# List of C4 plants

In botany, C$_4$ carbon fixation is one of three known methods of photosynthesis used by plants. C$_4$ plants increase their photosynthetic efficiency by reducing or suppressing photorespiration, which mainly occurs under low atmospheric CO$_2$ concentration, high light, high temperature, drought, and salinity. There are roughly 8,100 known C$_4$ species, which belong to at least 61 distinct evolutionary lineages in 19 families (as per APG IV classification) of flowering plants. Among these are important crops such as maize, sorghum and sugarcane, but also weeds and invasive plants. Although only 3% of flowering plant species use C$_4$ carbon fixation, they account for 23% of global primary production. The repeated, convergent C$_4$ evolution from C$_3$ ancestors has spurred hopes to bio-engineer the C$_4$ pathway into C$_3$ crops such as rice.
C$_4$ 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. C$_4$ 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 C$_4$ plants.
C$_4$ plants are usually identified by their higher $^{13}$C/$^{12}$C isotopic ratio compared to C$_3$ plants or their typical leaf anatomy. The distribution of C$_4$ lineages among plants has been determined through phylogenetics and was considered well known as of 2016. Monocots – mainly grasses (Poaceae) and sedges (Cyperaceae) – account for around 80% of C$_4$ species, but they are also found in the eudicots. Moreover, almost all C$_4$ plants are herbaceus, with the notable exception of some woody species from the Euphorbia genus, such as the tree Euphorbia olowaluana. The reason behind C$_4$ metabolism extreme rarity in trees is debated: hypotheses vary from a possible reduction in photosynthetic quantum yield under dense canopy conditions, coupled with an increased metabolic energy consumption (inherent to C$_4$ metabolism itself), to less efficient sunflecks utilization.
The following list presents known C$_4$ lineages by family, based on the overview by Sage (2016). They correspond to single species or clades thought to have acquired the C$_4$ pathway independently. In some lineages that also include C$_3$ and C$_3$–C$_4$ intermediate species, the C$_4$ pathway may have evolved more than once.

## Acanthaceae

The large acanthus family Acanthaceae includes one genus with C$_4$ species, found in dry habitats from Africa to Asia.
* Blepharis – 15 C$_4$ species, 1–4 origins

## Aizoaceae

While many species in the ice plant family Aizoaceae use crassulacean acid metabolism (CAM), one subfamily with drought-tolerant and halophytic plants includes C$_4$ species:
* Sesuvioideae – 30 C$_4$ species, 1–6 origins

## Amaranthaceae

The amaranth family Amaranthaceae (including the former goosefoot family Chenopodiaceae) contains around 800 known C$_4$ species, which belong to 14 distinct lineages in seven subfamilies. This makes Amaranthaceae the family with most C$_4$ species and lineages among the eudicots. Suaeda aralocaspica and species of the genus Bienertia use a particular, single-cell type of C$_4$ carbon fixation.
* Aerva (Amaranthoideae) – 4 C$_4$ species
* Alternanthera (Gomphrenoideae) – 17 C$_4$ species (also includes C$_3$–C$_4$ intermediates)
* Amaranthus (Amaranthoideae) – 90 C$_4$ species
* Atriplex (Chenopodioideae) – around 180 C$_4$ species
* Bassia–Camphorosma clade (Camphorosmoideae) – 24 C$_4$ species (also includes one C$_3$–C$_4$ intermediate), 1–2 origins
* Bienertia (Suaedoideae) – 3 C$_4$ species
* Caroxyleae (syn. Caroxyloneae, Salsoloideae) – 157 C$_4$ species
* Gomphrenoids (Gomphrenoideae) – 138 C$_4$ species
* Salsoleae (Salsoloideae) – 158 C$_4$ species, 2–4 origins
* Suaeda aralocaspica (Suaedoideae)
* Suaeda sect. Salsina – 30 C$_4$ species
* Suaeda sect. Schoberia – 9 C$_4$ species
* Tecticornia – (Salicornioideae) 2 C$_4$ species
* Tidestromia (Gomphrenoideae) – 8 C$_4$ species

## Asteraceae

The composite family Asteraceae contains three C$_4$ lineages, in two different tribes of subfamily Asteroideae. They include the model genus Flaveria with closely related C$_3$, C$_4$, and intermediate species.
* Flaveria (Tageteae) – 7 C$_4$ species, 2–3 origins (also includes C$_3$ and intermediate)
* Coreopsideae – 41 C$_4$ species
* Pectis (Tageteae) – 90 C$_4$ species

## Boraginaceae

The borage family Boraginaceae contains one widespread C$_4$ genus, Euploca, which has also been treated as part of a distinct family Heliotropiaceae.
* Euploca (also includes C$_3$–C$_4$ intermediates) – 130 C$_4$ species, 1–3 origins

## Cleomaceae

The Cleomaceae, formerly included in the caper family Capparaceae, contains three C$_4$ species in genus Cleome. These three species independently acquired the C$_4$ pathway; the genus also contains numerous C$_3$ as well as C$_3$–C$_4$ intermediate species.
* Cleome angustifolia
* C. gynandra
* C. oxalidea

## Caryophyllaceae

In the carnation family Caryophyllaceae, the C$_4$ pathway evolved once, in a clade within the polyphyletic genus Polycarpaea.
* Polycarpaea – 20 C$_4$ species

## Cyperaceae

The sedge family Cyperaceae is second only to the grasses in number of C$_4$ species. Prominent C$_4$ 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 C$_3$ carbon fixation in underwater leaves and C$_4$ carbon fixation in aerial leaves.
* Bulbostylis – 211 C$_4$ species
* Cyperus – 757 C$_4$ species
* Eleocharis ser. Tenuissimae – 10 C$_4$ species
* Eleocharis vivipara
* Fimbristylis – 303 C$_4$ species
* Rhynchospora – 40 C$_4$ species

## Euphorbiaceae

The spurge family Euphorbiaceae contains the largest single C$_4$ lineage among eudicots. The C$_4$ spurges are diverse and widespread; they range from weedy herbs to the only known C$_4$ trees – four species from Hawaii, including Euphorbia olowaluana (up to 10 m) and E. herbstii (up to 8 m).
* Euphorbia subgenus Chamaesyce section Anisophyllum (also treated as genus Chamaesyce) – 350 C$_4$ species (also including C$_3$ and C$_3$–C$_4$ intermediate species)

## Gisekiaceae

Contains a C$_4$ genus with a single species.
* Gisekia pharnaceoides

## Hydrocharitaceae

Includes the only known aquatic C$_4$ plants.
* Egeria densa
* Hydrilla verticillata

## Molluginaceae

The two C$_4$ species within the same genus have acquired the pathway independently.
* Mollugo – 2 C$_4$ species, 2 origins

## Nyctaginaceae

* Allionia – 2 C$_4$ species
* Boerhavia – 42 C$_4$ species

## Polygonaceae

* Calligonum – 80 C$_4$ species

## Portulacaceae

The single genus of this family forms one C$_4$ lineage. CAM photosynthesis is also known. Common purslane (Portulaca oleracea) is a major weed but also a vegetable.
* Portulaca – 100 C$_4$ species, 1–2 origins

## Poaceae

The grass family includes most of the known C$_4$ species – around 5000. They are only found in subfamilies of the PACMAD clade. Major C$_4$ crops such as maize, sugarcane, sorghum and pearl millet belong in this family. The only known species with C$_3$, C$_4$ and intermediate variants, Alloteropsis semialata, is a grass.
* Aristida – 288 C$_4$ species
* Stipagrostis – 56 C$_4$ species
* Chloridoideae (without Centropodieae) – 1596 C$_4$ species
* Centropodia – 4 C$_4$ species
* Eriachne – 50 C$_4$ species
* Tristachyideae – 87 C$_4$ species
* Andropogoneae – 1228 C$_4$ species (incl. maize, sugarcane, sorghum)
* Reynaudia filiformis
* Axonopus – 90 C$_4$ species
* Paspalum – 379 C$_4$ species
* Anthaenantia – 4 C$_4$ species
* Arthropoginae/Mesosetum clade – 35 C$_4$ species, 1–2 origins
* Arthropoginae/Onchorachis clade – 2 C$_4$ species
* Arthropoginae/Colaeteania clade – 7 C$_4$ species, 1–2 origins
* Anthephorinae – 286 C$_4$ species
* Echinochloa – 35 C$_4$ species
* Neurachne–Paraneurachne – 2 C$_4$ species, 2 origins
* Melinidinae–Panicinae–Cenchrinae – 889 C$_4$ species
* Alloteropsis – 5 C$_4$ species, 1–2 origins (incl. C$_3$ and intermediate)

## Scrophulariaceae

* Anticharis – 4 C$_4$ species

## Zygophyllaceae

* Tribuloideae – 37 C$_4$ species, 1–2 origins
* Tetraena simplex