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Nitrogen partitioning in orchard-grown Macadamia integrifolia

2009; Oxford University Press; Volume: 30; Issue: 2 Linguagem: Inglês

10.1093/treephys/tpp107

1758-4469

Andrew Fletcher, Heinz Rennenberg, Susanne Schmidt,

Banana Cultivation and Research

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 15N 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 15N-branch injection in winter, most 15N label was incorporated into flushing leaves and into bark. After 15N injection in spring, flushing leaves and flowers were most strongly 15N-labelled. In late spring, 15N label was equally incorporated by developing nuts that were retained or later abscised. Soil 15N application in summer resulted in 15N-labelling of outer and mid-canopy leaves. In the following spring, 15N 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.