Challenges to optimal macadamia (Macadamia spp.) kernel quality in a changing climate

Nut development and kernel maturity

Macadamia fruit require around 7 months to reach maturity (Jones 1939; Jones and Shaw 1943) (see Fig. 1), but this may vary according to cultivar, orchard practices, and climate. Within a geographic location, climate related warming may result in an earlier spring and maturity as described in other perennial crops such as grapes (Vitis spp.) (Rogiers et al. 2022), peach (Prunus persica) (Vanalli et al. 2021), and apple (Malus spp.) (Kunz and Blanke 2022). Macadamia kernels are considered mature when the oil content is 60–75% of the nut’s weight and the moisture content is 20–30% (Quinlan et al. 2008). Flower initiation occurs in autumn and early winter, when minimum temperatures reach 11–15°C, with anthesis in late winter (Trueman 2013). The abscission of fertilised ovules shortly after anthesis is common. The self-management of crop load results in extensive nut abortion at 5–6 weeks after anthesis (Stephenson and Gallagher 1986a). However, both biotic and abiotic stresses can intensify spring ovule shedding. Water stress is known to result in significant abortion, thus irrigation may be beneficial during those stages (Stephenson et al. 2003). Physical forces, such as high wind, and temperatures above 30°C can also exacerbate abortion (Stephenson and Gallagher 1986a). Moreover, nuts may abscise prematurely because of pest and disease incursions.

Fig. 1.

Timeline of macadamia nut development illustrating accumulation of oil, alongside changes in kernel volume, water, sugars and protein. These changes coincide with fluxes in vegetative and root growth. The period of spring ovule shedding, premature and mature nut drop are also indicated.

Having survived the spring ovule shedding phase, the retained fruit enlarge for 3 months after anthesis until the shell hardens. At this time, oil synthesis is initiated and continues rapidly for the following 2 months. Subsequently, the composition of polar lipids and fatty acids alters slowly from 5 months through to full maturity at 7 months after flowering (Koaze et al. 2002a). Hexose sugars are the precursors for short chain saturated fatty acids, which are subsequently converted into long chain unsaturated fatty acids (Jones and Shaw 1943). Accordingly, kernel sugar content declines during the oil accumulation phase. Protein levels remain low relative to the other compositional parameters (Fig. 1). Premature fruit drop may occur during these last phases of development. Immature kernels tend to have higher sucrose and reducing sugars, leading to more internal browning (Wall and Gentry 2007; Kader 2013).

Despite a short and defined flowering stage, the achievement of maximum oil content is not necessarily correlated with the onset of fruit abscission, and mature nut drop can extend over several months within a cultivar (Trueman et al. 2000). A delay in nut drop may have ramifications for the moisture and texture of the kernel. Oil, moisture and texture are relevant to both the raw kernel and nut-in-shell market. In one study, kernel shelf life was shorter for nuts that abscised 4 months later than those mature nuts that had abscised earlier (Gama et al. 2020).

Kernel size and shape

While oil content, flavour, and texture can serve as indicators of kernel quality, size and shape (including kernel integrity) are typically the primary characteristics monitored to assess economic value (Quinlan et al. 2008). The macadamia fruit is single seeded and composed of the kernel (embryo and endosperm, both progeny tissues), a particularly hard shell (testa), and husk (pericarp) (Hardner et al. 2001). Only one of two ovules develop within the globose follicle, and once mature, a single suture in the husk will release the nut, composed of the shell surrounding the kernel. High-quality macadamia kernels are uniform in size, shape, and colour. Macadamia kernels generally remain intact during nut development; however, smaller nuts tend to have lower kernel recovery (ratio of kernel to shell) and whole-kernel incidence than large nuts (Richards et al. 2020). Growers are paid a premium not only on oil content, but also for kernel recovery as a percentage of nut-in-shell. Kernels can be damaged and segmented during development, as well as after harvest when they are subjected to excessive force through dehusking and shell removal, or high-pressure during roasting. For instance, shoulder damage may occur as a result of dropping or dehusking (Walton and Wallace 2010). Moreover, the shell can crack due to changes in temperature or moisture levels, which can cause the kernel to break into smaller pieces. To maximise economic value, it is crucial for growers to carefully manage both the physical integrity of the kernels and the conditions under which they are processed, ensuring minimal damage and optimal recovery.

Kernel discolouration

Internal and external discolouration is a significant challenge affecting macadamia nut quality (Le Lagadec 2009). It can be present at the time of harvest, or may develop during handling, processing, roasting and storage. It has been established that internal discolouration, also referred to as kernel browning, may be the result of three different processes: (1) discolouration due to fungal or bacterial infection; (2) enzymatic browning; or (3) non-enzymatic browning (Penter et al. 2007; Quinlan et al. 2008; Martinez et al. 2024).

Discolouration due to infection can be caused by a variety of microorganisms, including Aspergillus spp., Penicillium spp., and Fusarium spp. Kernel browning can occur on the orchard floor and through to handling and storage stages. Infrequent harvesting and lengthy periods on the ground may exacerbate mould and this type of discolouration (Quinlan et al. 2008). Low temperature, low humidity, and low moisture content during storage will prevent and slow the likelihood of microbial degradation and improve the shelf life (Gama et al. 2018).

Enzymatic browning is a significant industry issue and can appear as a distinct line around the equator of the kernel, or progress to staining of the entire basal portion of the kernel to a depth of 5–10 cell layers (Penter et al. 2007). This stain is a water-soluble phenolic compound and is derived from the shell (Du Preez 2015). Polyphenol oxidase (PPO) and peroxidase (POD) are the primary enzymes responsible for enzymatic browning. These enzymes are located in the cell walls and are released when the cell walls are disrupted, leading to the oxidation of phenolic compounds and the formation of brown pigments. Enzymatic browning can occur during harvesting, processing, and storage of the nuts. For instance, post-harvest dropping of nuts can result in cellular damage, some of which is not revealed until roasting (Walton and Wallace 2010).

Non-enzymatic browning may also occur when the macadamia nuts are exposed to high temperatures and moisture during processing (Srichamnong and Srzednicki 2015). This type of browning is caused by the Maillard reaction, which is a chemical reaction between amino acids and reducing sugars. Maillard derived browning was more prevalent in immature fruits with high sugar and low oil content (Koaze et al. 2002b). Martinez et al. (2023) found that kernels with brown centres had a higher moisture content, and kernel browning could be induced in the lab by storage under high moisture and high temperatures (Martinez et al. 2024). The authors hypothesised that these conditions trigger the hydrolysis of sucrose into the reducing sugars, forming substrates for the Maillard reaction. Their studies also demonstrated that white kernels and brown centre kernels had similar fatty acid profiles and peroxide values, and that browning was not the result of non-enzymatic oil oxidation (Martinez et al. 2023). Confocal, X-ray tomography and MRI studies of cv. Daddow, a cultivar less prone to browning, revealed a smoother internal structure compared to cv. 842, which had more fractures (Srichamnong et al. 2013). The authors hypothesised that the structure of cv. Daddow would facilitate unrestricted diffusion and therefore high drying rates. Moreover, cv. Daddow exhibited lower shell and kernel density, also potentially accelerating drying rates, and therefore limiting chemical reactions related to browning. Non-enzymatic browning may alter the flavour and aroma of the nuts. Nonanoic acid, octanoic acid, and 2,3 butanediol, resulting from lipid oxidation, were associated with the brown centres (Martinez et al. 2024). It is evident that proper kernel drying to a moisture content of 1.5%, and subsequent storage at low humidity are prerequisite to warding off browning of macadamia kernels.

Calcium (Ca) is important to cell wall and membrane stabilisation; however, the low transpiration rates of structures such as seed kernels may result in deficiencies. Ca deficiency has been associated with browning in hazelnuts, and it was observed that the application of Ca to the soil resulted in a significant reduction in discolouration (Cacka and Sanguankeo 2014). Similarly, pecan opalescence, a discolouration ranging from white to brown, was found to be associated with high levels of oil and low levels of Ca (Wakeling et al. 2002). Unlike these two crops, this nutrient does not appear to be associated with kernel browning in macadamias. Martinez et al. (2024) found the brown regions had higher nutrient concentrations, including Ca. The authors suggested that the subcellular locations and concentrations may be critical, therefore the role of calcium and other nutrients requires further analysis.

Off-flavours and rancidity

Further quality defects in macadamia include off-flavour development caused by oxidation and rancidity (Wall 2013). Whilst the unsaturated fatty acids are nutritionally valuable, they are also prone to oxidation and result in the production of hydrogen peroxides. Peroxide value (PV) quantifies the amount of peroxides (primarily hydroperoxides) in the oil. Both the monounsaturated and polyunsaturated fatty acids are susceptible to oxidation when exposed to oxygen, moisture, heat and light (Borompichaichartkul et al. 2009). The oxidation process produces a variety of volatile compounds, including aldehydes, ketones, and alcohols, which can create off-flavours and aromas in the nuts. Compared with roasted kernels, raw kernels were found to have high levels of polyphenol oxidase, peroxide value and fatty acids with low concentrations of phenols, antioxidants and poor aroma (Buthelezi et al. 2021). In addition, the oxidation of unsaturated fatty acids can lead to the formation of free radicals, which can further accelerate the oxidation process and cause rancidity in the nuts (Shahidi and John 2013). Tocols are antioxidants that have the potential to kerb the onset of chronic diseases and have been shown to contribute to walnut (Juglans regia) and hazelnut (Corylus spp.) stability (Savage et al. 1997, 1999). Cultivar comparisons demonstrated that Hawaii-grown HAES 294 and HAES 835 had superior kernel oxidative stability, however this could not be attributed to the bioactive constituents tocopherols, tocotrienols or squalene (Wall 2010). Therefore, kernel stability may depend on interactions between specific fatty acids and other types of phytochemicals with antioxidant properties.

Cultivars, clones, and rootstocks

Cultivars, clones, and rootstocks have a significant impact on kernel weight and quality of macadamia nuts (Hardner et al. 2001; Penter et al. 2008; Topp et al. 2019). Macadamia has a short cultivation history, and most cultivars are only two to four generations from the wild (Lin et al. 2022). There are about 20 cultivars of macadamia planted globally (Trueman et al. 2024) Most of the breeding has focused on traits such as tree size, yield, and kernel recovery; however, little is known about differences in kernel quality parameters (Wall 2010). Further research into cultivar differences in propensity to insect damage, kernel discolouration and flavour is warranted.

Kernel recovery is the percentage of nut that is recovered as edible kernel and leads to better payment rates for growers. A negative relationship was evident between shell thickness and kernel recovery in wild macadamia germplasm (Mai et al. 2021). Cultivars also differ in shell hardness and thickness, and this impacts on the ease of cracking and drying rate. However, recent research has shown that pest control, particularly for Fruit Spotting Bug (FSB), is more effective with thicker shell and husk cultivars. If the combined shell and husk thickness exceeds 10 mm, FSB damage is significantly reduced, as the proboscis of the FSB is around 10 mm long. The cultivars A203, A268, A403 have a thick husk and shell and is reported to have less issues with ‘late insect’ damage (Bright and Alt 2024). Therefore, the trend towards higher kernel recovery may become less critical if the industry recognises the benefits of lower kernel recovery and reduced need for pest control measures.

Cultivar differences may play a significant role in resilience to climate change by providing various adaptations that help trees better cope with changing environmental conditions. The challenge lies in identifying the desirable traits and performance factors suited to the specific orchard situation and the potential climate influences on that particular site. Factors such tree shape, canopy density, length of flowering duration, compatibility with cross-pollinated cultivars, drop pattern of mature nut, and germination risk are some important considerations in the future that should be reviewed to ensure that the selected cultivar is well-suited to the specific orchard situation. For instance, dense canopies (344, 800 etc.) are difficult to penetrate with sprays and therefore may be more conducive to disease. Additionally dense canopies may lead to soil erosion due to low light levels on the orchard floor, especially in high rainfall areas. Cultivars that have a tendency for prolonged (849) or multiple nut drops may have reduced quality due to germination or kernel discolouration. In contrast, cultivars that have tendency for premature nut drop in hotter climates (741) will have lower oil content. Nuts of some cultivars (Beaumont) are prone to germination on the tree in wet conditions, reducing yield. Additionally, as described by Bright and Alt (2024), there are cultivars more susceptible to wind damage (A203, A268, 246 etc.), drier climates (cvs 660, A38 etc.), heat stress (cv. 344) abnormal vertical growth (cvs 344, 660, 741 etc.) and diseases such as Phytophthora (cvs H2, 816, 842, etc.), flower blight (849), Botryosphaeria (cvs 344, A268 etc.), and husk spot (Pseudocercospera macadamiae) (cvs 741, 816, A16, A4 etc.). This is useful information when planning to develop or maintain orchard into the future where climate variability is a certainty.

Climate-related stresses can lower productivity and rootstocks that provide better resistance to drought, heat, pests and disease will ensure farmers that their trees will remain productive. Beaumont is a common rootstock used in Australia but has a high-water requirement and suffers in drought (Bright and Alt 2024). H2 has a uniform and thick stem for grafting but is susceptible to phytophthora. Disease resilient rootstocks targeted for drought or flood prone regions will help macadamias thrive in these unpredictable climates. In addition, the development of vigour-reducing rootstocks to alleviate within tree competition for resources will provide new opportunities to improve orchard productivity and kernel quality. As evidenced in other crops such as apples, pears and stone fruits, high density orchards can increase yield per ha, are more efficient in water and nutrient use, and may result in improved quality due to better light penetration and pest and disease management with the proper plant architecture. Labour costs may also decline. Despite these potential benefits, research into specialised rootstocks is lacking.

Pollination

Macadamia trees can produce more than 3000 racemes, each bearing 100–300 flowers; however, like other Proteaceae species, fruit set is generally low (Trueman et al. 2022a). Once the flower has opened, the stigma is receptive for only 2 days (Sedgley 1983). Approximately 5% of flowers undergo fruit set, of which 1–2% become mature nuts (Trueman et al. 2022a, 2022b). The flowers are bisexual, but they are partially self-incompatible (Trueman 2013). Self-fertilisation is curtailed through impeded pollen tube growth (Sedgley 1983). Self-paternity was low at 0–3% in cvs Daddow, 816, and A4 (Richards et al. 2020). Similarly, in a varietal trial plot located in a large commercial orchard, self-pollination was not detected in cvs 246, 344, 800, A4, and A16, but ranged 20–40% in cvs 741, 791, 660, and 842 (Langdon et al. 2019). Cross pollination has positive effects on fruit set and nut retention (Wallace et al. 1996; Howlett et al. 2019) and may improve nut in shell (NIS) weight, kernel weight and kernel recovery (Trueman and Turnbull 1994; Wallace et al. 1996; Du Preez 2015; Trueman et al. 2024). A large study of 19 cultivars across 23 sites in Australia, Brazil, and South Africa revealed that 80–100% of fruit were cross pollinated, intermediate outcrossing (41–72%) was present in five sites, and low outcrossing (10%) was present in only one isolated cultivar at one site (Trueman et al. 2024). Self-pollinated fruitlets commonly abscise from the tree around 6 weeks after flowering (Trueman et al. 2022a). Macadamia’s pollen limitation was evidenced by a 63% increase in fruit set through supplemental pollen addition (Knight et al. 2006). Additionally, poor pollen quality (Van Etten 2022) and environmental conditions such as high or low temperature, low humidity and rain (Langdon et al. 2019) may play a role in abortion rates.

In Australia, the flowering racemes are pollinated by a wide variety of species including European (Apis mellifera) and native stingless bees (Tetragonula carbonaria) (Trueman et al. 2024). A study in Brazil found that butterflies were responsible for 50% of floral visits and that final fruit set was improved, but not kernel weight (Santos et al. 2020). In the three-country study (Trueman et al. 2024), outcrossed fruit were higher in oil concentration by 3.0 to 3.5% than fruit fertilised by pollen of the same cultivar. Additionally, a recent study found that both the seed parent and the pollen parent influence the fatty acid profile of the kernel (Hu et al. 2023). Therefore, outcrossing not only influences nut number, but also nut quality.

The extent of cross pollination in macadamia is important to orchard design (Anders et al. 2023) with the consideration of incorporating pollinating genotypes that bloom at the same phenological time in close proximity. With fruiting genotypes, stingless bee numbers within the orchard are dependent on the distance from the hive, and honeybee numbers are dependent on tree flower number (Evans et al. 2021). Therefore the physical distribution of hives, and the deployment of managed bee colonies at the correct phenological stage, can have a strong influence on pollination. Shifting climate patterns are putting pressure on particular pollinator species (Classen et al. 2015). For instance, honeybees are sensitive to extreme weather, resulting in reduced forage and increased disease, including parasites and pathogens (Ali et al. 2023). Pollinator species are not immune from the loss in biodiversity that is occurring at unprecedented rates because of habitat fragmentation. Considering that macadamia is pollinated by several species, these environmental shifts will have implications for future orchard design. Pollinator loss can be tackled, for instance, by incorporating diverse groundcovers, shrubs and trees in within or adjacent to the orchard to encourage, bees, flies and beetles. Reducing pesticide use, providing water sources and shelter for insects through deadwood and tree hollows will also encourage the presence of diverse pollinators.

Carbon allocation

Photosynthates are the primary source of energy for the tree, and their distribution and allocation to different carbon sinks can thus affect overall productivity (Spann et al. 2008). Temperature, light and water stress (too much or too little) inhibit photosynthesis, and consequently the accumulation and allocation of carbon (C). Notably, even under optimal conditions, macadamia is characterised by low rates of C assimilation relative to other perennial tree crops because of both stomatal and non-stomatal limitations (Smit et al. 2020). Light saturated net CO₂ assimilation rate (A_max) was 8.3 μmol CO₂ m⁻² s⁻¹ relative to other crops such as almond, pecan and pear which have values greater than 15 μmol CO₂ m⁻² s⁻¹ (Smit et al. 2020). However, A_max was 56% higher during the fruit bearing stage of the season despite greater evaporative demands (Smit et al. 2020). This is likely because nut development requires a significant amount of energy (Stephenson et al. 1989a) for building complex energy compounds such as fatty acids and proteins. C movement is dynamic, with two-way interactions between storage in roots and stems, and mobilisation to service cell maintenance, growing points and reproductive development (Auzmendi and Hanan 2020). Tree non-structural carbohydrates are highest prior to flowering and then decline during this phenological phase (Scholefield et al. 1985). Likewise, fruit set is at least partially dependent on stored carbohydrates (Scholefield et al. 1985). Fruitlet abscission after mass flowering occurs early in fruit development, possibly because of a direct resource limitation, or as an adaptation to adjust crop load in line with tree resources (Trueman et al. 2022a).

Competition for C between fruit, vegetative development, and below ground biomass can have a significant influence on yield, even in evergreen canopies such as macadamia. Vegetative flushes can occur several times per year, with peaks in spring and later summer (Fig. 1). However, if new growth is excessive, energy stores for reproductive growth are compromised (McFadyen et al. 2011). For example, excessive nitrogen (N) application prolonged vegetative flushing and decreased first grade kernels by 8–10% (Stephenson et al. 2002). Likewise, new shoot growth, stimulated by pruning, increased fruit abscission and was associated with reduced stem carbohydrates (McFadyen et al. 2011). The timing of pruning can be critical to flowering and yield (Wilkie et al. 2009). However, if the plant’s carbohydrate status is adequate to sustain a high crop load, this may lead to higher kernel recovery (Wilkie 2009) and low kernel discolouration (Du Preez 2015). Herbert et al. (2019) found that branch girdling (cincturing), a technique to prevent carbohydrates from leaving or entering the branch through the removal of phloem, increased fruit set and yield. Similarly, assimilates were redirected from vegetative growth to priority sinks such as fruit development as evidenced by trunk girdling at anthesis, which reduced shoot growth without compromising yield (McFadyen et al. 2013a).

A better understanding of carbon allocation in response to environmental changes will facilitate the development of appropriate agronomic practices to maximise yield and quality (Fig. 2). The fluxes between above and belowground biomass pools, metabolic sinks such as respiration and root activity, as well as primary and secondary metabolites are complex and alter according to the stage of the reproductive cycle. The allocation of C for long-term storage in the woody tissues of large trees like macadamias that live for decades may allow them to withstand future stresses such as drought, when net C assimilation is low as a result of stomatal closure (Hartmann et al. 2020). The interactions between roots and the soil, from which they draw nutrients and water, are also likely to alter under various climate related stresses. For instance, rising temperature is decreasing the soil organic carbon stock through microbial stimulation and greater CO₂ efflux, with ramifications for the chemical, physical and biological properties of the soil (Elbasiouny et al. 2022). Given the quantity and diversity of C molecules released from plant roots, in exchange for other resources such as nutrients (Selosse and Rousset 2011), fluxes in the population dynamics of root-associated microorganisms will likely have an impact on plant productivity. Therefore, the conservation of this C stock is an important component of sustainable land management.

Fig. 2.

Carbon competition between nut development and other sinks within the tree. Climate pressures such as cloud, heavy rain and the associated nutrient run-off, flooding, waterlogging, heat and water stress impact on how the non-structural carbohydrates (NSC) are allocated to the various growing and metabolically demanding sinks such as respiration, root nutrient and water uptake functions, and the formation of symbiotic relationships in the rhizosphere. NSCs may be stored and then mobilised to fuel areas of high demand when photosynthates are inadequate. Flower, ovule, and leaf abscission, as well as root senescence, represent a loss of plant C but can be incorporated into the soil as organic matter.

Solar radiation

Optimal light quality and duration are essential for producing high-quality nuts and maximising yield. This is partially because high light intensities are critical for optimal photosynthesis (Huett 2004), especially over the whole tree canopy. For example, cloudy conditions delay kernel filling and hence results in fruit quality concerns. Yield increases occur as canopy light interception increases (Huett 2004); notably higher yields occur in the upper part of the canopy. Low light incidence in the lower canopy generally supports new vegetative growth as opposed to reproductive growth (McFadyen et al. 2011), exacerbating reduced light penetration in the inner canopy. Ultimately yield per ha was compromised in orchard crowding (tree volumes >43,500 m³ ha⁻¹ and light interception <94%); however, percentage kernel recovery, unsound kernel, or grade 1 kernel were not affected (McFadyen et al. 2004). Changes in light quality may be significant to changes in kernel development and yield. It has been demonstrated (Bastías and Corelli-Grappadelli 2012) that a low red-to-far-red (R/FR) ratio, such as occurs in the lowest canopy, can cause peach trees to elongate and become tall and spindly, resulting in reduced yield and quality.

The manipulation of light interception within the canopy, through various canopy management strategies can have an enormous effect on metabolic processes and whole plant physiology including water and carbon relations. The control of crop load and shading can influence kernel output, nut size, and quality. Hedging, limb removal, limb rejuvenation, skirting, row removal, stumping, topping or heading are practices that may be implemented at different stages of production to improve or stabilise yield and quality. These practices are labour and resource intensive and may result in temporary loss of production. An economic analysis is required to better understand the trade-offs between immediate costs and long-term gains in productivity. This analysis should also cover the trade-offs between intensive one-off practices such as row removal with slower incremental changes such as sequential limb removal over many seasons.

Heatwaves and drought

Climate change has resulted in temperature variability and the timing and quantity of precipitation (Hughes 2011). Macadamias originate from a sub-tropical climate, with mild winters and warm summers, and rainfall (1200–2000 mm) in distinct wet and dry seasons. High and low orchard temperatures are amongst the most significant environmental factors affecting macadamia kernel development. The minimum mean temperature for vegetative growth is approximately 12°C (Trochoulias and Lahav 1983), while temperatures above 30°C may be detrimental (Carr 2013; Shabalala et al. 2022). Macadamia’s native range extends south into the Northern Rivers of NSW, an important macadamia growing region. Here summer daytime temperatures typically reach 25–30°C, while winter night-time temperatures fall to 8–12°C. Maximum photosynthesis occurs at 16–25°C (Allan and de Jager 1979) and is severely curtailed at temperatures above 40°C (Allan and de Jager 1979). High temperatures during nut development can lead to reduced kernel weight, diminished kernel quality, and extensive kernel browning. A controlled-environment glasshouse study, exposing trees to a range of temperatures between 15 and 35°C, demonstrated that 25°C resulted in optimal oil content and kernel recovery, whereas these parameters were adversely affected at 35°C (Stephenson and Gallagher 1986b). Limited energy for nut development because of sub-optimal photosynthesis at this high temperature likely retarded oil accumulation. Moreover, the breakdown of respiratory substrates, important for oil accumulation, is increased during high temperatures (Stephenson and Gallagher 1986b).

Macadamias can withstand dry conditions to some extent (Smit et al. 2020). This tolerance can partially be attributed to their sclerophyllous leaves that are xeromorphic with sclerified bundle sheath tissues, and stomata that are limited to the abaxial surface (Stephenson et al. 1989b, 2003) restricting water loss from leaves. Stomatal conductance declines in response to dry soils and to increasing evaporative demand (Lloyd et al. 1991) so that the plant remains isohydric, at least during the non-fruiting stages (Smit et al. 2020). Because grafted macadamia trees have a relatively shallow root system with a short taproot and most fibrous roots in the upper 0.40 m of soil (Firth et al. 2003; Carr 2013), water stress can initiate quickly. However, macadamia’s unique cluster roots (formerly known as proteoid roots) may promote water uptake during dry periods (Stephenson and Trochoulias 1994), which typically occur after harvest. The cluster roots are formed near the soil surface and consist of many fine rootlets clustered together in a small area, making it highly efficient in absorbing pockets of water and nutrients. This root architecture is especially valuable to macadamias in their native environment which often experiences seasonal fluctuations in rainfall. However, water deficit during the early stage of nut development (predawn leaf water potential of −2.0 MPa) can lead to reduced fruit set and greater nut drop (Carr 2013), which ultimately decreases nut yield (Mayer et al. 2006). Stress during the kernel-filling stage can result in a lower kernel-to-nut ratio, smaller kernel size, more discoloured and shrivelled kernels and overall lower quality (Stephenson et al. 2003; Shabalala et al. 2022). Irrigation is thus an indispensable practice in water-deficient areas, as it maintains adequate water and nutrient availability, resulting in larger nuts with higher quality (Stephenson et al. 2003; Shabalala et al. 2022).

Disease and pests occurring under dry conditions may be alleviated by adequate irrigation, but saturated soil can exacerbate diseases such as Phytophthora (Dron et al. 2022) and promote insect damage (Bouarakia et al. 2023). There is a need for research on irrigation techniques to optimize water use efficiency, reduce water usage and minimise environmental impact. The development of accurate crop and plant sensors, smart irrigation controllers for automated irrigation schedules, as well as remote monitoring, will enhance efficiencies, along with micro-irrigation techniques and renewables-based irrigation systems to promote sustainability. Further research on cultivar differences in water-use efficiency is also required to improve macadamia sustainability. Crop coefficients to estimate water use, in combination with technology such as soil moisture probes, sap flow sensors and dendrometers, may offer cost effective grower solutions to ensure optimum plant water status is managed. Information on hydrological conditions such as infiltration and run-off are also important to achieving sustainable production.

To enhance plant resilience in challenging environmental conditions, various protective solutions are employed to mitigate water loss and stress. Antitranspirants, sunscreens, and film-forming barriers provide a thin protective coating over the canopy, preserving water and reducing evapotranspiration rate. These coatings can potentially decrease sunburn, a key driver of oxidative stress (Munné-Bosch and Vincent 2019). For example, kaolin clay had positive effects on kernel recovery in 6-year-old trees (Du Preez 2015); however, the impact of adverse environmental conditions must be considered and balanced with the potential negative impacts on photosynthesis. Sometimes these negative side effects are minimal or short lived (Rosati et al. 2007).

Waterlogging

Waterlogging, resulting from heavy rainfall or overirrigation, occurs when soils become saturated with water, which results in hypoxia in the soil restricting root respiration (Nguyen et al. 2018). Waterlogging is known to reduce the yield of macadamia nuts and diminishes the quality of the kernel. Trochoulias and Johns (1992) found a negative relationship between individual nut weight and irrigation amount. Reproductive development is hampered because waterlogging reduces nutrient uptake (Salvatierra et al. 2020), increases disease susceptibility, and constrains the growth and development of the tree, including the nuts from reduced carbon supply. Root growth may slow and there may even be root decay. In partial submergence the upper leaves of the plants may be retained so that photosynthesis is maintained. However, over the total canopy, decreased photosynthesis results in the reliance and depletion of starch reserves (Olesen et al. 2008) and contributes to plant death, when these stores are exhausted.

Given the general trend for changes in rainfall quantities and timing (Dey et al. 2019), site selection and preparation for flooding is central to the success of an orchard. The consideration of slope during site selection is critical to orchard productivity (Bright and Alt 2022). A mild slope can improve drainage but during excessive rainfall, soil erosion may occur. In contrast, low-lying flat land, without mounding or ridging and susceptible to flooding, will result in root disease and necrosis, and thus decreased overall productivity (Drenth et al. 2009). Hence, drainage management is critical after heavy rains because it helps maintain the topsoil whilst preventing waterlogging, and maintains machinery access such as mowers, slashers and harvesters (Bright and Alt 2022). These drainage considerations are best implemented prior to orchard planting.

Soil properties and nutrition

Macadamia prefers a loam or sandy-loam soil that is slightly acidic with a pHCa of 6.0 (Bright and Alt 2022). Zuza et al. (2023) found that if the soil pH in macadamia orchards is too low (<5.5), the availability of essential macronutrients (Ca²⁺, K⁺, P, and N) declines, leading to decreased kernel size, weight and sub-optimal quality (Bright and Alt 2022). Among the critical micronutrients for nut set and quality are boron (B) and zinc (Zn) (Stephenson et al. 1986; Nagao et al. 1992). A recent study by De Silva et al. (2022) showed that while B can improve initial fruit set in cv. 816, it did not significantly influence yield or nut quality. Other cultivars were not tested. There is a need for research on critical nutrient thresholds, timing, form, movement and methods of nutrient application to optimize tree growth and yield while maintaining soil health and reducing nutrient loss from the orchard. Information on the interactions between the nutrients (e.g. cation antagonism) and nutrient dynamics in different soil types is limited. Once inside the plant, the specific macro or micronutrient will have an important influence on whole plant and nut physiology. In many agricultural industries, rapid, non-destructive tools for assessing plant nutrient status are available. For example, hyperspectral imaging of leaves has already shown potential for rapid nutrient assessment and requires further development for better accuracy and for a greater range of nutrients (De Silva et al. 2023).

Macadamia trees form cluster roots, which are particularly efficient in phosphorus (P) uptake, especially when the plant is P-deficient (Aitken et al. 1992; Hue 2009; Zhao et al. 2019, 2021). The role of cluster roots in the uptake of other nutrients, including N, requires investigation. Excessive N during oil production may lead to excessive vegetative growth, which competes with nut development (Stephenson et al. 2002). All four macadamia species are also high manganese (Mn) accumulators (Abubakari et al. 2022; Zhao et al. 2023), likely due to the exudates from cluster roots that not only solubilise P but also several micronutrients (Lambers et al. 2015). Conversely, Moodley et al. (2007) found that the uptake of metals and their distribution to the nuts were largely controlled by the plant’s inherent mechanisms and were low for toxic metals. Further investigation into how the presence of cluster roots alters soil chemistry and microbial activity will enhance the ability of macadamias to thrive in nutrient-poor soils, reducing the need for synthetic fertilisers.

A well-structured soil with good porosity is vital for proper root development and nutrient uptake, both of which are essential for healthy nut growth. Van Zwieten et al. (2003) observed that older macadamia orchards in NSW exhibited signs of soil compaction, erosion, and reduced organic carbon, especially in conventional farms, whereas these issues were less prominent in organic orchards. Practices like mulching and incorporating compost can enhance soil porosity and fertility. For instance, the application of poultry litter biochar in macadamia orchards has been shown to increase soil fertility, likely due to its slow-release fertiliser properties (Hosseini Bai et al. 2015). Assessments of different mulch and compost types, including the effects of temperature and moisture on decomposition rates and nutrient release rates, will help optimise their use. Likewise, carbon sequestration through mulching and composting requires further exploration. Moreover, research on the cost-effectiveness and scalability of specific soil amendments are required, especially in established orchards as previously alluded to.

Effective orchard floor management plays a key role in preventing soil erosion. One particular challenge in macadamia orchards is stem flow, which is the movement of water along the outside of the tree trunk that accumulates and contributes significantly to soil erosion at the tree base (Levia and Germer 2015). This is particularly challening during rainfall. To mitigate erosion, light hedging can promote light penetration to the orchard floor, encouraging the growth of ground cover and reducing soil loss (McFadyen et al. 2013b). In older orchards with low light conditions (Firth et al. 2002), pasture species are suitable for ground cover. However, from a farmer’s perspective, the orchard floor must also support efficient nut harvesting. Large mechanical harvesters rely on sweeping nuts into bins, a process that can be hindered by excessive vegetation and debris. Finding the right balance between orchard floor conditions and mechanical harvesting may be site specific and require experimentation and adjustments through trial and error.

Pest and disease pressure

Macadamia nut pests and diseases can significantly diminish the quality of the kernel. Ellis et al. (2023) have emphasised increased interest in integrated pest and disease control in the macadamia industry. However, shifts in frequency and intensity of precipitation, as well as temperature, may alter the dynamics between pests and their natural predators, compromising integrated pest management. Furthermore, the direct and indirect effects of climate change drive changes in the temporal and spatial distribution of insects and microbes (Skendžić et al. 2021). Temperature is especially important to population dynamics (Zvereva and Kozlov 2006) because it affects the metabolic rates, development, and reproduction of both insects and microbes. For instance, many insect species have specific temperature thresholds for development, and as temperatures rise, these species may experience accelerated growth rates, shorter life cycles, and increased reproductive output. Additionally, other climatic effects such as water and heat stress reduce the plant’s resilience so that they are more susceptible to pests and diseases. Examples include the fruit spotting bug and the banana-spotting bug (Amblypelta spp.), the foremost cause of nut rejection at the factory (Bright 2019). They feed on the kernel tissue from pea size until harvest, causing it to become discoloured, misshapen and translucent. The fruit spotting bug and the macadamia nut borer (Cryptophlebia spp.), another pest of great concern in the industry (Smith et al. 2022), can cause nuts to fall prematurely. Amblypelta spp. pierce young nuts and these drop readily, whereas older pierced nuts tend to stay on the tree (Ellis et al. 2023). The green vegetable bug (Nezara viridula) causes similar damage to the kernel but there are no external symptoms (Bright 2019). Adults inject digestive fluids into the kernel, causing entry points for pathogens and contributing to rancidity (Jamieson et al. 2004). Diversifying understory plants in macadamia orchards provides habitats for beneficial insects, which can help control pests like the felted coccid (Eriococcus ironsidei) (Gutierrez-Coarite et al. 2018). Reduced mowing, combined with the removal of unwanted weeds, has also been shown to increase non-pest arthropod numbers in orchards (Bright 2019), thereby promoting natural pest control.

Fungal diseases are common following wet weather and are more likely in dense, shaded canopies, as well as the upper zone of tall trees outside of spray coverage. Disease incidence can lead to kernel necrosis or discolouration, diminishing both kernel weight and quality (Doster and Michailides 1999). Husk spot can cause a significant increase in immature rejects (Nunn et al. 2023). Phytophthora root rot can reduce kernel quality by causing the tree to produce smaller, lower-quality nuts (Hunter et al. 1971). Husk spot results in immature abscission and therefore reduces yields (Akinsanmi et al. 2008). Frequent monitoring of weather, orchard conditions and pest presence will be required, alongside predictive modelling to prepare for incursions. An environmentally conscious integrated pest and disease management program that is responsive to weather variability will reduce the risk of significant economic losses, when the necessary research is completed to facilitate such a program.

Effects of harvest delays

Aside from the phenology of crop growth, climate change has an impact on the entire food system, including harvesting, post-harvest handling, and storage. If not managed appropriately, these factors contribute to wastage through losses in quality and spoilage. For instance, unseasonal heavy rains can hamper orchard accessibility by farm equipment such as harvesters and can contribute to a delayed harvest. Prolonged exposure on the orchard floor will result in sun damage, fungal diseases, viviparous germination, or other physiological changes within the nut. Viviparous germination is more likely to occur during sunlight exposure and warm temperatures (Quinlan et al. 2008). Enzymatic browning, leading to higher levels of discolouration, is the result of movement of tannin from the shell (Penter et al. 2007; Walton and Wallace 2009). Apparently, the more water is available in the nut, the higher the rate of this enzymatic browning (Borompichaichartkul et al. 2009). It is unclear if this a direct effect of rain or relative humidity, or how wet the nuts are before going into storage. It was noted that frequent harvesting (less than every 4 weeks) improves sound kernel recovery (Liang et al. 1996; Quinlan et al. 2008). Once harvested, it is critical that the husk is removed from the rest of the nut within 24 h of harvest. Its presence initiates the translocation of sugars in preparation for germination and will result in the formation of a dark halo around the circumference of the nut (Mereles et al. 2022).

Moisture content and storage conditions

During storage, nuts with high moisture content are more likely to undergo kernel browning, possibly because of higher metabolic activity (Walton et al. 2013). Silo size and shape can have an influence on rates of drying. For example, nuts stored in smaller silos (less than 20 tonnes) had reduced unsound kernel recovery relative to larger silos (Quinlan et al. 2008). Depth of nuts within the silo as well as efficiency of air flow through the bed of nuts is also important. Nuts are typically harvested with a 20–25% moisture content, and rapid drying to 10% moisture is recommended prior to transport for processing to alleviate enzyme activity, chemical reactions or microbial activity (Wall and Gentry 2007; Walton and Wallace 2010). Nuts are further dried down to 3.5% moisture content before the kernel is removed from the shell (Duduzile Buthelezi et al. 2019). By contrast, nuts that are stored in high humidity or moisture conditions can cause the nut to stick to the shell, resulting in a lower kernel recovery rate. High temperature storage can lead to an increase in free fatty acids, resulting in the loss of vitamin E and other antioxidants that are important for retaining the quality of macadamia nuts. Vitamin E is known for its antioxidant properties and plays a crucial role in inhibiting lipid oxidation in nuts. Some research has found that storage temperatures higher than 25°C can lead to a reduction in vitamin E content in macadamia nuts (Cavaletto et al. 1966). Temperature fluctuations during storage can also affect the quality of macadamia kernels. Exposure to fluctuating temperatures can cause moisture migration within the nut, which can lead to quality deterioration such as rancidity and discolouration. Therefore, macadamia processors should store kernels at a constant low temperature (around 4°C) to preserve their quality and freshness.

Roasting and its impact on nut quality

Despite disagreement by processors on the shelf life of raw versus roasted nuts, roasting of kernels results in higher flavonoid, phenols and antioxidant activity and can improve the sensory quality and shelf life of macadamia (Duduzile Buthelezi et al. 2019). Roasting results in crisper kernels (Tu et al. 2021); however, over-roasting can cause the kernels to become dry and brittle, whereas under-roasting can result in a softer, chewier texture (Aruwajoye et al. 2023). Browning during roasting may be a consequence of the Maillard reaction resulting from high reducing sugar and amino acid content of the kernel (Srichamnong and Srzednicki 2015). Roasting may thus result in brown centres if the nuts are not fully mature and still high in sugar content (Wall and Gentry 2007). The high sugar content, combined with differing temperature and moisture between inner and outer parts of the kernel, may result in brown centres (Wall and Gentry 2007). After-roast darkening may occur because of enzymatic changes (cell wall invertase) in sugar composition driven by a loss in membrane integrity through high incubation temperatures (Albertson et al. 2006). Kernels susceptible to after-roast darkening were found to contain higher glucose and fructose concentrations relative to cream-coloured kernels. The processing of immature kernels with high sugar levels should thus be avoided, and sugar assessments at harvest may be warranted.

Advanced methods for assessing nut quality

Traditional wet chemistry methods to assess nut quality can be costly and time consuming and may require expensive laboratory equipment and the associated expertise. Accurate, simple and cost-effective technologies targeted at kernel discolouration, lipid and moisture content aids in the detection of sub-standard nuts. The Australian industry predominantly uses colour (RGB) sorters to assess nut defects but is currently also accepting X-ray technology for sorting nuts in shell at both farm and processor levels. The development of AI in sorting and defect identification has progressed rapidly and is often used in conjunction with a range of spectroscopic methods. For instance, nut internal defects can be detected with Near InfraRed (NIR) spectroscopy (Guthrie et al. 2004) and chemometric modelling (Rahman et al. 2021). Furthermore, nuclear magnetic resonance (NMR) was also useful for detecting external defects in unshelled macadamia kernels (Carvalho et al. 2019); however, high cost and complexity of data interpretation are major drawbacks. Meanwhile, hyperspectral imaging and machine learning was used to detect kernel moisture content, with the advantages in that it is simpler, non-destructive, and faster than the oven-drying technique during post-harvest processing (Farrar et al. 2024). However, some of these promising techniques require further development before they can be used at the commercial scale.