What if plants had more carbon dioxide?

A green leaf is like a tiny kitchen. It uses light to cook up sugar.

One of its ingredients is a gas in the air called carbon dioxide. What if the leaf had much more of it?

In photosynthesis, a leaf takes in carbon dioxide from the air and uses the energy in sunlight to join it with water into sugar. The leaf gives off oxygen as it works.

Carbon dioxide is only a tiny part of the air: about 420 parts in every million. Some greenhouse growers pump in extra, to around 1,000 parts per million. This lab asks: if a leaf gets more than twice as much carbon dioxide, does it make sugar more than twice as fast?

The carbon a plant builds into sugar comes from atmospheric CO₂, fixed in the Calvin cycle using energy captured by chlorophyll. Global mean CO₂ was 422.8 ppm in 2024, according to NOAA Climate.gov, up from 280 ppm or less before the Industrial Revolution.

Greenhouse CO₂ enrichment to around 1,000 ppm has produced positive responses in many crops (UC Agriculture and Natural Resources). This lab asks how much of that 2.4-fold increase a leaf can actually use, and what takes over as the limit.

Olly's model leaf has bright light, warm air and today's air.

It is busy making sugar. But it could go faster with more carbon dioxide.

Our model leaf sits in a glass growth chamber with bright light (800 units), a comfortable 25 °C, and today's air at 420 ppm. Its net photosynthesis is 11.7, which means it takes in 11.7 micromoles of carbon dioxide per square meter of leaf every second, after paying for its own breathing.

That makes about 1.27 grams of sugar per square meter of leaf each hour. The 'Holding it back most' readout says carbon dioxide: of the three things the leaf needs from outside, that is the one furthest below its best.

At I = 800 µmol/m²/s, C = 420 ppm and T = 25 °C, the model's gross uptake is 12.7 µmol CO₂/m²/s, respiration 1.0, net 11.7, or 1.27 g glucose/m²/h; a two-hour run makes 2.54 g/m².

The factors are I/(I + 300) = 0.73 for light and C/(C + 300) = 0.58 for CO₂, with the temperature factor at its peak of 1. CO₂ has the lowest factor, so the readout names carbon dioxide as the limit.

Now Olly gives the leaf lots more carbon dioxide.

The leaf makes more sugar! But not twice as much. Now it needs more light to go faster.

Raise the carbon dioxide to 1,000 ppm, more than twice as much, and net photosynthesis climbs from 11.7 to 15.8. That is about 35 percent more, not double. Sugar goes from 1.27 to 1.71 grams per square meter per hour.

Now look at 'Holding it back most': it says light. The leaf has plenty of carbon dioxide, so light has become the limit. Turn the light up to 2,000 and net photosynthesis reaches 19.1. In dim light (200), extra carbon dioxide helps much less: from 6.0 to 8.2.

At 1,000 ppm the CO₂ factor rises to 0.77, past the light factor of 0.73, and the limit switches to light. Net photosynthesis rises from 11.7 to 15.8 µmol/m²/s (×1.35) for a 2.4-fold CO₂ increase; sugar from 1.27 to 1.71 g/m²/h, and a two-hour run from 2.54 to 3.41 g/m².

The response is co-limited. At I = 2,000, 1,000 ppm gives net 19.1, and the limit returns to CO₂; at I = 200, the same CO₂ rise lifts net only from 6.0 to 8.2. And heat cancels the gain: at 40 °C, 1,000 ppm gives just 3.3.

A leaf needs light, carbon dioxide and warmth all together.

If one is missing, the leaf slows down. Giving extra of the others does not help much.

Photosynthesis is like a team. Light, carbon dioxide and a good temperature all have to work together. When one is in short supply, it holds the others back; it is called the limiting factor. Adding more of something that is already plentiful helps less and less.

The model is a simplification. It describes one sunlit leaf of a typical plant, like wheat, rice, beans or most trees, with plenty of water. Real leaves are more varied. On a hot, dry day, for example, a real leaf closes its tiny pores to save water, and less carbon dioxide gets in (OpenStax Biology).

The model multiplies three saturating or bell-shaped factors, so the rate is pulled down most by whichever factor is furthest below 1, a simple form of the limiting-factor idea. Each CO₂ response is a hyperbola C/(C + 300): doubling C from 300 to 600 lifts the factor from 0.50 to only 0.67.

The model's limits: it is one sunlit C3 leaf, not a whole plant or crop, and other kinds of plant are not modeled; the curves are smooth stand-ins, not measurements; water and nutrients are assumed plentiful; and the stomata never close, though real guard cells close them on hot, dry days, lowering the CO₂ available (OpenStax Biology). It says nothing about how much a crop will yield.

Some farmers grow plants inside glass greenhouses. Some add extra carbon dioxide to help the plants grow.

It helps most when it is cold outside and the windows stay shut.

Greenhouse growers really do add carbon dioxide. Steven Tjosvold, writing for University of California Agriculture and Natural Resources, explains that in cold places, sunny greenhouses with the windows shut can run low on carbon dioxide as the plants use it up. Adding carbon dioxide to about 1,000 ppm helped many crops there.

But it does not always pay. In California's coastal greenhouses, the vents are open whenever the sun shines, so it is hard to keep extra carbon dioxide inside. In his tests with roses, carbon dioxide added in a careful way, all day, near the leaves, gave 12 percent more roses.

Tjosvold (UC Agriculture and Natural Resources) reports that in unvented, cold-climate greenhouses, fast-growing crops depleted CO₂ below ambient on sunny days, and enrichment to around 1,000 ppm gave positive responses in many crops. In vented coastal California houses, enrichment was possible only for a few hours. Canopy CO₂ around roses fell as low as 225 ppm; all-day enrichment delivered through drip tubing raised rose production by 12%.

He also notes that a fall below ambient hurts photosynthesis more than enrichment above it helps, and cites an Ontario study in which summer enrichment of tomatoes gave little or no extra yield, valuable only under favorable growing conditions. The model's lesson matches: CO₂ helps only when light and temperature allow.