In botany, C₄ carbon fixation is one of three known methods of photosynthesis used by plants. C₄ plants increase their photosynthetic efficiency by reducing or suppressing photorespiration, which mainly occurs under low atmospheric CO₂ concentration, high light, high temperature, drought, and salinity.^[2][3] There are roughly 8,100 known C₄ species, which belong to at least 61 distinct evolutionary lineages in 19 families (as per APG IV classification^[4]) of flowering plants.^[1] Among these are important crops such as maize, sorghum and sugarcane, but also weeds and invasive plants.^[1] Although only 3% of flowering plant species use C₄ carbon fixation, they account for 23% of global primary production.^[5] The repeated, convergent C₄ evolution from C₃ ancestors has spurred hopes to bio-engineer the C₄ pathway into C₃ crops such as rice.^[1][5]

C₄ photosynthesis probably first evolved 30–35 million years ago in the Oligocene, and further origins occurred since, most of them in the last 15 million years. C₄ plants are mainly found in tropical and warm-temperate regions, predominantly in open grasslands where they are often dominant. While most are graminoids, other growth forms such as forbs, vines, shrubs, and even some trees and aquatic plants are also known among C₄ plants.^[1]

C₄ plants are usually identified by their higher ¹³C/¹²C isotopic ratio compared to C₃ plants or their typical leaf anatomy.^[5] The distribution of C₄ lineages among plants has been determined through phylogenetics and was considered well known as of 2016^[update]. Monocots – mainly grasses (Poaceae) and sedges (Cyperaceae) – account for around 80% of C₄ species, but they are also found in the eudicots.^[1]

The following list presents known C₄ lineages by family, based on the overview by Sage (2016).^[1] They correspond to single species or clades thought to have acquired the C₄ pathway independently. In some lineages that also include C₃ and C₃–C₄ intermediate species, the C₄ pathway may have evolved more than once.^[1]

The large acanthus family Acanthaceae includes one genus with C₄ species, found in dry habitats from Africa to Asia.^[8]

• Blepharis – 15 C₄ species, 1–4 origins

While many species in the ice plant family Aizoaceae use crassulacean acid metabolism (CAM), one subfamily with drought-tolerant and halophytic plants includes C₄ species:^[9]

• Sesuvioideae – 30 C₄ species, 1–6 origins

The amaranth family Amaranthaceae (including the former goosefoot family Chenopodiaceae) contains around 800 known C₄ species, which belong to 14 distinct lineages in seven subfamilies. This makes Amaranthaceae the family with most C₄ species and lineages among the eudicots.^[1] Suaeda aralocaspica and species of the genus Bienertia use a particular, single-cell type of C₄ carbon fixation.^[1][10]

• Aerva (Amaranthoideae) – 4 C₄ species^[11]
• Alternanthera (Gomphrenoideae) – 17 C₄ species (also includes C₃–C₄ intermediates)^[11]
• Amaranthus (Amaranthoideae) – 90 C₄ species^[11]
• Atriplex (Chenopodioideae) – around 180 C₄ species^[12]
• Bassia–Camphorosma clade (Camphorosmoideae) – 24 C₄ species (also includes one C₃–C₄ intermediate), 1–2 origins^[13]
• Bienertia (Suaedoideae) – 3 C₄ species^[10][14]
• Caroxyleae (syn. Caroxyloneae, Salsoloideae) – 157 C₄ species^[15][16]
• Gomphrenoids (Gomphrenoideae) – 138 C₄ species^[11]
• Salsoleae (Salsoloideae) – 158 C₄ species, 2–4 origins^[15][16]
• Suaeda aralocaspica (Suaedoideae)^[10]
• Suaeda sect. Salsina – 30 C₄ species^[10]
• Suaeda sect. Schoberia – 9 C₄ species^[10]
• Tecticornia – (Salicornioideae) 2 C₄ species^[17]
• Tidestromia (Gomphrenoideae) – 8 C₄ species^[11]

The composite family Asteraceae contains three C₄ lineages, in two different tribes of subfamily Asteroideae.^[1][18] They include the model genus Flaveria with closely related C₃, C₄, and intermediate species.^[1]

• Flaveria (Tageteae) – 7 C₄ species, 2–3 origins (also includes C₃ and intermediate)^[1][19][20]
• Coreopsideae – 41 C₄ species^[18]
• Pectis (Tageteae) – 90 C₄ species^[21]

The borage family Boraginaceae contains one widespread C₄ genus, Euploca, which has also been treated as part of a distinct family Heliotropiaceae.^[22]

• Euploca (also includes C₃–C₄ intermediates^[23]) – 130 C₄ species, 1–3 origins^[1]

The Cleomaceae, formerly included in the caper family Capparaceae, contains three C₄ species in genus Cleome. These three species independently acquired the C₄ pathway; the genus also contains numerous C₃ as well as C₃–C₄ intermediate species.^[1][24][25]

• Cleome angustifolia
• C. gynandra
• C. oxalidea

In the carnation family Caryophyllaceae, the C₄ pathway evolved once, in a clade within the polyphyletic genus Polycarpaea.^[1][26]

• Polycarpaea – 20 C₄ species

The sedge family Cyperaceae is second only to the grasses in number of C₄ species. Prominent C₄ sedges include culturally important species such as papyrus (Cyperus papyrus) and chufa (C. esculentus) but also purple nutsedge (C. rotundus), one of the world's major weeds. Eleocharis vivipara uses C₃ carbon fixation in underwater leaves and C₄ carbon fixation in aerial leaves.^[1]

• Bulbostylis – 211 C₄ species
• Cyperus – 757 C₄ species
• Eleocharis ser. Tenuissimae – 10 C₄ species
• Eleocharis vivipara
• Fimbristylis – 303 C₄ species
• Rhynchospora – 40 C₄ species

The spurge family Euphorbiaceae contains the largest single C₄ lineage among eudicots. The C₄ spurges are diverse and widespread; they range from weedy herbs to the only known C₄ trees – four species from Hawaii, including Euphorbia olowaluana (up to 10 m) and E. herbstii (up to 8 m).^[1][6]

• Euphorbia subgenus Chamaesyce section Anisophyllum (also treated as genus Chamaesyce) – 350 C₄ species (also including C₃ and C₃–C₄ intermediate species)

Contains a C₄ genus with a single species.

• Gisekia pharnaceoides^[27]

Includes the only known aquatic C₄ plants.^[1]

• Egeria densa
• Hydrilla verticillata

The two C₄ species within the same genus have acquired the pathway independently.

• Mollugo – 2 C₄ species, 2 origins

• Allionia – 2 C₄ species
• Boerhavia – 42 C₄ species

• Calligonum – 80 C₄ species

The single genus of this family forms one C₄ lineage. CAM photosynthesis is also known. Common purslane (Portulaca oleracea) is a major weed but also a vegetable.^[1]

• Portulaca – 100 C₄ species, 1–2 origins

The grass family includes most of the known C₄ species – around 5000. They are only found in subfamilies of the PACMAD clade. Major C₄ crops such as maize, sugarcane, sorghum and pearl millet belong in this family. The only known species with C₃, C₄ and intermediate variants, Alloteropsis semialata, is a grass.^[1]

• Aristida – 288 C₄ species
• Stipagrostis – 56 C₄ species
• Chloridoideae (without Centropodieae) – 1596 C₄ species
• Centropodia – 4 C₄ species
• Eriachne – 50 C₄ species
• Tristachyideae – 87 C₄ species
• Andropogoneae – 1228 C₄ species (incl. maize, sugarcane, sorghum)
• Reynaudia filiformis
• Axonopus – 90 C₄ species
• Paspalum – 379 C₄ species
• Anthaenantia – 4 C₄ species
• Arthropoginae/Mesosetum clade – 35 C₄ species, 1–2 origins
• Arthropoginae/Onchorachis clade – 2 C₄ species
• Arthropoginae/Colaeteania clade – 7 C₄ species, 1–2 origins
• Anthephorinae – 286 C₄ species
• Echinochloa – 35 C₄ species
• Neurachne–Paraneurachne – 2 C₄ species, 2 origins
• Melinidinae–Panicinae–Cenchrinae – 889 C₄ species
• Alloteropsis – 5 C₄ species, 1–2 origins (incl. C₃ and intermediate)

• Anticharis – 4 C₄ species

• Tribuloideae – 37 C₄ species, 1–2 origins
• Tetraena simplex