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] Moreover, almost all C4 plants are herbaceus, with the notable exception of some woody species from the Euphorbia genus, such as the tree Euphorbia olowaluana.[6] The reason behind C4 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 C4 metabolism itself), to less efficient sunflecks utilization.[7]
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 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][12]
The composite family Asteraceae contains three C4 lineages, in two different tribes of subfamily Asteroideae.[1][20] They include the model genusFlaveria with closely related C3, C4, and intermediate species.[1]
The borage family Boraginaceae contains one widespread C4 genus, Euploca, which has also been treated as part of a distinct family Heliotropiaceae.[24]
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][26][27]
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]
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][8]
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]
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]
^Fisher, A.E.; McDade, L.A.; Kiel, C.A.; et al. (2015). "Evolutionary history of Blepharis (Acanthaceae) and the origin of C4 photosynthesis in section Acanthodium". International Journal of Plant Sciences. 176 (8): 770–790. doi:10.1086/683011. ISSN1058-5893.
^Bohley, K.; Joos, O.; Hartmann, H.; et al. (2015). "Phylogeny of Sesuvioideae (Aizoaceae) – biogeography, leaf anatomy and the evolution of C4 photosynthesis". Perspectives in Plant Ecology, Evolution and Systematics. 17 (2): 116–130. doi:10.1016/j.ppees.2014.12.003. ISSN1433-8319.
^ abcdeSage, R.F.; Sage, T.L.; Pearcy, R.W.; Borsch, T. (2007). "The taxonomic distribution of C4 photosynthesis in Amaranthaceae sensu stricto". American Journal of Botany. 94 (12): 1992–2003. doi:10.3732/ajb.94.12.1992. ISSN0002-9122. PMID21636394.
^Kadereit, G.; Mavrodiev, E.V.; Zacharias, E.H.; Sukhorukov, A.P. (2010). "Molecular phylogeny of Atripliceae (Chenopodioideae, Chenopodiaceae): Implications for systematics, biogeography, flower and fruit evolution, and the origin of C4 photosynthesis". American Journal of Botany. 97 (10): 1664–1687. doi:10.3732/ajb.1000169. ISSN0002-9122. PMID21616801.
^Kapralov, M.V.; Akhani, H.; Voznesenskaya, E.V.; Edwards, G.; Franceschi, V.; Roalson, E.H. (2006). "Phylogenetic Relationships in the Salicornioideae / Suaedoideae / Salsoloideae s.l. (Chenopodiaceae) clade and a clarification of the phylogenetic position of Bienertia and Alexandra using multiple DNA sequence datasets". Systematic Botany. 31 (3): 571–585. doi:10.1600/036364406778388674. ISSN0363-6445.
^ abPyankov, V.; Ziegler, H.; Kuz'min, A.; Edwards, G. (2001). "Origin and evolution of C4 photosynthesis in the tribe Salsoleae (Chenopodiaceae) based on anatomical and biochemical types in leaves and cotyledons". Plant Systematics and Evolution. 230 (1–2): 43–74. Bibcode:2001PSyEv.230...43P. doi:10.1007/s006060170004. ISSN0378-2697. S2CID25370346.
^ abAkhani, H.; Edwards, G.; Roalson, E.H. (2007). "Diversification of the Old World Salsoleae s.l. (Chenopodiaceae): Molecular phylogenetic analysis of nuclear and chloroplast data sets and a revised classification". International Journal of Plant Sciences. 168 (6): 931–956. doi:10.1086/518263. ISSN1058-5893.
^Bissinger, K.; Khoshravesh, R.; Kotrade, J.P.; et al. (2014). "Gisekia (Gisekiaceae): Phylogenetic relationships, biogeography, and ecophysiology of a poorly known C4 lineage in the Caryophyllales". American Journal of Botany. 101 (3): 499–509. doi:10.3732/ajb.1300279. ISSN0002-9122. PMID24638165.
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