In the dry, sun-baked landscapes of Africa, where water is scarce and droughts are common, farmers face immense challenges growing crops. However, a remarkable plant called Agave sisalana is changing the game. Known for its tough fibers and resilience, this plant has a secret weapon: CAM photosynthesis.
A groundbreaking 2024 study reveals how Agave sisalana uses this unique survival strategy to thrive in drought conditions, offering hope for sustainable farming in arid regions.
Understanding CAM Photosynthesis and Its Role in Drought Survival
CAM photosynthesis, short for crassulacean acid metabolism, is a specialized process that allows plants to grow in extremely dry environments. Unlike most plants, which open their leaf pores (stomata) during the day to absorb carbon dioxide (CO₂), CAM plants like Agave sisalana do the opposite.
They open their stomata at night when temperatures are cooler and humidity is higher. This simple shift helps them avoid losing large amounts of water through evaporation, a common problem for daytime-active plants in hot climates.
During the night, CAM plants absorb CO₂ and store it as malic acid in their cells. When the sun rises, they close their stomata and use the stored CO₂ to produce sugars through photosynthesis. This two-step process makes CAM plants incredibly water-efficient.
In fact, studies show CAM plants use 80% less water than traditional crops like corn or wheat. For regions battling drought, this efficiency is a game-changer.
A Groundbreaking Study on Agave Sisalana in Kenya
To understand how CAM photosynthesis works in real-world drought conditions, researchers studied a sisal plantation in Kenya’s Taita Hills. The region experiences a semi-arid climate with unpredictable rainfall, making it ideal for observing how Agave sisalana adapts to water stress.
Over 65 days, scientists used advanced tools like eddy covariance towers to measure gas exchange between the plants and the atmosphere. These towers tracked CO₂ absorption, water loss, and environmental factors like soil moisture and temperature. The study period covered both wet and dry seasons.
- During the wet season, the plantation received 123 mm of rain, keeping the soil moist.
- In contrast, the dry season saw only 103 mm of sporadic rainfall, causing the soil to dry out completely.
By comparing data from these periods, researchers uncovered how Agave sisalana switches its photosynthesis strategy to survive.
Agave Sisalana’s Dual Strategy: Flexibility in Wet and Dry Seasons
When water is plentiful, Agave sisalana behaves like a hybrid plant. During the wet season, it combines CAM photosynthesis with limited daytime CO₂ uptake, similar to regular crops.
This dual approach allows it to grow faster by absorbing more carbon when conditions are favorable. For example, the plantation acted as a carbon sink during the wet season, absorbing -1.1 µmol of CO₂ per square meter every second.
However, as the soil dries, the plant shifts to strict CAM mode. In the dry season, daytime CO₂ uptake stops entirely, and the stomata remain closed to prevent water loss.
Remarkably, nighttime CO₂ absorption stays strong even as the soil moisture drops below 10%, a level where most plants wilt.
This ability to “switch gears” ensures Agave sisalana survives prolonged droughts without sacrificing growth entirely.
Why CAM Photosynthesis Matters for Drought-Resistant Crops
The study’s findings have far-reaching implications for agriculture in arid regions. First, CAM plants like Agave sisalana require significantly less water than traditional crops.
For instance, during the dry season, the plantation’s water loss (evapotranspiration) dropped by 41%, yet the plants continued to produce fibers. This makes sisal a reliable crop for farmers in areas where water shortages are common.
Second, CAM photosynthesis helps combat climate change. During the wet season, the sisal plantation absorbed CO₂ at rates comparable to tropical forests, acting as a carbon sink.
While dry conditions reduced this capacity, the plant’s ability to store carbon at night still contributes to lowering atmospheric CO₂ levels.
Finally, Agave sisalana grows well on marginal lands unsuitable for food crops. In Kenya, sisal estates coexist with native bushland, preserving biodiversity while providing economic opportunities. This balance is critical for regions where expanding farmland often leads to deforestation and soil degradation.
Challenges and Future Research on CAM Plants
Despite its advantages, CAM photosynthesis has limitations. The Kenyan study, for example, lasted only 65 days, leaving questions about how Agave sisalana performs over years of drought.
Additionally, the research focused on topsoil moisture, but the plant’s deep roots (reaching up to 50 cm) might access hidden water reserves not detected by sensors.
Future studies could explore how irrigation or mulching affects CAM plasticity. For example, small amounts of water during critical growth phases might boost yields without wasting resources.
Researchers are also curious about transferring CAM traits to food crops like sorghum or millet, which could revolutionize farming in dry areas.
Practical Tips for Farmers Growing CAM Crops
For farmers interested in drought-resistant crops like Agave sisalana, timing is key. Planting during rainy seasons helps young plants establish strong roots and leverage daytime CO₂ uptake.
Once the dry season begins, reducing leaf harvesting ensures the plant retains enough water reserves to survive. Soil management also plays a role. Organic mulches, such as sisal waste or crop residues, can lock in moisture and delay the shift to strict CAM mode.
Governments can support these efforts by offering subsidies for CAM crops and funding research into water-efficient farming techniques.
The Future of Agriculture in a Drier World
As climate change intensifies droughts, CAM photosynthesis offers a lifeline for farmers in arid regions. Plants like Agave sisalana prove that it’s possible to grow crops sustainably even in harsh conditions. By blending ancient plant wisdom with modern science, we can create farming systems that are resilient, water-efficient, and environmentally friendly.
The Kenyan study is just the beginning. With further research, CAM crops could transform agriculture, turning barren lands into productive ecosystems. For now, Agave sisalana stands as a symbol of hope—a reminder that nature, when understood and respected, provides solutions to humanity’s greatest challenges.
Power Terms
Crassulacean Acid Metabolism (CAM): A specialized form of photosynthesis where plants take in carbon dioxide at night and store it as malic acid for use during the day. This adaptation helps plants conserve water in arid environments. For example, agave plants and cacti use CAM photosynthesis to survive in deserts where water is scarce.
Agave sisalana: A species of drought-resistant succulent plant cultivated for its strong fibrous leaves. These plants are grown commercially in semi-arid regions like Kenya, where their fibers are processed into ropes, twines, and textiles. The plant’s ability to thrive with minimal water makes it valuable for agriculture in dry areas.
Eddy Covariance (EC): A scientific technique that measures the exchange of gases like carbon dioxide and water vapor between ecosystems and the atmosphere. Researchers use tall towers with sensitive instruments to track these flows, helping us understand how plants respond to environmental changes. This method was used in the study to monitor the sisal plantation’s gas exchange.
Net Ecosystem Exchange (NEE): The balance between the carbon dioxide absorbed by plants during photosynthesis and the CO₂ released through respiration by plants and soil organisms. When NEE is negative, the ecosystem is absorbing more carbon than it releases, acting as a carbon sink. The study found the sisal plantation switched from being a carbon sink during wet periods to a slight carbon source during dry periods.
Stomatal Control: The process by which plants regulate the opening and closing of tiny pores called stomata on their leaves. CAM plants like agave keep their stomata closed during the day to prevent water loss, then open them at night to take in CO₂. This adaptation is crucial for survival in dry conditions.
Photosynthetic Plasticity: The ability of certain plants to adjust their photosynthetic processes in response to environmental changes. Agave sisalana demonstrates this by shifting between daytime C3 photosynthesis during wet periods and nighttime CAM photosynthesis during droughts, allowing it to adapt to varying water availability.
Succulent: Plants with thick, fleshy tissues adapted to store water. These water-storing adaptations help plants survive extended dry periods. Agave plants are succulents that store water in their thick leaves, enabling them to withstand the arid conditions of their native habitats.
C3 Pathway: The most common form of photosynthesis, where plants directly fix carbon dioxide during daylight hours using the enzyme RuBisCO. While efficient in moderate climates, this process leads to significant water loss in hot, dry environments. Many crops like wheat and rice use the C3 pathway.
C4 Pathway: An alternative photosynthetic process where carbon dioxide is first incorporated into a 4-carbon compound before entering the Calvin cycle. This adaptation helps plants conserve water in hot climates. Crops like corn and sugarcane use the C4 pathway, which is more efficient than C3 under high light and temperature conditions.
RuBisCO: An enzyme (Ribulose-1,5-bisphosphate carboxylase/oxygenase) that catalyzes the first major step of carbon fixation in photosynthesis. While essential for plant growth, this enzyme can be inefficient in hot conditions, leading some plants like agave to develop alternative strategies like CAM photosynthesis.
PEP Carboxylase: A key enzyme in CAM and C4 plants that initially fixes carbon dioxide into organic acids. In CAM plants like agave, PEP carboxylase works at night to capture CO₂ when the stomata are open, storing it as malic acid for use in photosynthesis the next day.
Malic Acid: An organic compound that serves as temporary carbon storage in CAM plants. At night, CO₂ is fixed into malic acid and stored in vacuoles, then broken down during the day to release CO₂ for photosynthesis. This storage system allows CAM plants to keep their stomata closed during hot daylight hours.
Water-Use Efficiency (WUE): The ratio of carbon gained through photosynthesis to water lost through transpiration. CAM plants like agave have exceptionally high WUE because they minimize water loss by keeping stomata closed during the day. This makes them ideal crops for arid regions where water is scarce.
Ecosystem Respiration: The total release of carbon dioxide by all living organisms in an ecosystem, including plants, animals, and microorganisms in the soil. In the sisal plantation study, researchers measured how respiration rates changed between wet and dry periods to understand the carbon balance.
Soil Moisture Deficit: A condition where the amount of water in the soil is insufficient to meet plant needs. The study observed that when soil moisture dropped below 10%, the sisal plants shifted completely to nighttime CAM photosynthesis and reduced their daytime activity to conserve water.
Xerophyte: Plants specially adapted to grow in dry environments through features like water-storing tissues, reduced leaf surfaces, and specialized photosynthesis. Agave sisalana is a xerophyte that thrives in semi-arid regions where other crops would struggle to survive.
Latosols: A type of deep, reddish tropical soil with low organic matter content. The sisal plantation in the study grew in these nutrient-poor soils, demonstrating how agave can be cultivated in marginal lands unsuitable for traditional agriculture.
Permanent Wilting Point: The soil moisture level at which plants can no longer extract enough water to recover from wilting. When soil moisture in the sisal plantation dropped below this point (pF 4.2), the plants showed signs of drought stress but maintained some nighttime CO₂ uptake through CAM photosynthesis.
Vapor Pressure Deficit (VPD): The difference between the amount of moisture in the air and how much moisture the air could hold at saturation. High VPD indicates dry air that increases plant water loss. The study found VPD was a major controller of gas exchange, especially during dry periods.
Canopy Conductance: A measure of how easily water vapor passes through a plant canopy, reflecting the degree of stomatal opening. Researchers calculated this to understand how the sisal plants regulated water loss in response to changing soil moisture conditions.
Biomass: The total mass of living plant material in a given area. The study noted that sisal plantations accumulate about 10.6 megagrams of aboveground biomass per hectare, similar to natural bushlands in the region, making them important for carbon storage.
Bioenergy Crops: Plants cultivated specifically for fuel production rather than food. Agave species are being investigated as potential bioenergy crops because they can grow on marginal lands with minimal water requirements while still producing substantial biomass.
Carbon Sequestration: The long-term storage of carbon in plants, soils, or other reservoirs to mitigate atmospheric CO₂ increases. While the sisal plantation alternated between being a carbon sink and source depending on soil moisture, its ability to grow in dry conditions makes it potentially valuable for carbon sequestration in arid regions.
Land Degradation: The deterioration of land quality through human activities like overgrazing, deforestation, or poor farming practices. The study highlights how drought-resistant crops like sisal could help prevent land degradation in vulnerable semi-arid regions.
Desertification: The process by which fertile land becomes desert, typically due to drought, deforestation, or inappropriate agriculture. The research suggests that cultivating CAM plants like agave could be a sustainable land-use option in areas at risk of desertification from climate change.
Reference:
Skogberg, M., Kohonen, K.-M., Lohila, A., Merbold, L., Rasanen, M., Vuorinne, I., Pellikka, P., Vesala, T., & Kübert, A. (2025). Ecosystem-scale crassulacean acid metabolism (CAM) gas exchange of a sisal (Agave sisalana) plantation. Agriculture, Ecosystems & Environment, 381, 109435. https://doi.org/10.1016/j.agee.2024.109435
