Ultrastructure of Macadamia (Proteaceae) embryos: implications for their breakage properties

BACKGROUND AND AIMS: Macadamia integrifolia, M. tetraphylla and their hybrids are cultivated for their edible kernels. Whole kernels, i.e. intact mature embryos with cotyledons fused together, are highly valued and breakage of embryos into halves results in loss of value for the commercial macadamia industry. The morphology and ultrastructure of the mature macadamia embryo, with particular emphasis on the break zone between cotyledons, were investigated. Differences in breakage between different macadamia cultivars were also examined. METHODS: Manual cracking was used to compare breakage in five cultivars and the ultrastructure of the break zone between the cotyledons was examined using light and transmission electron microscopy. KEY RESULTS: Breakage of macadamia embryos was strongly dependent on genotype of the female parent, with cultivars 'HAES 344' and 'HAES 741' much more likely to break than 'HV A16' and 'HAES 835'. Cotyledons were surrounded by a layer of cuticle resulting in a double cuticle in the break zone between the cotyledons. Three major differences have been found in the ultrastructure of the double cuticle between cultivars: a thicker cuticle in the low-whole cultivar; convolutions in the cuticle of a low-whole cultivar, and the presence of more electron-dense objects in the high-whole cultivar. CONCLUSIONS: Breakage of macadamia embryos depends on the cultivar, with clear ultrastructural differences in the break zone between cultivars. To ensure commercial benefits, macadamia breeding programs should identify germplasm with structural characteristics that ensure high percentages of whole kernel.