The most familiar plants that use C4 photosynthesis are corn (maize), sugarcane, and sorghum, three of the world’s most productive crops. But C4 photosynthesis isn’t limited to a handful of agricultural stars. It appears in roughly 3% of all plant species, spread across 23 plant families, and those species punch well above their weight: C4 vegetation accounts for nearly 20% of all photosynthetic carbon captured on Earth.
Major Crops That Use C4 Photosynthesis
C4 species are dramatically overrepresented among the world’s most important food and fuel crops. Corn and sugarcane rank at the very top of global commodity production, and both use the C4 pathway. Sorghum, millet, and miscanthus (a biofuel grass) are also C4 plants. So is switchgrass, increasingly cultivated for cellulosic ethanol.
This isn’t a coincidence. These crops dominate warm-climate agriculture precisely because C4 photosynthesis makes them exceptionally efficient at converting sunlight into biomass, especially under hot, dry, or nutrient-poor conditions where other plants struggle.
Wild Grasses, Weeds, and Other C4 Species
Beyond the farm, C4 photosynthesis is common in wild grasses and some of the most aggressive weeds in the world. Crabgrass (both large and smooth varieties), foxtail species (giant, green, and yellow), barnyardgrass, johnsongrass, and witchgrass all use the C4 pathway. If you’ve ever battled a lawn weed that thrives in midsummer heat while your cool-season turf goes dormant, there’s a good chance it was a C4 plant.
C4 photosynthesis also shows up outside the grass family. Palmer amaranth, redroot pigweed, and waterhemp are C4 broadleaf weeds in the amaranth family, and they’re among the most problematic weeds in North American agriculture. Yellow nutsedge and purple nutsedge, members of the sedge family, round out the list of common C4 weeds.
Which Plant Families Contain C4 Species
Three plant families account for the vast majority of C4 species. Grasses (Poaceae) make up about 57% of all known C4 plants. The amaranth family (Amaranthaceae), which includes pigweeds, spinach relatives, and many desert-adapted shrubs, contributes around 17%. Sedges (Cyperaceae) account for roughly 13%. The remaining C4 species are scattered across 20 other families, but those three groups contain the bulk of the diversity.
This scattered distribution is a clue to how C4 photosynthesis arose. It didn’t evolve once and spread. Instead, it evolved independently dozens of times across unrelated plant lineages, always arriving at a similar solution to the same underlying problem.
How C4 Photosynthesis Works
All plants need to capture carbon dioxide from the air and feed it to an enzyme called Rubisco, which builds it into sugars. The problem is that Rubisco is slow and easily confused. In hot conditions, it frequently grabs oxygen instead of CO2, triggering a wasteful process called photorespiration that can reduce a plant’s photosynthetic output by 20% to 50%.
C4 plants solve this by adding a preliminary step. In their outer leaf cells (mesophyll cells), a faster, more selective enzyme grabs CO2 first and attaches it to a small molecule, creating a four-carbon acid (hence “C4”). That acid is then shuttled inward to a ring of inner cells (bundle sheath cells) wrapped tightly around the leaf veins. There, the acid releases its CO2 directly to Rubisco in a small, sealed compartment where CO2 concentrations are high and oxygen is low.
This two-step relay acts as a biological CO2 pump. Rubisco works in an environment where it almost never makes the oxygen mistake, so photorespiration is virtually eliminated. The physical arrangement is essential: the bundle sheath and mesophyll cells are packed closely together and connected by large numbers of tiny channels that allow molecules to pass between them rapidly.
Why C4 Plants Thrive in Heat and Drought
Because C4 plants concentrate CO2 internally, they don’t need to keep their leaf pores (stomata) open as wide or as long to get enough carbon. That means they lose less water through evaporation. The result is striking: C4 plants use roughly half as much water per unit of growth as C3 plants. Their nitrogen efficiency follows the same pattern, with C4 plants producing about twice as much biomass per unit of nitrogen compared to C3 species.
These advantages are most pronounced in hot, sunny, and seasonally dry environments. That’s why C4 grasses dominate tropical and subtropical grasslands, savannas, and open agricultural land in warm climates. In cooler or shadier conditions, the extra energy cost of running the CO2 pump erases the benefit, which is why C4 plants are relatively rare in forests, high mountains, and northern latitudes.
When C4 Photosynthesis Evolved
C4 photosynthesis first appeared in grasses between 25 and 32 million years ago, during the Oligocene epoch. The trigger was a long decline in atmospheric CO2 levels over the preceding tens of millions of years. As CO2 dropped, Rubisco’s oxygen problem became worse, creating strong evolutionary pressure for a workaround. The falling CO2 lowered the temperature at which C4 photosynthesis becomes advantageous from above 40°C down to roughly 17 to 21°C, opening a huge range of warm environments where C4 plants could outcompete their C3 relatives.
Molecular evidence shows at least four independent origins of C4 photosynthesis in different grass lineages around this time, with C4 broadleaf plants (eudicots) appearing somewhat later, between 14 and 21 million years ago. The repeated, independent evolution of essentially the same solution across dozens of plant lineages is one of the most striking examples of convergent evolution in biology.
Engineering C4 Traits Into Other Crops
Rice, the staple food for roughly half the world’s population, uses the less efficient C3 pathway. An international research effort known as the C4 Rice Project has been working to install the C4 machinery into rice, which could substantially boost yields in tropical growing conditions. Researchers have identified the core enzymes and membrane transporters needed, and have successfully introduced five key C4 genes into rice plants that can now produce one of the critical intermediate molecules (malate) in the pathway.
The project has also made progress on getting these genes to turn on in the right cell types, a major challenge since the whole system depends on strict separation between mesophyll and bundle sheath cell functions. Certain gene promoters from C4 species can direct proteins to accumulate specifically in bundle sheath or mesophyll cells when placed in rice. Overexpressing specific regulatory genes from corn in rice has increased the chloroplast content of bundle sheath cells and improved the connectivity between cell types. Still, the full assembly of a working C4 pathway in rice remains incomplete. Just crossing the necessary gene combinations into a single rice variety has taken nearly six years in one phase of the project, and more genes for metabolite transporters are still being added.