Phenotypic Association of Multi-scale Architectural Traits with Canopy Volume and Yield: Moving Toward High-density Systems for Macadamia

Abstract

The vigorous growth and large canopy size of commercial macadamia (Macadamia integrifolia, M. tetraphylla, and hybrids) cultivars generally restricts macadamia orchards to low-density planting. Little is known of the detailed interactions between plant architecture and yield components specific to macadamia. This chapter examines how dependent traits such as canopy size and yield might be determined by direct and indirect interactions between traits at different scales within the canopy. Fifteen genotypes (n = 3) were phenotyped in two growing seasons for architectural and reproductive traits, around the age of their transition from juvenility to maturity. Genotypes varied in canopy volume, cumulative yield, and canopy efficiency, and particular genotypes with low canopy volume and high yield were considered potentially useful for future high-density orchard systems. There was high variability in architectural, floral, and yield traits at multiple scales. Direct and indirect effects of architectural traits on the variability of yield and tree size were quantified using path coefficient analysis. Canopy volume was subject to positive direct effects from trunk cross-sectional area (TCA; 0.72), lateral branching (0.24), and branch unit (BU) length (0.24). Other traits showed significant indirect effects with canopy volume via TCA, such as branch cross-sectional area (BCA; 0.43), BU length (0.40), lateral branching (0.35), and internode length (0.32). Branch angle had a significant indirect negative effect on canopy volume via BU length (−0.11). Nut number had the strongest direct effect on yield (0.97), and this relationship was significantly indirectly influenced by raceme number (0.47), raceme length (0.50), nut number per raceme (0.33), canopy volume (0.37), and branch angle (0.35). In these relatively young trees, early yield was directly and positively influenced by canopy volume (0.12), presumably due to increased early light interception, which suggests that early canopy vigor contributes to early yield. This study suggests that yield and canopy size are determined by complex phenotypic interactions between architectural traits at different scales. Therefore, preplanting (i.e., scion and rootstock selections) and postplanting (i.e., pruning and training) manipulations that specifically manage architectural traits such as shoot length, branching, branch angle, raceme length, and nuts per raceme may result in the creation of efficient macadamia canopies.