Phenology of flowering and nut production in macadamia

Macadamia is an Australian native tree that produces a highly valued, edible kernel. Quantitative information on the phenology of flowering and nut-drop in this species will assist orchard design and management of production orchards and breeding programmes. Three replicates of 12 varieties were selected for study at a single orchard in northern New South Wales, Australia (NSW). In early August 1999, 6 unopened racemes were tagged at each of 1.5 and 3 in heights above the ground and on both northerly and southerly aspects. Percentage of flowers open per raceme was recorded every three days. Nut-set was quantified as the number of nuts per raceme on March 21(st) 2000 and nut-drop was monitored to late July. All flowers opened over a 33-day period, commencing on August 27(th). There was significant genetic variation for start of flowering, the duration of flowering, nut-set and the start of nut-drop, but little genetic variation for nut-drop length. The onset of flowering was under the strongest genetic control. However, the largest source of variation for all traits was among racemes within a tree. There was no effect of aspect or within-tree height on any trait except for nut-set, which was greater on the northern side. Since stigmas are receptive for several days, there appears to be opportunity for genetic exchange between all but the extreme varieties. Nut-set was not strongly related with start of flowering or flower length. However, racemes that set more nuts tended to begin dropping those nuts later and over a longer period. The implications of these results for breeding and some aspects of orchard management are discussed.