South Africa’s thriving macadamia industry faces a growing challenge: water scarcity. As droughts intensify due to climate change, farmers are under pressure to adopt sustainable practices.
A groundbreaking 2025 study published in the South African Journal of Botany reveals how specific macadamia cultivars use unique leaf structures to thrive in dry conditions.
Understanding the Role of Leaf Anatomy in Drought Survival
Plants survive droughts through two main strategies: avoiding water loss and tolerating dry conditions. For macadamia trees, leaf anatomy plays a critical role in both strategies.
Firstly, stomatal density—the number of tiny pores on a leaf’s surface—determines how efficiently a plant balances carbon dioxide intake and water loss.
Macadamia leaves naturally have stomata only on their undersides, a trait that reduces evaporation.
However, cultivars like ‘814’ and ‘695’ take this further with higher stomatal density, enabling better photosynthesis without excessive water loss.
Secondly, mesophyll thickness—the inner tissue layers of a leaf—affects how well a plant uses sunlight and carbon dioxide.
The palisade layer, packed with chloroplasts, captures light, while the spongy layer allows gases to move freely. Thicker mesophyll, as seen in cultivar ‘814’, provides more space for photosynthesis, a key advantage during droughts.
Thirdly, vein density—the network of tiny veins in a leaf—ensures water reaches every cell quickly. Cultivar ‘695’ excels here, with dense veins that shorten water’s travel path, making it ideal for arid regions.
Finally, the petiole Huber value—a measure of water supply capacity in the leaf stem—determines how well a plant keeps its leaves hydrated. Cultivar ‘814’ leads here, with robust petioles that act like water highways during dry spells.
How the Study Was Conducted: Regions, Cultivars, and Methods
Researchers from the University of Pretoria analyzed four macadamia cultivars (‘695’, ‘814’, ‘816’, and ‘A4’) across three South African regions: Komatipoort (hot and dry), Nelspruit (moderate), and KwaZulu–Natal (KZN) South Coast (cool and humid).
Leaves were collected during spring and summer to account for seasonal changes. To measure stomatal density, researchers used nail polish to create imprints of leaf undersides, counting pores under a microscope.
For mesophyll thickness, cross-sections of leaves were examined to compare palisade and spongy layers. Vein density was calculated by clearing leaves with chemicals to expose veins, which were then measured for length.
The petiole Huber value was determined by comparing the water-carrying area of leaf stems to leaf size. Advanced statistical tools, including PCA and PERMANOVA, helped identify how climate and genetics influenced these traits.
Key Findings: Traits That Define Drought-Resistant Cultivars
The study revealed significant differences in leaf anatomy across cultivars and regions. For example, stomatal density was highest in ‘814’ (500 stomata/mm²) and ‘695’ (450/mm²), particularly in humid KZN.
This suggests these cultivars optimize carbon intake in milder climates. In contrast, ‘A4’ and ‘816’ had lower densities (300–320/mm²), likely to conserve water in drier areas like Komatipoort.
Surprisingly, stomatal size increased slightly with density—a rare trait that hints at unique adaptations in macadamias. When it came to mesophyll thickness, ‘814’ consistently outperformed others.
Its palisade layer measured 250 µm in Nelspruit, compared to 180 µm in ‘A4’. Similarly, its spongy layer (150 µm) was thicker than ‘816’s 100 µm. These differences allow ‘814’ to maintain photosynthesis even with limited water.
Vein density was another standout trait. ‘695’ had the densest veins (0.028 mm/mm²) in Komatipoort, ensuring rapid water delivery under heat stress.
Meanwhile, ‘A4’ had the lowest density (0.015 mm/mm²), making it less suited for arid zones. Finally, petiole Huber values highlighted ‘814’s superiority, with values 3x higher than ‘A4’. This means ‘814’ can supply more water to its leaves, a lifesaver during droughts.
Climate and Genetics: How Environment Shapes Anatomy
The study found that both climate and genetics influence leaf traits. For instance, hot, dry regions like Komatipoort promoted thicker mesophyll and higher stomatal density, likely as adaptations to heat.
In contrast, humid KZN saw lower stomatal density due to reduced evaporation pressure. Seasonal changes also mattered: spring leaves had thinner mesophyll but higher vein density compared to summer leaves.
However, genetics played a consistent role. Cultivar ‘814’ maintained thick mesophyll across all regions, while ‘695’ retained high vein density regardless of climate.
This stability suggests breeders can develop drought-tolerant macadamia cultivars by selecting these traits early.
Practical Tips for Farmers: Selecting and Managing Cultivars
For farmers in dry regions like Komatipoort, ‘814’ and ‘695’ are top choices. Their thick mesophyll and dense veins ensure productivity under water stress. In contrast, ‘A4’ and ‘816’ perform better in humid KZN but require careful irrigation in droughts.
Irrigation strategies should adjust with seasons. Summer flushes, for example, need more water for cultivars like ‘A4’ with low Huber values.
Shade leaves, which have 20% fewer stomata than sun leaves, also require less water—a detail farmers can use to optimize canopy management.
Breeders, meanwhile, should focus on traits like high vein density and Huber values. Mapping genes linked to these traits could accelerate the development of water-efficient macadamia cultivars, reducing reliance on irrigation.
Limitations and Future Research Directions
While the study offers valuable insights, gaps remain. For instance, it didn’t measure photosynthesis rates or stomatal conductance, leaving questions about how anatomy translates to real-world performance. Long-term studies are also needed to see if these traits persist as trees age.
Future research should test South Africa’s 20+ commercial cultivars and integrate physiological data. This would provide a fuller picture of drought tolerance, helping farmers and breeders make informed decisions.
Conclusion: Building a Drought-Resistant Future for Macadamia Farming
This study highlights how drought-tolerant macadamia cultivars use leaf anatomy to combat water scarcity. By choosing cultivars like ‘814’ and ‘695’, farmers can reduce irrigation needs while maintaining yields.
For researchers, the next step is linking anatomy to genetics and physiology, paving the way for smarter breeding. As climate change reshapes agriculture, such innovations are vital.
By understanding nature’s designs, South Africa can protect its macadamia industry—and ensure its nuts remain a global favorite for decades to come.
Power Terms
Stomatal Density: The number of stomata (tiny pores) per unit area on a leaf surface. Stomata allow plants to take in carbon dioxide for photosynthesis and release oxygen and water vapor. Higher stomatal density can improve gas exchange but may also increase water loss. For example, drought-tolerant plants often have higher stomatal density to balance CO₂ intake and water conservation.
Mesophyll: The inner tissue of a leaf where photosynthesis occurs. It is divided into two layers: the palisade parenchyma (tightly packed cells near the top) and the spongy parenchyma (loosely arranged cells with air spaces). Thicker mesophyll helps plants capture more light and CO₂, improving photosynthesis under stress.
Palisade Parenchyma: A layer of tall, tightly packed cells in the mesophyll that contain many chloroplasts for photosynthesis. Thicker palisade layers mean more sunlight absorption, which is useful in dry conditions where light may be intense.
Spongy Parenchyma: A layer of loosely arranged cells in the mesophyll with air spaces that help CO₂ move freely. These spaces allow gases to diffuse efficiently, supporting photosynthesis even when water is scarce.
Vein Density: The total length of leaf veins per unit area. Higher vein density means water and nutrients are distributed more efficiently, helping plants stay hydrated during drought. It is measured in millimeters per square millimeter (mm/mm²).
Petiole Huber Value: The ratio of the petiole’s xylem (water-conducting tissue) area to the leaf area it supplies. A higher value means more water is delivered to the leaf, which helps prevent dehydration. Formula: Petiole Huber Value = Xylem Area / Leaf Area.
Drought Tolerance: A plant’s ability to survive with limited water. Some plants store water, while others reduce water loss through thick leaves or deep roots. Macadamia trees, for example, have thick leaves and sunken stomata to conserve water.
Drought Avoidance: Strategies plants use to prevent water loss, such as closing stomata, shedding leaves, or growing deeper roots. Unlike drought tolerance, avoidance focuses on maintaining water rather than enduring dry conditions.
Hypostomatic Leaves: Leaves with stomata only on the underside (abaxial surface). This reduces water loss by shielding stomata from direct sunlight and wind, common in drought-resistant plants like macadamia.
Sclerophyllous Leaves: Thick, tough leaves with a waxy coating to reduce water loss. Found in dry climates, these leaves help plants like macadamia survive long periods without rain.
Transpiration: The process of water movement through a plant and evaporation from leaves. It helps pull water and nutrients from roots but can lead to dehydration if not controlled.
Photosynthesis: The process by which plants use sunlight to convert CO₂ and water into sugars and oxygen. Efficient photosynthesis is crucial for growth, especially in water-limited environments.
Xylem: Plant tissue that transports water and nutrients from roots to leaves. A larger xylem area (as in high Huber value) supports better water supply during drought.
Abaxial Surface: The underside of a leaf. In macadamia, stomata are only found here to minimize water loss while allowing gas exchange.
Guard Cells: Specialized cells that surround stomata and control their opening and closing. They help balance CO₂ intake and water loss, especially in dry conditions.
Vapour Pressure Deficit (VPD): The difference between the amount of moisture in the air and how much it can hold. High VPD increases water loss from leaves, stressing plants.
Reference Evapotranspiration (ET₀): The rate at which water evaporates from soil and plant surfaces. It helps estimate irrigation needs, especially in drought-prone areas.
Hydraulic Conductance: How easily water moves through a plant. Higher vein density and Huber value improve this, ensuring leaves stay hydrated.
Stomatal Conductance: The rate at which stomata allow CO₂ in and water vapor out. Plants adjust this to balance photosynthesis and water loss.
Minor Veins: Small veins in leaves that distribute water to cells. Higher minor vein density improves water delivery, helping plants cope with drought.
Plasticity: A plant’s ability to change its traits (like stomatal density) in response to the environment. This helps macadamia adapt to varying water conditions.
Saturation Vapour Pressure: The maximum water vapor air can hold at a given temperature. Used with VPD to assess drought risk in different climates.
Permutational Analysis of Variance (PERMANOVA): A statistical method to compare groups (e.g., cultivars or regions) based on multiple traits. It showed season had the biggest impact on macadamia leaf anatomy.
Principal Component Analysis (PCA): A technique to simplify complex data by identifying key patterns. In this study, PCA helped group traits like stomatal density and vein density to compare drought adaptations.
Box-Cox Transformation: A statistical method to make data more normal (balanced) for analysis. It was used here to properly compare traits like Huber value and vein density across cultivars.
Reference:
Maumela, K., Taylor, N. J., Oberlander, K. C., & Kritzinger, A. (2025). Leaf anatomical traits of macadamia cultivars with potential links to drought tolerance. South African Journal of Botany, 180, 378–386.
https://doi.org/10.1016/j.sajb.2025.03.032
