Humic acid (HA) is a kind of organic substance that has shown good effects in regard to promoting crop growth. In the current study, the influences of three kinds of fertilizers mainly containing humic/fulvic acids, including water-soluble fertilizer containing humic acid (WHA), fulvic acid potassium (FAP), and fulvic acid distillate (FAD), on the qualities of ‘Eureka’ lemon fruits were investigated systematically at their different harvest times. As demonstrated by the results, the indexes used to assess the quality of lemon fruit showed that all lemons treated with those three humic/fulvic acid fertilizers at different harvest times improved in quality. Notably, the single fruit weight, edible rate, and juice yield of lemon fruit in the WHA- and FAP-treated groups, as well as the contents of vitamin C, total acid, total sugar, and total soluble solid, were higher than those in the blank control (CK) group (p < 0.05). Furthermore, the contents of total flavonoids and phenols in the peels, pulps, and seeds of lemons in humic/fulvic acid fertilizer–treated groups were all higher than those in the CK group at the three harvest times (p < 0.05). This result indicated that humic/fulvic acid fertilizers produced positive effects on the quality of lemon fruits and could be used in lemon planting to improve the quality and added value of lemons.

Citrus limon (L.) Burm. f. is a horticultural crop known for its abundance of valuable secondary metabolites, including flavonoids, which are found in its fruits and leaves. Our previous research has shown that treating C. limon with fulvic acid (FA) can enhance the levels of vitamin C, total acid, total sugar, total flavonoids, and phenols in its fruits as well as the volatiles and total flavonoids in its leaves. In this current study, we established a method to analyze eight specific flavonoids in lemon leaves and evaluated the impact of irrigation with FA on the content of these flavonoids over a six-month period using HPLC-DAD/MS analysis. Moreover, we investigated the potential mechanisms of FA through ELISA and q-PCR methods. The results indicated that FA increased the contents of four flavonoids, namely, isoorientin, eriocitrin, vitexin, and rutin, and promoted the activity and gene expression of phenylalanineammonialyase (PAL), 4-coumaric acid coenzyme A ligase (4CL), chalcone synthase (CHS), flavonoid 3′-hydroxylase (F3′H), and flavonol synthase (FLS). Furthermore, the relationship between flavonoid content and the activities of biosynthetic enzymes was analyzed using orthogonal partial least squares discriminant analysis (OPLS-DA), which revealed a positive correlation between seven flavonoid levels and the activity of five biosynthetic enzymes under FA treatment.

Fatty acids tend to undergo dynamic changes during the growth and development of fruits. In this study, we analyzed the variations in fruit morphology and kernel fatty acid fractions and contents at seven post-flowering stages in the fruit of ‘A4’ and ‘OC’, two main macadamia cultivars in Yunnan, China. The single fruit weight and longitudinal and transverse diameters showed a ‘fast–slow–stable’ growth trend, and the fruit shape index gradually decreased with fruit development. A total of 13 saturated fatty acids, 18 monounsaturated fatty acids, and 10 polyunsaturated fatty acids were detected in macadamia kernels at seven developmental stages. The total fatty acid content in ‘OC’ and ‘A4’ tended to first increase and then decrease. The fatty acid content accounted for 8.81% and 6.33% of the total fatty acids at 50 days after flowering (DAF), and peaked at 95 DAF and 125 DAF (the fatty acid content accounted for 25.61% and 20.69% of the total fatty acids), indicating that these two periods are critical for fatty acid accumulation in the two cultivars. In addition, oleic acid, palmitoleic acid, cis-Vaccenic acid, and hexadecenoic acid were determined as the main fatty acids. This study reveals the dynamic changes in fatty acid composition and content in ‘OC’ and ‘A4’ during fruit development, providing a scientific basis for determining the appropriate harvesting time for macadamia nuts.

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.