Carbon dioxide is one of the two raw ingredients plants need for photosynthesis, alongside water, so raising its concentration in the air generally speeds the process up. For many common crops and trees, higher atmospheric CO₂ means faster carbon fixation, at least in the short term. But the relationship is far from a simple “more is better” story. How much a plant benefits depends on its photosynthetic type, whether it has enough nitrogen and water, how warm it is, and how long the elevated CO₂ persists.
How CO₂ Enters the Photosynthetic Machinery
Inside a leaf, photosynthesis depends on an enzyme called RuBisCO to grab CO₂ molecules and attach them to a sugar molecule, kicking off the chain of reactions that builds carbohydrates. RuBisCO is famously inefficient: it also grabs oxygen by mistake, triggering a wasteful side process called photorespiration that burns energy without producing useful sugars. When CO₂ concentrations rise, RuBisCO is more likely to latch onto CO₂ instead of oxygen, so photosynthesis runs more smoothly and photorespiration drops.
This competitive relationship between CO₂ and oxygen at RuBisCO’s active site is the single most important reason extra CO₂ boosts photosynthesis. Under today’s atmospheric conditions (roughly 425 parts per million of CO₂), RuBisCO still wastes a meaningful fraction of the plant’s energy on photorespiration. Raising CO₂ tips the odds in favor of productive carbon fixation.
There is a ceiling, though. At very high CO₂ levels, photosynthesis can become limited by how fast the plant recycles a molecule called triose phosphate. When the plant cannot export or use its sugars quickly enough, excess carbohydrates accumulate in the leaf, and the whole system slows down. Research on rice has shown that this bottleneck becomes relevant under elevated CO₂ when the plant does not have enough places to send its sugars, a situation called sink limitation.1PubMed. Is triose phosphate utilization involved in the feedback inhibition of photosynthesis in rice under conditions of sink limitation?
Why C3 Plants Gain More Than C4 Plants
Not all plants respond to rising CO₂ equally, and the split falls along the lines of how they handle carbon internally. The majority of the world’s plant species, including wheat, rice, soybeans, and most trees, use what is called C3 photosynthesis. Their RuBisCO is directly exposed to whatever CO₂ concentration diffuses into the leaf, which means they stand to gain the most when atmospheric CO₂ goes up. In experiments where CO₂ was doubled, well-watered C3 species increased their rate of carbon fixation by roughly 15 to 25 percent.2HAYATI Journal of Biosciences. Photosynthesis of C3 and C4 Species in Response to Increased CO2 Concentration and Drought Stress
C4 plants, which include maize, sugarcane, and many tropical grasses, have evolved an internal carbon-concentrating mechanism. They shuttle CO₂ into specialized bundle-sheath cells where RuBisCO operates in a locally CO₂-rich environment, effectively suppressing photorespiration under normal conditions.3Frontiers in Plant Science. The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in Higher Plants Because their RuBisCO is already bathed in high CO₂, raising atmospheric levels does not help C4 plants nearly as much. In those same doubled-CO₂ experiments, C4 species showed no significant increase in carbon fixation when water was plentiful.2HAYATI Journal of Biosciences. Photosynthesis of C3 and C4 Species in Response to Increased CO2 Concentration and Drought Stress
A third group, CAM plants (think cacti, agaves, and pineapples), have a more complex relationship with rising CO₂. CAM plants fix CO₂ at night using a different enzyme and then re-fix it during the day using RuBisCO. Under elevated CO₂, CAM species generally accumulate more biomass, but the mechanism varies from species to species: some increase nighttime CO₂ uptake while others shift more of their carbon fixation to daytime C3-style photosynthesis.4PubMed Central. Evolution of Crassulacean acid metabolism in response to the environment: past, present, and future In fact, elevated CO₂ tends to reduce the advantage of the nighttime fixation pathway, making the distinctly “CAM” portion of their metabolism less important.5Annals of Botany. Atmospheric CO2 decline and the timing of CAM plant evolution
The Acclimation Problem
The initial burst in photosynthesis that plants show under elevated CO₂ does not always last. Over weeks to months, many species dial back their photosynthetic capacity, a process researchers call downregulation or photosynthetic acclimation. The plant effectively adjusts its internal machinery so that it produces less RuBisCO and invests fewer resources in running the photosynthetic engine at full throttle.
One major driver is nitrogen. Building RuBisCO requires a lot of nitrogen, and when a plant is growing faster because of elevated CO₂, it can outstrip the soil’s ability to supply that nutrient. But the picture is not as simple as the soil running out of nitrogen. Research has shown that in some cases, the downregulation is better explained by a dilution effect: the plant accumulates so much extra starch and sugar in its leaves that the nitrogen already present gets spread thinner on a per-weight basis, even though the total amount of nitrogen per unit leaf area has not actually dropped.6Journal of Plant Physiology. Down-regulation of photosynthesis and its relationship with changes in leaf N allocation and N availability after long-term exposure to elevated CO2 concentration
Does this acclimation wipe out the CO₂ benefit entirely? Not according to global modeling informed by field data. A study projecting changes through 2100 found that even though the maximum rate of carbon fixation per unit RuBisCO declined by more than 30 percent in most regions, net photosynthesis at the ecosystem level stayed the same or still increased, because the higher CO₂ compensated for the reduced enzyme capacity.7Global Change Biology. Acclimation of Photosynthesis to CO2 Increases Ecosystem Carbon Storage due to Leaf Nitrogen Savings The plant spends less nitrogen on RuBisCO while still fixing as much or more carbon. Whether that nitrogen savings matters for the larger ecosystem depends on how tightly nitrogen cycles through the soil.
Nitrogen and the Long-Term Nutrient Squeeze
The concern about nitrogen goes beyond what happens inside a single leaf. As forests and ecosystems take up extra carbon under elevated CO₂, they lock more nitrogen into woody biomass and slowly decomposing organic matter. Over years and decades, this can tighten the supply of plant-available nitrogen in the soil, a concept known as progressive nitrogen limitation. The worry is that ecosystems might eventually lose their ability to respond to rising CO₂ because they simply run short of the nitrogen needed to build new tissue.8BioScience. Progressive Nitrogen Limitation of Ecosystem Responses to Rising Atmospheric Carbon Dioxide
Field experiments tell a more nuanced story. A decade-long study in a warm-temperate forest found that while more nitrogen did accumulate in tree biomass and the forest floor under elevated CO₂, the ecosystem’s carbon-to-nitrogen ratio widened faster than expected. In practical terms, the trees were fixing more carbon per unit of nitrogen, which could delay the onset of serious nitrogen limitation.9PubMed. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest Meanwhile, faster root growth and more rapid litter decomposition released nitrogen back into the soil, keeping the productivity boost going for at least ten years. That study documented a roughly 26 percent increase in net primary productivity that was sustained across the full decade.10PubMed. Forest productivity under elevated CO₂ and O₃: positive feedbacks to soil N cycling sustain decade-long net primary productivity enhancement by CO₂
When plants are nitrogen-limited, they tend to funnel the extra carbohydrates from elevated CO₂ into storage and root growth rather than into taller stems and bigger leaves. A study on a deciduous shrub found exactly this pattern: nitrogen-limited plants under high CO₂ put their extra carbon belowground and into chemical defense compounds rather than into aboveground growth.11PubMed. Effects of elevated CO(2) on growth and carbon/nutrient balance in the deciduous woody shrub Lindera benzoin (L.) Blume (Lauraceae)
How Extra CO₂ Helps Plants Handle Heat and Drought
One of the less obvious effects of elevated CO₂ is that it changes how plants manage water. When CO₂ is abundant, leaf pores called stomata do not need to open as wide to let enough CO₂ in. Partially closing stomata means less water escapes, so the plant loses less moisture through transpiration. A global meta-analysis found that elevated CO₂ improved leaf water status under drought conditions, with leaves retaining more water and showing less negative water potential, primarily because of reduced stomatal opening and a shift toward a higher root-to-shoot ratio.12Journal of Ecology. Elevated CO2 alleviates adverse effects of drought on plant water relations and photosynthesis: A global meta‐analysis Research on soybeans has confirmed this at the physiological level, showing that elevated CO₂ thickened leaf tissue and stimulated cuticular wax production, further reducing water loss.13Plant Physiology and Biochemistry. Elevated CO2 concentration enhances drought resistance of soybean by regulating cell structure, cuticular wax synthesis, photosynthesis, and oxidative stress response
The interaction with heat is more complicated. Elevated CO₂ can partially offset heat damage by maintaining higher rates of carbon fixation than would otherwise be possible at high temperatures. In maize hybrids grown under high temperatures, elevated CO₂ helped several varieties recover their net photosynthesis to levels similar to those seen at normal temperatures.14Scientific Reports. Elevated CO2 ameliorates the high temperature stress effects on physio-biochemical, growth, yield traits of maize hybrids Tomato research has linked this heat-protective effect to melatonin signaling within the plant, finding that elevated CO₂ enhanced heat tolerance in normal plants but lost that protective ability in plants engineered to be melatonin-deficient.15PubMed. Melatonin mediates elevated carbon dioxide-induced photosynthesis and thermotolerance in tomato
But the protection has limits. In wheat, elevated CO₂ slowed the decline in photosynthetic capacity at high temperatures but did not actually shift the temperature at which photosynthesis peaked. And the electron-transport side of photosynthesis was actually more constrained at high temperatures under elevated CO₂, meaning the plant’s ability to use light energy efficiently dropped off more steeply above the optimal temperature.16PubMed Central. Elevated CO2 alleviates the negative impact of heat stress on wheat physiology but not on grain yield Critically, that same study found that elevated CO₂ did not rescue grain yield under heat stress, even though it helped the photosynthetic machinery cope. The gap between photosynthetic rate and final yield is a reminder that the pathway from CO₂ fixation to harvestable food involves many other steps.
What Happens to Crop Nutritional Quality
A persistent finding across field experiments is that when C3 crops grow under the CO₂ levels projected for mid-century, their grain and seed become less nutritious. Zinc and iron concentrations drop in C3 grains and legumes, and protein content falls in C3 crops other than legumes. C4 crops appear less affected.17PubMed Central. Increasing CO2 threatens human nutrition The mechanism is likely related to the same carbohydrate accumulation that drives photosynthetic downregulation: as the plant packs more starch into its grain, the minerals and protein present get diluted.
Rice, the staple food for billions of people, shows additional nutritional losses beyond minerals and protein. Experiments growing rice under elevated CO₂ found consistent declines in several B vitamins (B1, B2, B5, and B9 specifically), though vitamin E bucked the trend and increased.18PubMed Central. Carbon dioxide (CO2) levels this century will alter the protein, micronutrients, and vitamin content of rice grains with potential health consequences for the poorest rice-dependent countries For populations that depend heavily on rice and have limited dietary diversity, these changes could matter for public health. The irony is hard to miss: the same CO₂ that makes plants grow faster may make the resulting food less nourishing per calorie.
Shifts in Plant Chemistry Beyond Nutrition
Rising CO₂ does not only change the nutritional profile of crops. It also reshapes the chemical defense compounds that wild plants use to deter herbivores. A study on the perennial herb Aristolochia contorta found that elevated CO₂ suppressed overall growth but increased concentrations of toxic defensive chemicals. The specifics depended on the plant’s age: younger plants ramped up one class of defensive compound (aristolochic acids) while mature plants invested more heavily in a different set of alkaloids.19PubMed. Elevated CO2 alters age-dependent variation in secondary metabolites, C/N ratio and growth of perennial herb, Aristolochia contorta Changes like these ripple through food webs. An insect that feeds on a plant with lower nitrogen and higher defensive chemicals gets a worse meal, which can alter herbivore populations and, in turn, the animals that eat them.
Forest and Ecosystem Productivity Over Decades
Moving from individual plants to whole ecosystems, the question becomes whether the CO₂ fertilization effect is real and lasting at scale. Flux-tower measurements across extratropical forests provide some of the most direct evidence available. An analysis of these long-term monitoring sites found a median stimulation rate in gross primary production of about 16 percent per 100 parts per million increase in atmospheric CO₂.20Journal of Geophysical Research: Biogeosciences. Estimating the CO2 Fertilization Effect on Extratropical Forest Productivity From Flux‐Tower Observations That study’s authors emphasize that the photosynthetic stimulation was maintained under long-term CO₂ increases across multiple sites, suggesting the effect is not just a short-lived bump.
Shade-tolerant trees may benefit in particular ways. Research on red oak and red maple found that elevated CO₂ tended to improve photosynthetic efficiency under low light conditions, with the strongest effect seen in shade-grown red maple. The increase in light-use efficiency was statistically significant only in certain species-by-light combinations, but the pattern suggests that understory trees could capture more carbon per photon under rising CO₂.21Tree Physiology. Effects of elevated CO2 and light availability on the photosynthetic light response of trees of contrasting shade tolerance
What Happens Belowground
Elevated CO₂ does not just change what happens in leaves. It changes the soil. When plants fix more carbon, they send more of it to their roots, and those roots release more carbon-rich compounds into the surrounding soil. This can accelerate the breakdown of existing soil organic matter through a process called rhizosphere priming, where root exudates stimulate soil microbes to decompose carbon that was previously stable.
The extent of this priming varies dramatically by plant species. An experiment comparing lupin, canola, and wheat found that elevated CO₂ roughly doubled rhizosphere priming in lupin-grown soil but had no significant effect in soil growing canola or wheat. The lupin roots released a much higher ratio of dissolved organic carbon to mineral nitrogen, which appeared to energize specific groups of soil bacteria and fungi that specialize in breaking down organic matter.22Plant and Soil. Clarifying the role of microbial communities in carbon loss from rhizosphere priming of contrasting crop species under elevated atmospheric CO2 This matters for climate projections: if elevated CO₂ drives plants to pump more carbon into soils but that carbon just stimulates the release of older stored carbon, the net climate benefit is smaller than it looks from aboveground productivity alone.
Photosynthesis Underwater
The CO₂-photosynthesis relationship is not limited to land plants. In the ocean, phytoplankton and seagrasses also depend on dissolved CO₂ (or bicarbonate) for carbon fixation. Many marine photosynthesizers have evolved carbon-concentrating mechanisms to cope with the relatively low dissolved CO₂ in seawater, and the energetic cost of running those mechanisms could shift as ocean CO₂ levels change.23PubMed. Carbon concentrating mechanisms in eukaryotic marine phytoplankton
Seagrasses offer a clear example of carbon limitation in the marine world. Experiments on temperate seagrass species found that photosynthetic rates were substantially higher when dissolved CO₂ was abundant than under normal seawater conditions. When forced to rely on bicarbonate (the dominant form of dissolved inorganic carbon at typical ocean pH), photosynthesis ran 1.6 to 6 times slower than when the same species had access to high dissolved CO₂.24Journal of Experimental Marine Biology and Ecology. Inorganic carbon sources for seagrass photosynthesis: an experimental evaluation of bicarbonate use in species inhabiting temperate waters As ocean acidification increases dissolved CO₂ in seawater, seagrasses could see a genuine boost in photosynthesis, which is one reason they are sometimes described as potential “winners” in a high-CO₂ ocean. However, the knock-on effects of acidification on the broader marine ecosystem, including the organisms seagrasses depend on for nutrient cycling, make the net outcome far from straightforward.