Uptake efficiency and internal allocation of nitrogen in apple trees

Improved nitrogen (N) uptake efficiency (NUE) can lead to better economic and environmental outcomes. The physiological processes of N storage and remobilisation within deciduous fruit trees are relatively well understood. However, much can be gained through better understanding of management and environmental factors on these processes. This study aimed to determine the influence of pre- and post-harvest N application on NUE and the partitioning of N within the tree determined at winter dormancy. Fifteen-year-old potted ‘Jonagold’ trees, grafted on M26 rootstock were allocated to either pre- or post-harvest 5% ¹⁵N enriched calcium nitrate application. Fruit was harvested at commercial harvest. At winter dormancy, each tree was destructively harvested with plant material allocated to either roots, stem or branches. 15N recovery from dried plant samples was determined using Isotope Ratio Mass Spectroscopy. Nitrogen uptake efficiency was significantly greater with pre-harvest N application at 64.5% of applied N compared to post-harvest applied N at 50.5%. Timing of N application significantly influenced N distribution throughout the tree. Over 30% of pre-harvest N was recovered from fruit. The proportion of recovered N allocated to roots, stem and branches was significantly greater for post – harvest N application with leading to increased N storage reserves. This study shows that greater uptake efficiency can be achieved with pre-harvest N application, but that the majority of this was allocated to the fruit. This may present some risk to fruit quality outcomes as high fruit N has been shown to delay maturity, limit red colouration and lead to softer fruit that stores relatively poorly. Results from this study improve the current understanding of the influence of timing of N application on allocation and storage within deciduous fruit trees.