Physiological Responses of Cowpea to Drought and High Night Temperatures
Cowpeas, often called “black-eyed peas,” are more than just a humble legume. For millions of people in Africa, Asia, and the Americas, they are a critical source of protein and a lifeline in harsh, dry climates. However, climate change is now pushing this resilient crop to its breaking point. A groundbreaking 2025 study reveals that the combination of drought and rising nighttime temperatures—two escalating threats fueled by global warming—can slash cowpea yields by 63% and reduce seed protein content by 25%.
The Growing Threat to Cowpeas
Cowpeas thrive in hot, dry conditions where other crops fail, making them indispensable for farmers in regions like West Africa and the southern United States.
However, climate change is intensifying two major threats: drought (prolonged dry spells) and high night temperatures (unusually warm nights).
Over the past century, nighttime temperatures have risen 1.4 times faster than daytime temperatures due to greenhouse gas emissions. At the same time, unpredictable rainfall patterns are causing more frequent droughts.
For cowpeas, these changes are disastrous. The plants require cool nights (around 16.5°C) during their reproductive stage—the phase when flowers develop into pods and seeds.
Even a 1°C increase in nighttime temperatures can reduce yields by 13.6%, according to earlier research. When combined with drought, the damage is far worse.
To understand why, researchers at Mississippi State University conducted a detailed study simulating these stressors in controlled greenhouses.
How the Study Worked
The team grew three genetically distinct cowpea varieties under four conditions: Control: Normal irrigation and nighttime temperatures (24°C). Drought: 40% less water than normal.
High Night Temperature (HNT): Nighttime temperatures raised to 28°C. Combined Stress: Drought + 28°C nights. The varieties tested were EpicSelect.4 (drought-tolerant), LOVI (high-yielding), and UCR 369 (heat-resilient).
Stress treatments began at the flowering stage and lasted 30 days—a critical window for pod and seed development. Researchers measured key traits like leaf function, photosynthesis efficiency, and seed quality to assess the impact.
Key Findings: A Devastating Toll on Cowpeas
1. Leaves Struggle to “Breathe” Under StressWhen plants face drought, their leaves close tiny pores called stomata to save water. However, this also blocks carbon dioxide (CO₂)—a key ingredient for photosynthesis.
In the study, stomatal conductance (a measure of how open these pores are) dropped by 91.5% under drought and 99% under combined stress. With CO₂ intake nearly halted, the plants couldn’t produce enough energy to grow pods and seeds.
At the same time, hotter nights worsened the problem. Plants under combined stress had 3.5°C warmer leaves than the control group. For example, the LOVI variety saw leaf temperatures rise to 4.6°C above normal. This overheating disrupted cellular processes, further crippling growth.
2. Photosynthesis Breaks Down
Photosynthesis—the process plants use to convert sunlight into energy—collapsed under stress. Efficiency dropped by 73% under drought and 17% under heat alone.
Even more alarming, the electron transport rate (ETR), which reflects energy production in leaves, fell by 60% in the heat-tolerant UCR 369 variety under combined stress.
Surprisingly, chlorophyll levels—the green pigments that capture sunlight—increased by 47-48% in stressed plants. Researchers believe this was a desperate attempt to compensate for poor photosynthesis.
However, without functional stomata and efficient energy transport, extra chlorophyll couldn’t save the plants.
3. Yields and Nutrition Plummet
The combined stress of drought and heat caused catastrophic losses: Pod weight dropped by 64.5%. Seed numbers per plant fell by 66%. Total seed yield decreased by 63%. Seed quality also suffered. Protein content—critical for human nutrition—fell by 25%, while starch levels rose by 11%.
This shift from protein to starch is a survival tactic: stressed plants prioritize quick-energy carbohydrates over nutrient-rich proteins. For communities relying on cowpeas as a protein source, this decline threatens to worsen malnutrition.
Why Some Cowpea Varieties Survived Better Than Others
The three cowpea varieties responded differently to stress, offering clues for future crop breeding:
1. EpicSelect. 4: The Drought Fighter
EpicSelect.4, bred for drought tolerance, handled water scarcity better than others. Its stomatal conductance dropped only 17% under drought, allowing it to maintain some CO₂ intake. However, it struggled under combined stress, losing 53.7% of its yield.
2. UCR 369: The Heat Survivor
UCR 369, a Kenyan variety, excelled in high nighttime temperatures. Its pod weight fell just 5.7% under heat alone, and it maintained stable seed numbers. However, even UCR 369 couldn’t withstand the double stress of drought + heat, suffering a 62.2% yield loss.
3. LOVI: The High-Yield Casualty
LOVI, a high-yielding variety from Cyprus, thrived in ideal conditions but collapsed under stress. Its yields dropped 72% under combined stress—the worst performance of the three. This highlights a harsh trade-off: varieties bred for maximum yields often lack resilience to climate shocks.
What This Means for Farmers and Food Security
The study’s findings have urgent implications for agriculture, particularly in regions already struggling with hunger:
1. Climate Adaptation Is Non-Negotiable
With global protein demand projected to rise 51% by 2050, cowpeas must remain a reliable crop. Farmers in drought-prone areas like Niger and Nigeria—which produce 66% of the world’s cowpeas—need access to heat- and drought-tolerant seeds.
2. Breeding Smarter Crops
The resilience of UCR 369 and EpicSelect.4 suggests that key traits can be combined into hybrid varieties. For example, cross-breeding UCR 369’s heat tolerance with EpicSelect.4’s drought resistance could create a “super cowpea” suited to harsher climates.
3. Rethinking Farming Practices
Simple changes can reduce losses:
- Drip irrigation: Delivers water directly to roots, minimizing waste.
- Shade nets: Lowers leaf temperatures during hot nights.
- Crop rotation: Improves soil health and reduces pest pressure.
4. Policy Action Is Critical
Governments and organizations must fund climate-smart agriculture, including:
- Seed banks to preserve genetic diversity.
- Subsidies for drought-resistant seeds and irrigation tools.
- Education programs to teach farmers stress-management techniques.
While the study provides vital insights, it has limitations. The experiments were conducted in greenhouses, which lack real-world variables like pests, wind, and fluctuating humidity. Field trials are needed to confirm the results. Additionally, the genetic mechanisms behind stress tolerance remain unclear. Future research could identify specific genes responsible for traits like heat resilience, enabling faster breeding of hardy varieties.
Conclusion: Protecting Cowpeas for a Hungrier World
The message from this research is clear: climate change is not a single threat but a cascade of challenges. For cowpeas, the combination of drought and hot nights is catastrophic, destroying yields and eroding nutritional value. However, the study also offers hope. By leveraging resilient varieties like UCR 369 and EpicSelect.4, we can develop crops that withstand tomorrow’s climates.
Frequently Asked Questions (FAQs)
Drought Stress (DS): Drought stress occurs when plants don’t receive enough water to meet their needs, leading to reduced growth and yield. In this study, cowpea plants were given only 40% of the normal irrigation to simulate drought. Drought affects processes like photosynthesis and can cause wilting, leaf drop, and lower seed production. For example, cowpea yields dropped by 30–66% under drought.
High Night Temperature (HNT): This refers to warmer-than-usual temperatures during nighttime, which disrupt plant metabolism. Here, HNT was set at 28°C (vs. the control at 24°C). Warmer nights increase respiration, depleting energy reserves and reducing yields. For instance, cowpea seed yield fell by 23% under HNT alone.
Stomatal Conductance (gₛ): A measure of how open stomata (tiny leaf pores) are, allowing gas exchange for photosynthesis. Under drought, stomata close to save water, reducing CO₂ uptake. In the study, gₛ dropped by 91.5% under combined drought and HNT, severely limiting growth.
Transpiration (E): The process of water movement through a plant and evaporation from leaves. It cools plants and aids nutrient uptake. Drought and HNT reduced transpiration by up to 99%, raising leaf temperatures and stressing cowpeas.
Photosystem II Efficiency (PhiPS2): Measures how well plants convert light energy into chemical energy during photosynthesis. Stress lowers PhiPS2; here, it declined by 17.4% under HNT, reducing the plant’s energy production.
Electron Transport Rate (ETR): Reflects the speed of electrons moving through photosynthesis pathways. A high ETR means efficient energy use. Drought and HNT slowed ETR by 57–60%, weakening cowpea growth.
Chlorophyll Index: Indicates chlorophyll content, crucial for photosynthesis. Surprisingly, stressed cowpeas had 42–48% higher chlorophyll, possibly due to thicker leaves or adaptive responses.
Flavonol Index: Measures flavonols, antioxidants that protect plants from stress damage. Drought reduced flavonols by 22.6%, making cowpeas more vulnerable to oxidative harm.
Pod Weight: The total weight of seed-containing pods per plant. Combined stress caused a 64.5% drop in pod weight, directly reducing yield.
Harvest Index (HI): The ratio of seed yield to total plant biomass. A high HI means efficient energy use for seeds. UCR 369 maintained a better HI under stress, showing resilience.
Volumetric Water Content (VWC): The amount of water in soil. Control plants had 0.19 m³/m³ VWC, while drought-stressed ones had 0.078 m³/m³, severely limiting water availability.
Respiration: The process where plants break down sugars for energy, increasing at night. HNT raised respiration, depleting sugars needed for growth and seeds.
Oxidative Stress: Damage caused by reactive molecules when plants can’t balance energy production under stress. Drought and HNT increased oxidative stress in cowpeas.
Genotype: Genetic variants of a species (e.g., EpicSelect.4, LOVI, UCR 369). Here, UCR 369 tolerated HNT better, while EpicSelect.4 resisted drought.
Phenological Traits: Growth stages like flowering or seed development. Cowpeas were stressed during reproduction, the most yield-sensitive phase.
Biomass: Total plant weight, including stems, leaves, and pods. HNT increased biomass by 68% in EpicSelect.4, but combined stress reduced it by 44%.
Seed Protein Content: Nutritional quality metric. Combined stress cut protein by 25.1%, making seeds less nutritious.
Antioxidant Defense: Molecules like flavonols that protect cells from stress. Drought lowered these defenses in cowpeas.
Carbon-to-Nitrogen Balance: The ratio of carbon (from photosynthesis) to nitrogen (for proteins). Drought disrupted this balance, hurting seed development.
Pod Abortion: When plants drop pods due to stress, reducing yield. HNT increased pod abortion in cowpeas by impairing flower retention.
Leaf Temperature: Stressed leaves heat up due to closed stomata. Combined stress raised leaf temps by 3.5°C, worsening damage.
Starch Content: A carbohydrate stored in seeds. Stress increased starch by 11% but reduced proteins, altering seed quality.
Resilience: Ability to withstand stress. UCR 369 showed resilience to HNT, making it a candidate for breeding programs.
Synergistic Effect: When combined stressors (e.g., drought + HNT) cause worse damage than individual ones. Cowpea yields fell 63% under combined stress vs. 30% (drought alone).
Stability Index: Compares trait performance under stress vs. normal conditions. EpicSelect.4 had higher stability in physiological traits, while UCR 369 excelled in yield stability.
Reference:
1. Chakravaram, A., Sankarapillai, L. V., Poudel, S., & Bheemanahalli, R. (2025). Interactive effects of drought and high night temperature on physiology and yield components of cowpea (Vigna unguiculata (L.) Walp.). Journal of Agriculture and Food Research, 101844. https://doi.org/10.1016/j.jafr.2025.101844
