Potential use of triazole growth retardants as a tool for canopy management in Macadamia

Abstract

Intensive, high-density orchard systems have allowed a transformational leap forward in crop production of several tree crop species. Recently, it has also been demonstrated that they provide potential advantages for macadamia (Macadamia integrifolia x M. tetraphylla) production. The higher planting density reduces the allocated orchard space for each tree, dictating a need to grow smaller trees. Macadamia trees naturally grow large and tall, which is not a suitable trait for high-density orchards and requires extensive canopy management. To support macadamia orchard intensification, we evaluated the potential application of the growth retardants from the triazole family, paclobutrazol and uniconazole, as a tool for canopy management. We hypothesise that these chemicals will reduce vegetative vigour and, as a result, will potentially lower pruning requirements and increase resource allocation to reproductive development. A spray application following pruning of either paclobutrazol or uniconazole significantly reduced the regrowth flush length by shortening the stem internodes. However, a late application, four weeks from pruning, did not a ect shoot elongation but caused significant distortion of the leaves. Similar to the spray application, a soil drench application of paclobutrazol resulted in a substantial reduction in the elongation of the flush growth. The canopy volume of trees that received a soil application, measured using a LiDAR system, was lower compared to control trees, also demonstrating the reduction in vegetative vigour. In addition to field experiments, a functional-structural plant model was used to predict the long-term e ect of a paclobutrazol soil drench. The model was able to demonstrate the potential long-term reduction in canopy volume and thus pruning requirements. Orchard intensification requires considerable adjustments to the canopy management. Our results suggest that the application of triazole growth retardants may be adopted as part of the canopy management strategy for intensive macadamia orchards.