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Self-fertility is a commercially valuable trait in crop species, enabling fruit set without reliance on pollinators or external pollinisers. In macadamia, most cultivars are self-infertile, though some can produce nuts with self-pollen. The mechanisms underlying this variation remain unclear. This study investigates herkogamy, dichogamy and in vivo pollen tube growth to investigate self-fertility. The traits were measured on cultivars from self-fertile and self-infertile groups. Herkogamy was assessed by pistil length (PL), stamen length (SmL), and stigma-anther distance (SAD). Significant interactions were observed between fertility groups and cultivars (nested within groups), with PL ranging from 11.0 to 14.5 mm, SmL from 6.5 to 9.0 mm, and SAD from 6.6 to 7.9 mm. The non-significant differences in SAD, together with the presence of approach herkogamy (stigma positioned above anthers) in both the self-fertile and self-infertile groups, demonstrated that spatial separation does not explain fertility differences. Temporal separation was assessed via pollen viability and stigma receptivity across six floral stages, where all cultivars exhibited protandry (dichogamy). The overlap in male and female reproductive maturity in all cultivars confirmed that temporal differences do not account for self-fertility. Fluorescence microscopy revealed inhibited pollen tube growth within styles of incompatible pollinations, indicated stylar self-incompatibility. Pollen tube progression ranged from 0 to 73.3% pistils where pollen tube reached the lower style, with significant general, specific, and reciprocal effects. We concluded that self-fertility in macadamia is not governed by spatial or temporal reproductive differences, but instead reflects a homomorphic stylar self-incompatibility mechanism, providing a foundation for future molecular and genetic investigations.

Abstract Macadamia is cultivated globally, and its nuts contain abundant monounsaturated fatty acids. Oil bodies, storage units of lipids in plant seeds, exhibit favourable stability, emulsification, and weak gelation properties. Macadamia oil body (MOB) emulsion gels were prepared by using κ-carrageenan (κ-Car) and konjac glucomannan (KGM) of different ratios (1:0, 1:0.5, 1:1, 0.5:1, 0:1) and different composite polysaccharide contents (0, 0.1, 0.2, 0.3, 0.4 wt%) through microgel particle compaction (particulate gels) and natural gelation (filled gels), respectively. Characteristics of MOB emulsion gels were investigated by confocal laser scanning microscopy, rheology, storage stability, and oxidative stability. MOB emulsion particulate gels exhibited enhanced G′ and apparent viscosity with the incorporation of microgel particles (MGPs) or an elevated KGM proportion. However, MGPs caused a deterioration of storage stability (30/60/90 days), while KGM improved storage stability. MOB emulsion-filled gels presented lower G′ and apparent viscosity than particulate gels with equivalent composite polysaccharide content and complex ratio (except 1:0). Nevertheless, filled gels exhibited superior storage stability and antioxidant properties than particulate gels. Moreover, KGM can lead to a decline in G′, apparent viscosity, and storage stability of filled gels. Alternatively, the antioxidant properties of MOB emulsion gels could be significantly enhanced by loading resveratrol. Graphical Abstract Graphical Abstract

Macadamia nuts have high nutritional value and positive health attributes. Changes to the composition and availability of these compounds during roasting contribute to product quality. In this study, changes to the chemical composition of lipids (fatty acids, triglycerides, and free fatty acids) and other phytochemicals were analyzed, and a sensory evaluation was carried out of two major varieties of macadamia nuts planted in China, after roasting. Only small changes in fatty acid (FA) content and a slight decrease in total triglycerides (TAGs) were observed after roasting. The free fatty acid (FFA) content and the peroxide value were increased by roasting. The total available polyphenol content increased by 25.6% and the oxidative stability index of kernels increased by 21.6%. The sensory scores for taste and aroma were doubled by roasting. Overall, the sensory, nutritional quality, and oxidative stability of roasted macadamia nuts were greatly improved, compared with raw nuts. This research provided a deep insight into the quality control of macadamia nuts during roasting and would help set guidelines for selecting the suitable roasting conditions for macadamia nuts.

The global production of macadamia nuts has witnessed a significant increase, resulting in the accumulation of large quantities of discarded nutshells. These nutshells possess the properties of remarkable hardness and toughness, which are comparable to those of aluminum. Incorporating natural fillers to enhance the properties of composite materials for various applications, including light duty, structural, and semi-structural purposes, is a common practice. Given their inherent hardness and toughness, macadamia nutshells present an intriguing choice as fillers, provided that the manufacturing conditions are economically viable. With the urgent need to shift toward natural fillers and reduce reliance on synthetics, exploring macadamia nutshells as components of natural fiber composites becomes imperative. This review aims to comprehensively examine the existing body of knowledge on macadamia nutshells and their bio-synthetic polymer composites, highlighting key research findings, achievements, and identifying knowledge gaps. Furthermore, the article will outline prospective areas of focus for future research endeavors in this domain, aligning with the universal goal of minimizing synthetic materials.

The consumption of macadamia nuts has increased due to their cardioprotective and antioxidant properties. However, this rise is consistent with an increase in the cases of macadamia nut allergy, leading to severe reactions. Although two Macadamia integrifolia allergens (Mac i 1 and Mac i 2) have been identified in Australian and Japanese patients, the allergenic sensitization patterns in Western European populations, particularly in Spain, remain unclear. For this purpose, seven patients with macadamia nut allergy were recruited in Spain. Macadamia nut protein extracts were prepared and, together with hazelnut and walnut extracts, were used in Western blot and inhibition assays. IgE-reactive proteins were identified using MALDI-TOF/TOF mass spectrometry (MS). Immunoblotting assays revealed various IgE-binding proteins in macadamia nut extracts. Mass spectrometry identified three new allergens: an oleosin, a pectin acetylesterase, and an aspartyl protease. Cross-reactivity studies showed that hazelnut extract but not walnut extract inhibited macadamia nut oleosin-specific IgE binding. This suggests that oleosin could be used as marker for macadamia–hazelnut cross-reactivity. The results show an allergenic profile in the Spanish cohort different from that previously detected in Australian and Japanese populations. The distinct sensitization profiles observed highlight the potential influence of dietary habits and environmental factors exposure on allergenicity.

Global fruit and nut yields are affected by shortfalls in pollinator populations, and pollen limitation is most prevalent among tropical, bee-pollinated and self-incompatible plants. Macadamia is a subtropical, bee-pollinated crop in which some cultivars have been found to be highly outcrossing. We aimed to determine the extent of outcrossing and its effects on nut quality across a wide range of international macadamia cultivars in three countries. We sampled fruit from 19 macadamia cultivars across 23 sites in Australia, Brazil and South Africa. We used genotype-by-sequencing and MassARRAY methods to assign paternity to individual fruit and we assessed pollen-parent effects on nut quality. Macadamia was highly outcrossing, producing 80–100% of fruit by cross-pollination, at 17 of the 23 sites. Mixed mating (41–72% outcrossing) was identified at five sites, and low outcrossing (10%) was identified in one cultivar at one site where it was isolated from other flowering macadamia trees. Outcrossed fruit often had significantly better quality than selfed fruit, with 1.61–3.39 g higher nut-in-shell mass, 0.53–1.55 g higher kernel mass, 3.3–6.4% higher kernel recovery, and 3.0–3.5% higher oil concentration. The differences in kernel recovery equated to differences in value of USD 433–841 per ton of nut-in-shell at prices of USD 3000 per ton. In summary, macadamia cultivars were mostly highly outcrossing, and outcrossed nuts often had higher quality than selfed nuts. Growers should consider interplanting different cultivars more closely and distributing bee hives more widely to maximise cross-pollination, produce high yields, and optimise nut quality.

Macadamia integrifolia is a vital cash crop. The shells of its nuts serve multiple purposes in both agricultural practices and waste management initiatives. In this research, transcriptome analysis was carried out on three macadamia nut varieties with significantly different shell thicknesses, namely ‘A38’, ‘Guire No.1’ (‘GR1’), and HAES842 (‘842’), at the same stage of maturity. The results revealed remarkable differences in their gene expression profiles. A total of 4311 novel genes were identified, among which 1631 were functionally annotated. Analyses using Gene Ontology (GO), Clusters of Orthologous Groups (COGs), and the Kyoto Encyclopedia of Genes and Genomes (KEGG) indicated that the main categories of differentially expressed genes (DEGs) were associated with plant–pathogen interactions. Additionally, 10 members of the heat shock protein 90 (MiHSP90) family were identified and classified into subgroups A, B, and C by comparing them with the HSP90 gene family members of Arabidopsis and rice. Among these, the MiHSP90.1, MiHSP90.2, and MiHSP90.9 proteins were differentially highly expressed in the three macadamia nut varieties. These findings provide fundamental insights into the regulatory mechanisms underlying shell formation in macadamia nuts.

In this study, adsorptive removal of benzene, toluene, ethylbenzene and xylenes (BTEX) from synthetic water using activated carbon adsorbent derived from macadamia nut shells was investigated. The surface functional groups of the synthesized adsorbents were assessed by Fourier transform infrared spectra. The specific surface area, pore size and pore volume at 77 K nitrogen adsorption, surface morphology, and the crystalline structure of the adsorbents were determined using Brunauer-Emmett-Teller, scanning electron microscopy and x-ray difraction, respectively. Batch adsorption mode was used to evaluate the performance of the activated carbon. The stock solutions of synthetic wastewater were prepared by dissolving 100 mg/L of each of the BTEX compound into distilled water in a 250 mL volumetric flask. Efect of initial concentration of BTEX compounds, contact time, and mass of adsorbent on the removal of BTEX compounds from the synthetic wastewater was investigated. The macadamia nut shell-derived activated carbon (MAC) proved to be an efective adsorbent for BTEX compounds, with a large surface area of 405.56 m2/g. The exposure time to reach equilibrium for maximum removal of BTEX was observed to be 20 min. The adsorption capacity of the BTEX compounds by MAC followed the following adsorption order: benzene > toluene > ethylbenzene > xylene.

Macadamia nut butter is a value‐added product, promoting lower‐grade nuts to premium products. The commercial grade of macadamia nuts includes the kernel quality (i.e., maturity and oil content) and style (i.e., size). However, whether immature nuts are suitable for nut butter production remains under‐explored. The study aimed to determine the influence of macadamia nut qualities and styles on the sensory profile of macadamia nut butter. Individual butters from two high oil, mature qualities (Style 1, Style 8) and two low oil, immature qualities (Style 2, Style 4) were compared to a commercial control. Descriptive sensory analysis was performed with an experienced panel (n = 13), a combination of ballot and consensus training, and reference standards. A sensory lexicon of 40 descriptors was developed, where 35 attributes differed (p < 0.050) between treatments. Principal component analysis (70.42% explained variance) confirmed variability in the sensory profiles of commercial control batches. High oil, mature kernels produced more replicable butter quality. Low oil, immature kernels produced inconsistent quality butters, with higher association with positive attributes (e.g., nutty aroma/flavor), but also with off‐notes (e.g., fruity aroma/flavor). The results suggest that kernel quality (maturity and oil content), rather than style, has the greatest influence on the final sensory quality of nut butter. These findings provide practical guidance for manufacturers in the procurement of macadamia kernel grades for butter production and the implications on product sensory quality. Further investigation is needed to customize roasting and grinding times based on kernel quality, which may improve product consistency. This study investigated how macadamia nut quality (maturity and oil content) and style (size) influence the sensory profile of macadamia nut butter. Descriptive sensory analysis showed that kernel quality, rather than style, most strongly affected butter characteristics, with mature, high‐oil nuts producing more consistent quality. Immature nuts yielded both desirable and off‐flavor attributes, indicating the need for tailored processing to optimize product quality. Graphical , for sensory quality of nut butter produced from different commercial macadamia grades, with sample photos belonging to the authors. The jar stock image, broken macadamia nut image, and icons were sourced from canva.com.

Peptides with xanthine oxidase (XOD)-inhibitory activities are potential dietary interventions for the treatment of hyperuricemia and gout. In this study, Macadamia integrifolia antimicrobial protein 2 (MiAMP2) was chosen for in silico proteolysis with pepsin, trypsin and chymotrypsin through ExPASy PeptideCutter, and the obtained peptides were further screened by water solubility, ADMET prediction and molecular docking. Four novel peptides, namely RPLY, PGPR, HGGR and GPY, were supposed to be non-toxic and have XOD-inhibitory potential. Molecular docking analysis showed that these peptides were able to bind to the active sites (such as Leu873, Phe914 and Thr1010) of XOD through interactions mainly including conventional hydrogen bond, alkyl and pi-interaction. These peptides were further synthesized to verify their in vitro XOD-inhibitory activities, and the IC50 values of PGPR, GPY and HGGR were shown to be 24.84 ± 0.02, 30.44 ± 0.33 and 24.89 ± 0.19 mM, respectively. Kinetic experiments demonstrated that PGPR and HGGR are mixed-type inhibitors, and GPY is a competitive inhibitor toward XOD with the inhibitory constant (Ki) values ranging from 1.09 to 8.98. Our results suggested that MiAMP2 is an important source of bioactive peptides for the management of hyperuricemia.