Broad-sense heritability and inter-trait relationships in young macadamia architecture, flowering and yield

Macadamia in the orchard environment is relatively unaltered from its natural form, and there is considerable scope to change vegetative and reproductive architecture for improving yield and related traits through breeding. An understanding of genetic and environmental control mechanisms and the dynamics between vegetative and reproductive characteristics are useful to identify the most important traits for improvement. In breeding programs, considerations of the heritability of vegetative traits are usually limited to tree scale measurements such as canopy volume. Here we studied the broad-sense heritability (H) of characteristics that constitute the macadamia canopy and yield at multiple architectural scales, and also documented relationships between vegetative and reproductive traits. Cutting-grown clones of 15 macadamia genotypes were subsampled from a breeding trial planted in 2011 in South East Queensland, Australia. At the tree scale, canopy volume had very low H (0), suggesting a strong environmental influence. Detailed measurements of individual canopy components give insight to more complex interactions within the canopy. Some vegetative architectural characteristics, such as branch number, average internode length and growth unit (GU) length had medium H (0.31-0.39), and node number per GU had high H (0.57). Other canopy components such as length of primary branch axes and primary branch cross-sectional area (BCA) displayed very low heritability (0-0.05), and are likely to be mainly controlled by environmental conditions. H values were low for yield and nut counts (0.16-0.21), except nuts per raceme (0.43). Heritability was medium to high for other reproductive characteristics (0.34-0.70) that may indirectly relate to yield. Understanding the relationships within and between vegetative and yield traits at different architectural scales informs the choice of multiple useful traits, and will aid the breeding of future elite macadamia cultivars. Genetic correlations and principal component analysis provided an insight to genetic relationships between limb angle and nodes per GU; nodes per GU and GU length; GU length and yield traits; and raceme length and yield