Can greater understanding of macadamia canopy architecture lay the foundation for orchard productivity improvements?

Abstract

Macadamia canopies tend to grow large and complex due to vigorous recurrent flushes of vegetative growth. New shoot growth can occur at any time of the year from apical and axillary buds, elongating existing axes and forming new axis orders, respectively. The macadamia canopy is relatively unmodified and low yielding, thus yield efficiency and management practices would benefit from an increased understanding of canopy growth dynamics and architecture. Here we detail a range of investigations into the architectural development of macadamia over different scales, and methods for modifying architecture with a view to this information being incorporated into the design of improved orchard systems. For isolated shoots we have documented the elongation of internodes and the whole growth unit (GU), and their relationship to thermal time. We have also undertaken a detailed architectural study on two cultivars of young trees leading up to their first flowering, in an attempt to compare and understand patterns of early vegetative development. At the tree scale there were no differences in canopy volume or tree height between cultivars, however, detailed scales revealed differences in canopy components such as GU number, GU length and branching patterns. Shoot bending can modify vegetative architecture and reproductive development in temperate crops, although responses in subtropical crops could be more complicated. Bending first-order shoots in macadamia reduced apical growth and induced axillary release. When shoots are bent at the time of floral initiation raceme number was increased on the first-order shoot axis, possibly due to a coincidence between bending-induced axillary bud release and time-dependent floral signals. Observing interactions between components at different scales aids understanding of the mechanisms and relationships controlling the structure and function of the growing macadamia canopy, thus providing opportunities to modify macadamia growth for the benefit of production efficiency.