The Australian native rainforest tree Macadamia integrifolia has been cultivated for its edible kernel since the early 1900's. High variability in yield and quality between trees, years and regions poses a significant problem for producers. To date, factors strongly correlated to yield have not been identified. There is little information on the N physiology of sub-tropical evergreen tree crops such as Macadamia. Three methods were used to evaluate N characteristics of mature, orchard grown Macadamia. Firstly, N compounds present in the xylem sap were characterised over two years from December 1999 to November 2001 in upper and lower canopy regions. Secondly, 15N fertiliser was applied to mature trees and 15N enrichment was determined in different tissues. Finally, 15NH4+ was injected into the xylem tissue of individual branches to identify N distribution patterns at particular times of the phenological cycle. There were distinct changes in xylem sap N and sugar concentration and composition with relation to phenology and tree management. Asparagine (ASN), glutamine (GLN), arginine (ARG), aspartate (ASP) and glutamate (GLU) comprised the majority of xylem sap amino N in mature Macadamia. A positive correlation was found between xylem sap sucrose and aspartate + glutamate content implying a metabolic link between these compounds. Canopy reduction is currently required in orchard grown Macadamia in order to maintain equipment access. Within one week, hedging of the lower canopy reduced xylem sap amino-N and sucrose concentrations by 5-6 fold and 2 fold respectively in the hedged portions of the canopy. Glutamine-N concentrations in the xylem sap increased 2-3 fold from February to May in this study suggesting that GLN may be a product of recent N assimilation. Close correlation between upper and lower canopy glutamine-N (GLN-N) concentrations up to 6 months after hedging suggest an even distribution of recently assimilated N throughout the canopy. Xylem sap glucose: fructose ratios close to 1:1 during flowering in spring and in the year following hedging suggest that sucrose is the primary C source of glucose and fructose. All trees were hedged at the same time so it is not possible to distinguish seasonal from treatment effects with certainty.
Mature tree N uptake, transport and cycling were investigated using soil application of 15N labeled fertiliser. Highest mean 15N incorporation was detected in outer and mid canopy leaves 8 weeks after soil application. Decreases in 15N incorporation into outer and mid canopy leaves during vegetative dormancy suggest that soil derived N present in mature leaves remains part of a larger cycling pool of N. The close correlation between outer/mid canopy leaves and flush/flowers suggest the presence of a large mobile pool of N rather than discrete stores. 15N incorporation was significantly higher in actively growing tissues than in mature leaves or bark following introduction of 15NH4+ into individual branches using a novel xylem injection method. The relative N content and 15N incorporation patterns of outer, mid and inner canopy leaves suggest that reallocation of cycling N is the primary role of mature Macadamia leaves. Developing nuts that were prematurely abscised within 7 days of 15N injection had very low 15N incorporation. In contrast, 15N incorporation into nuts prematurely abscised 8-14 days after injection were similar to retained nuts implies that 15N transport to nuts ceased between 7 and 14 days prior to nut drop. It is likely that the reduced 15N incorporation is a result of reductions in xylem and phloem connectivity within the abscission zone resulting in a general loss of resource supply.
Xylem sap collection, soil 15N application and 15N injection techniques allowed examination of mobile N forms and distribution at different temporal and spatial resolutions. The previously proposed role of mature leaves as N storage organs was revised and an alternative model of N cycling and reallocation is presented.