Macadamia (Macadamia integrifolia), Macadamia tetraphylla and hybrids, a crop of high economic and nutritional importance, faces challenges with low fruit set rates and severe fruit drop. To address this, we investigated the effects of exogenous plant growth regulators (PGRs) and boron fertilizer on the development, fruit set, and yield of the A4 macadamia variety. The study was conducted in 2024 at the Lujiangba research base (China, Yunnan Province). Five treatments were applied during key growth stages: boron (B), brassinosteroids (BR), N-(2-Chloro-4-pyridyl)-N’-phenylurea (CPPU), 6-benzylaminopurine (6-BA), and gibberellic acid (GA₃). Growth stages include flower bud formation, peak flowering, and fruiting. Our findings revealed that B treatment significantly increased pollen viability (95.69% improvement) and raceme length (23.97% increase), while BR enhanced flower count per raceme (26.37% increase) and CPPU improved flower retention (10.53% increase). Additionally, GA₃ and 6-BA promoted leaf expansion in new shoots, increasing leaf length by 39.83% and 31.39%, respectively. Notably, B application significantly improved total yield (43.11% increase) and fruit number (39.12% increase), whereas BR maximized nut shell diameter (5.7% increase) and individual nut weight (19.9% increase). Furthermore, CPPU and 6-BA markedly improved initial fruit set rates, while GA₃, BR, and B effectively reduced early fruit drop. Physiological analyses indicated that elevated soluble sugars and proteins in flowers correlated with higher initial fruit set, whereas increased endogenous cytokinin and GA₃ levels improved fruit retention and reduced drop rates. Based on these findings, we propose an integrated approach to optimize productivity: applying 0.02% B at the floral bud stage, 2 mg/L 6-BA at full bloom, and a combination of 0.02% B and 0.2 mL/L BR during early fruit set. This strategy not only enhances yield but also mitigates fruit drop, offering practical solutions for macadamia production.

Severe flower and fruit abscission leading to low yields makes improving fruit set and increasing production critical challenges in Macadamia cultivation. Irrigation and artificial pollination during the flowering period significantly influence the fruiting and yield of macadamia. However, the synergistic effect of these two factors on macadamia production and yield remains unclear. This study investigated the effects of irrigation and artificial pollination on fruit set and yield using 11-year-old ‘A16’ Macadamia trees. Four treatments were applied: drought (DC), drought with artificial pollination (DC + AP), irrigation (I), and irrigation with artificial pollination (I + AP). Each treatment included three biological replicates, with a total of 12 trees. We assessed fruit set and yield, analyzing underlying mechanisms by evaluating changes in pollen viability, leaf morphology, inflorescence characteristics, and leaf/inflorescence physiology. Results revealed that DC + AP, I, and I + AP treatments exhibited significantly higher pollen viability and raceme length compared to DC. The I + AP treatment also resulted in the longest summer shoot internode length. Racemes were more sensitive to drought stress than leaves. Soluble protein and soluble sugar content in racemes were significantly higher in I + AP than in I and DC + AP, and lowest in DC. The DC treatment showed significantly higher superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and malondialdehyde (MDA) contents compared to I and I + AP. Leaf physiological traits exhibited inconsistent changes across treatments. Both artificial pollination and irrigation significantly increased fruit set. Treatment efficacy ranked as follows: I + AP (102.00% increase) > DC + AP (56.00% increase) > I (14.00% increase) > DC. Consequently, the I + AP treatment achieved significantly higher yield, fruit numbers, and fruit diameters than the other treatments. In terms of yield, treatment efficacy ranked as follows: I + AP (77.72% increase) > DC + AP (41.14 increase) > I (27.54% increase) > DC. These findings provide a scientific basis for enhancing yield in high-yield Macadamia cultivation systems.