. Peach leaves rapidly abs orbed urea-N irrespective of application date, and transport of urea-N to perennial tree parts occurred primarily within 4-7 d after applica tion. Between 48 and 58% of the urea-N applied was recovered in abscin ded leaves and perennial organs. Leaves exported greater than or equal to 60% of the foliar-applied urea-N following application in early au tumn (September or October), but <50% export occurred when applied sho rtly before leaf fall (November). Of the urea-N translocated, most was recovered in roots (greater than or equal to 38%) following applicati on in September or October. Urea-N applied in November, however, remai ned largely in the current year wood (ca. 45%). Thus, export and trans location of foliar applied urea-N diminished during the final stages o f leaf senescence. Foliar application of urea in September or October supplied the equivalent of about 20% of crop nitrogen content, but onl y 14% (i.e. ca. 30% lower) when applied shortly before leaf senescence in November.