Developmental Changes in Raceme Carbohydrates and Nutrients During Flowering and Fruit Set in Macadamia

Daddow is a macadamia cultivar that yields poorly because flowers and fruitlets frequently abort early in development. The objective was to determine whether resource availability limits fruitlet retention in this cultivar. Racemes of Daddow and three other cultivars, 849, A38, and A203, were sampled weekly during flowering and fruit set, and trends in nutrients and non-structural carbohydrates were assessed. Starch concentrations in the flowers and rachis were lower in Daddow than in the other cultivars before fruit set. Rachis concentration of glucose, an important signalling molecule, was also lower in Daddow at flowering. Most flower and fruitlet nutrient concentrations in Daddow were comparable to those of the other cultivars. However, potassium concentrations were lower in both the rachis and leaves of Daddow, while boron concentration, important for pollen tube growth, was higher. These results suggest altered signalling mechanisms, inadequate carbohydrate supply and deficiencies in specific nutrients may have been contributing factors to the high flower and fruitlet abortion rates of Daddow.