For many years, scientists and farmers alike have known that rising atmospheric carbon dioxide (CO₂) can stimulate plant growth. In C₃ crops such as wheat, rice, and soybeans, elevated CO₂ often boosts photosynthesis, increases biomass, and raises yields. This is sometimes called the “CO₂ fertilisation effect.” However, alongside these yield gains, researchers have uncovered a less publicised reality: higher CO₂ levels can reduce the concentration of essential nutrients in grains, pulses, and vegetables. This means that even as we grow more food, its nutritional quality may decline — a concern that has profound implications for global health.
The physiological explanation for this “carbon–nutrient penalty” begins with carbohydrate dilution. When photosynthesis speeds up under elevated CO₂, plants accumulate more sugars and starch relative to proteins and minerals. If nutrient uptake does not keep pace with the extra carbon, the concentrations of nitrogen, iron, zinc, and other micronutrients fall. Field-scale Free-Air CO₂ Enrichment (FACE) experiments, which simulate future atmospheric conditions, consistently show this pattern: yields increase, but protein content and key minerals decline, especially in staple grains. This is not just a statistical curiosity — in regions where diets are heavily reliant on a few staple crops, such nutrient dilution could exacerbate “hidden hunger” and micronutrient deficiencies.
Another driver of nutrient loss lies in the plant’s water relations. Under elevated CO₂, stomata partially close, which reduces water loss and improves water-use efficiency. While this is beneficial for drought tolerance, it also means less water — and fewer dissolved nutrients — flow from the soil into the plant through mass flow. Nutrients like nitrate, which depend heavily on this process, may therefore reach the roots in smaller quantities. Changes in nitrogen assimilation, including reduced activity of enzymes like nitrate reductase and a downregulation of nitrogen-rich proteins such as Rubisco, further limit the nitrogen available for grain filling.
Evidence from recent multi-site rice FACE trials is particularly striking. Under CO₂ levels projected for the end of this century, rice grains showed lower concentrations of protein, iron, zinc, and even several B-vitamins, including B₁, B₂, B₅, and B₉. These vitamins are vital for energy metabolism and neurological development. A decline of this magnitude, even if only 5–10 percent, can have serious consequences for populations in which rice is a major dietary staple. Similar patterns have been observed in wheat, barley, and legumes, reinforcing the conclusion that nutrient dilution is a broad and robust phenomenon.
Addressing this challenge will require coordinated strategies that combine plant physiology insights, agronomic practices, and plant breeding innovations. Biofortification is one of the most promising solutions. Through both conventional breeding and modern biotechnology, scientists are developing varieties of rice, wheat, beans, and maize that have naturally higher levels of zinc, iron, and other nutrients. Agronomic biofortification, which involves applying micronutrients through soil or foliar sprays, can complement genetic approaches, especially when timed to coincide with key reproductive stages. Importantly, new biofortified lines are now being tested under elevated CO₂ conditions to ensure their nutrient advantages are preserved in future climates.
Nutrient management also plays a central role. Adequate and well-timed nitrogen supply can help offset the decline in protein content under elevated CO₂, although it rarely restores it completely. Split nitrogen applications, controlled-release fertilisers, and balanced nutrient blends that include sulfur and micronutrients can sustain nutrient flows to developing grains. Supporting root and rhizosphere traits — such as deeper rooting, greater root hair length, and stronger associations with beneficial soil microbes — can further improve nutrient acquisition. Soil health improvements, including building organic matter and maintaining optimal pH, provide a longer-term foundation for nutrient-rich harvests.
Ultimately, breeding targets will need to evolve. In a high-CO₂ world, yield alone is not enough. We must select for nutritional resilience — varieties that can maintain high protein and micronutrient concentrations even under future atmospheric conditions. This means integrating nutrient density into mainstream breeding pipelines and testing candidates in realistic field conditions that reflect the climate of the coming decades.
The stakes are not only agricultural but also public health. Small percentage drops in nutrient content may seem insignificant in isolation, but when multiplied across billions of daily meals, they can slow or reverse progress in combating malnutrition. As climate change accelerates and CO₂ continues to rise, the goal of plant physiology and crop science must shift from simply feeding the world to nourishing it. Through the combined efforts of breeders, agronomists, soil scientists, and policymakers, we can work to ensure that our food remains as rich in nutrition as it is in calories.
