Uptake and internal allocation of nitrogen in apple trees

Objective: 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 the; NUE and quantity of N allocated to storage and; the partitioning of N within the tree.

Methods: Fifteen-year-old potted 'Jonagold' trees, grafted on M26 rootstock were allocated to either pre- or post-harvest 5% N¹⁵ enriched calcium nitrate application. Pre-harvest N application trees were allocated to either summer or dormant destructive sampling. An additional set of trees was allocated to monthly sampling of leaf N¹⁵ content throughout the season. N¹⁵recovery from samples was determined using Mass Spectroscopy.

Results: No significant difference in NUE or proportion of N allocated to storage was found between trees receiving pre- and post-harvest N¹⁵ application. Determining potential losses found the efficiency of N¹⁵ resorption from leaf material was close to 100%. Timing of N application significantly influenced N distribution throughout the 'tree. Trees receiving post-harvest N application contained a significantly greater proportion of N¹⁵in the below ground organs in comparison with pre-harvest application, 52% and 23% respectively. However, trees receiving pre-harvest N¹⁵ distributed significantly more to the canopy than trees receiving post-harvest application, 52% and 26%, respectively.

Conclusion: N distribution was found to vary significantly between summer active growth and dormancy, with the N¹⁵ content of the trunk increasing from 13 to 35% and below ground N¹⁵ content increasing from 25 to 36%. Results from this study improve the current understanding of the influence of timing of N application on allocation and storage within deciduous fruit trees.