Résumé
Nut yield is highly variable in commercial macadamia production, and to ensure that nitrogen (N) Supply does not limit yield, high rates of N fertilizer are generally applied. To elucidate N source and sink relations in mature Macadamia integrifolia Maiden et Betche trees, we traced N-15 label after injection into individual branches and, after soil application, analysed xylem sap and examined the effects of hedging on tree N relations. Xylem sap N and sugar composition and concentration changed in relation to phenology and tree management. Canopy position did not affect xylem sap N concentration but sampling date had a significant effect. Hedging in spring was associated with a rapid and dramatic reduction of the concentration of xylem sap N until the following autumn, but unhedged trees were not available to unequivocally assess the significance of the results. Following N-15-branch injection in winter, most N-15 label was incorporated into flushing leaves and into bark. After N-15 injection in spring, flushing leaves and flowers were most strongly N-15-labelled. In late spring, N-15 label was equally incorporated by developing nuts that were retained or later abscised. Soil N-15 application in summer resulted in N-15-labelling of outer and mid-canopy leaves. In the following spring, N-15 label was translocated to flushing leaves, flowers and developing nuts. The results indicate that outer and mid-canopy leaves are the main N sink for soil-derived N during the vegetative phase and a N Source for developing tissues during the reproductive phase. Our study provides evidence that N supply to developing nuts is not a primary cause for nut abscission, supporting the notion that high N fertilizer application rates do not improve nut retention. We propose that current orchard design and hedging practices should be reviewed in context of the role of outer canopy leaves as a source of N for reproductive tissues.