Uptake of 15N by Kiwifruit Vines from Applications of Nitrogen Fertilizer Prior to Budbreak

Abstract

Mature field-grown kiwifruit vines (Actinidia deliciosa var. deliciosa cv. Hayward) were fertilized with 15N-labelled fertilizer (ammonium sulphate, 10 atom % 15N, 50 kgN ha-1) to investigate the timing of uptake of fertilizer nitrogen (N) and its availability for new season's growth. Treatments were applied on four occasions, representing 2, 6, 10 and 14 weeks prior to budbreak. Samples of root, stem, cordon, fruiting cane, vacuum-extracted xylem sap, and new season's growth were collected at fortnightly interval from early winter until 2 months after budbreak.Two weeks after application of each treatment, 15N equivalent to an average of 7% of the applied label was recovered in root material. Although label was taken up by roots, there was no movement of 15N within the plant until about 1 month prior to budbreak when it was measured in the stem and cordon. Fertilizer nitrogen was not detected at the distal end of fruiting canes, and in new season's growth until 3-4 weeks after budbreak. Before budbreak, all nitrogen in the xylem sap was in amino forms. Nitrate appeared 4 weeks after budbreak, and although more enriched with 15N than the amino nitrogen, accounted for only 19% of the label. Eight weeks after budbreak, nitrate nitrogen accounted for 57% of the label.There were no major treatment effects of 15N on vines in either spring or at harvest, although enrichments in fruit and leaves from the earliest treatment tended to be less at the end of the season than those from the later applications.