C4 plants use the C4 carbon fixation pathway to 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 C4evolution 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. 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 C4pathway independently. In some lineages that also include C3 and C3–C4 intermediate species, the C4 pathway may have evolved more than once.