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

Abstract

Additional index words. path analysis, plant architecture, planting density, intensification, vigor, productivity 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 (L0.11). 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.

Global Macadamia Science: Overview of the Special Section

“…Areas of research included propagation, tree physiology, soils and nutrition, pollination biology, pest and disease biology, orchard management, genetics and breeding, developing production areas, and product development. Seven of the presentations are published in this special section Hardner et al, 2019;Howlett et al, 2019;O'Connor et al, 2019;O'Hare et al, 2019;Toft et al, 2019;Zhao et al, 2019).…”

“…Several presentations (J. Wilkie et al, personal communication; T.G. Thorp and A. Smith, personal communication; B. Toft et al, personal communication) (Toft et al, 2019) discussed the physiology of young (<4 years of age) trees to support development of a new high-density intensive production system and underpin greater productivity. Early yield per tree (and per hectare) was 50 times greater for the cultivar A203 than HAES 741 at age 4, primarily due to early flowering (J. Wilkie et al, personal communication) (Fig.…”

“…Branching characteristics were quantified at different scales (i.e., whole tree, primary branches, secondary branches, vegetative growth flush units, node/internode) within trees of 5 to 6 years of age and correlated with per tree yield (B. Toft et al, personal communication) (Toft et al, 2019). Canopy volume was strongly associated with trunk cross-sectional area (phenotypic correlation, r P = 0.82) but only moderately (r P = 0.43) with branch cross-sectional area.…”

Selection strategies to improve yield in macadamia using component traits and genomics

“…Few other studies have measured these component traits on a diverse collection of germplasm. In their study of 15 varieties and selections at 4-5 years since planting, Toft et al (2019) found that phenotypes for RL ranged from 2-46 cm, with an average of 16 cm, whilst NPR varied from 0.2-2.6. These findings do not reflect the phenotypic ranges observed in the current study (RL 3.6-32.3 cm, NPR 1-10.4), which may be explained by different germplasm, environments and slightly different measuring techniques.…”

“…However, cultivars with thin shells may have lower kernel quality due to defects caused by damaging pests and diseases (Hardner et al 2009). With the focus turning towards smaller trees with high yields, planting densities, and, consequently, yield per hectare, are likely to increase (Toft et al 2019). Trunk circumference (stem girth) or trunk cross-sectional area, are easily-measured traits commonly used as an estimate of tree size (Hardner et al 2002;Toft et al 2018); trunk size may affect nut yield due to uptake of resources available for growth and fruit production.…”