Fruit Set, Abscission and Dry Matter Accumulation on Girdled Branches of Macadamia

The patterns of fruit growth, abscission and dry matter accumulation were determined for ungirdled macadamia branches and compared with the patterns on girdled branches. The relationships between fruit set, fruit drop, and the number of available leaves were also assessed. Approximately 50 leaves were required to support development of each fruit on girdled branches, so that final fruit numbers on girdled branches were higher or lower than on ungirdled controls, depending on leaf number. High leaf numbers did not cause any increase in fruit size on girdled branches, but low leaf numbers resulted in fruit volumes up to 23% lower than the controls. The number of fruits set on ungirdled branches was independent of the number of leaves on the branch. Defoliation of ungirdled branches had no effect on the number of fruits set, with fruit set on these branches apparently being supported almost entirely from carbohydrates obtained elsewhere in the tree. Three phases of fruit drop were observed, with maxima at 2, 6-7, and 10 weeks post-anthesis, separated by phases of relatively low fruit drop at 4 and 8 weeks post-anthesis. The maximum dry weight increase of the crop occurred between 10 and 20 weeks post-anthesis, when there was very little fruit drop. Therefore, although limited availability of assimilates may influence fruit retention in macadamia, there is no simple relationship between structural carbon demand and fruit drop.