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Nitrogen partitioning in orchard-grown Macadamia integrifolia
Andrew Fletcher1, Heinz Rennenberg, Susanne Schmidt
1School of Biological Sciences, University of Queensland, Brisbane, 4072 QLD, Australia.
Nitrogen (N) fertilizer in macadamia trees is mainly utilized by canopy leaves, not directly by nuts. This suggests current orchard practices may not optimize N use for improved nut retention.
Area of Science:
- Agricultural Science
- Plant Physiology
- Nutritional Ecology
Background:
- Macadamia nut yield is highly variable, often leading to excessive nitrogen (N) fertilizer application to prevent yield limitation.
- Understanding nitrogen source-sink dynamics is crucial for optimizing nutrient management in macadamia production.
Purpose of the Study:
- To elucidate nitrogen source and sink relationships in mature Macadamia integrifolia trees.
- To investigate the impact of hedging on nitrogen relations within the tree.
Main Methods:
- Used stable isotope tracing ((15)N) via branch injection and soil application.
- Analyzed xylem sap for nitrogen and sugar composition.
- Examined the effects of canopy position and hedging on nitrogen distribution.
Main Results:
- Nitrogen-15 ((15)N) label was primarily incorporated into flushing leaves and bark after winter injection, and into leaves and flowers after spring injection.
- Developing nuts showed (15)N labeling in late spring, regardless of retention or abscission.
- Soil-applied (15)N was primarily assimilated by outer and mid-canopy leaves during the vegetative phase and translocated to reproductive tissues in the following spring.
Conclusions:
- Outer and mid-canopy leaves act as the main nitrogen sink during vegetative growth and a nitrogen source during reproduction.
- Nitrogen supply to developing nuts is unlikely to be the primary driver of nut abscission.
- Current macadamia orchard design and hedging practices may need re-evaluation considering the role of canopy leaves in nitrogen redistribution for reproductive success.
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