Aspect and Rootstock Genotype Influence Leaf Stomatal Density in Macadamia sp.

Stomata, the pores on the leaf epidermis, contribute to the exchange of water and CO2 in plants. They play important roles in sensing and adaptation to environmental changes by altering in size, distribution, and behaviour. To adapt to a changing climate, planting cultivars with optimised stomatal density in the target environments can be useful to improve water use efficiency. As a clonally propagated crop, orchard production of macadamia largely depends on rootstocks. The effect of rootstocks on stomatal density in macadamia remains unclear. This study, for the first time in macadamia, aimed to investigate the effects of aspect and rootstock genotype on leaf stomatal density. We investigated the effects of three rootstocks (Beaumont, Daddow and B-29), and two aspects (north (N) and south (S)), on leaf stomatal density. An existing phenotyping protocol was adapted to create an imprint of the leaf surface. This imprint was mounted to a microscope slide for imaging and stomatal density calculation using manual (M) and artificial intelligence (AI) methods. A highly significant interaction was observed between rootstock and stomatal density (P < 0.001 (M), P = 0.003 (AI)). Additionally, stomatal density was found to be significantly higher on the northern aspect of the tree than the southern aspect (P < 0.001 (M), P < 0.001 (AI)). These findings will assist in the development of climate-smart macadamia varieties with superior water use efficiency.