c4 carbon fixation

The first experiments indicating that some plants do not use {{C3}} carbon fixation but instead produce malate and aspartate in the first step of carbon fixation were done in the 1950s and early 1960s by Hugo Peter Kortschak and Yuri Karpilov.{{cite journal | vauthors = Nickell LG | title = A tribute to Hugo P. Kortschak: The man, the scientist and the discoverer of C4 photosynthesis | journal = Photosynthesis Research | volume = 35 | issue = 2 | pages = 201–4 | date = February 1993 | pmid = 24318687 | doi = 10.1007/BF00014751 | bibcode = 1993PhoRe..35..201N | s2cid = 40107210 }}{{cite journal | vauthors = Hatch MD | title = C₄ photosynthesis: discovery and resolution | journal = Photosynthesis Research | volume = 73 | issue = 1–3 | pages = 251–6 | year = 2002 | pmid = 16245128 | doi = 10.1023/A:1020471718805 | bibcode = 2002PhoRe..73..251H | s2cid = 343310 }} The {{C4}} pathway was elucidated by Marshall Davidson Hatch and Charles Roger Slack, in Australia, in 1966. While Hatch and Slack originally referred to the pathway as the "C₄ dicarboxylic acid pathway", it is sometimes called the Hatch–Slack pathway.

File:Cross section of maize, a C4 plant..jpg leaf, a {{C4}} plant. Kranz anatomy (rings of cells) shown]]

{{C4}} plants often possess a characteristic leaf anatomy called kranz anatomy, from the German word for wreath. Their vascular bundles are surrounded by two rings of cells; the inner ring, called bundle sheath cells, contains starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. The primary function of kranz anatomy is to provide a site in which {{CO2}} can be concentrated around RuBisCO, thereby avoiding photorespiration. Mesophyll and bundle sheath cells are connected through numerous cytoplasmic sleeves called plasmodesmata whose permeability at leaf level is called bundle sheath conductance. A layer of suberin{{cite book|last=Laetsch|url=https://archive.org/details/photosynthesisph0000hatc|title=Photosynthesis and Photorespiration|date=1971|publisher=New York, Wiley-Interscience|isbn=9780471359005 |editor1-last=Hatch|editor2-last=Osmond|editor3-last=Slatyer |url-access=registration}} is often deposed at the level of the middle lamella (tangential interface between mesophyll and bundle sheath) in order to reduce the apoplastic diffusion of {{CO2}} (called leakage). The carbon concentration mechanism in {{C4}} plants distinguishes their isotopic signature from other photosynthetic organisms.

Although most {{C4}} plants exhibit kranz anatomy, there are, however, a few species that operate a limited {{C4}} cycle without any distinct bundle sheath tissue. Suaeda aralocaspica, Bienertia cycloptera, Bienertia sinuspersici and Bienertia kavirense (all chenopods) are terrestrial plants that inhabit dry, salty depressions in the deserts of the Middle East. These plants have been shown to operate single-cell {{C4}} {{CO2}}-concentrating mechanisms, which are unique among the known {{C4}} mechanisms.{{cite journal| vauthors = Freitag H, Stichler W |year=2000|title=A remarkable new leaf type with unusual photosynthetic tissue in a central Asiatic genus of Chenopodiaceae|journal=Plant Biology|volume=2|issue=2|pages=154–160|doi=10.1055/s-2000-9462|bibcode=2000PlBio...2..154F |s2cid=260250537 }}{{cite journal | vauthors = Voznesenskaya EV, Franceschi VR, Kiirats O, Artyusheva EG, Freitag H, Edwards GE | title = Proof of C4 photosynthesis without Kranz anatomy in Bienertia cycloptera (Chenopodiaceae) | journal = The Plant Journal | volume = 31 | issue = 5 | pages = 649–62 | date = September 2002 | pmid = 12207654 | doi = 10.1046/j.1365-313X.2002.01385.x | s2cid = 14742876 | doi-access = free }}{{cite journal| vauthors = Akhani H, Barroca J, Koteeva N, Voznesenskaya E, Franceschi V, Edwards G, Ghaffari SM, Ziegler H |year=2005|title=Bienertia sinuspersici (Chenopodiaceae): A New Species from Southwest Asia and Discovery of a Third Terrestrial {{C4}} Plant Without Kranz Anatomy|journal=Systematic Botany|volume=30|issue=2|pages=290–301|doi=10.1600/0363644054223684|s2cid=85946307}}{{cite journal| vauthors = Akhani H, Chatrenoor T, Dehghani M, Khoshravesh R, Mahdavi P, Matinzadeh Z |year=2012|title=A new species of Bienertia (Chenopodiaceae) from Iranian salt deserts: a third species of the genus and discovery of a fourth terrestrial {{C4}} plant without Kranz anatomy|journal=Plant Biosystems|volume=146|pages=550–559|doi=10.1080/11263504.2012.662921|s2cid=85377740}} Although the cytology of both genera differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two separate areas. Carboxylation enzymes in the cytosol are separated from decarboxylase enzymes and RuBisCO in the chloroplasts. A diffusive barrier is between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath-type area and a mesophyll-type area to be established within a single cell. Although this does allow a limited {{C4}} cycle to operate, it is relatively inefficient. Much leakage of {{CO2}} from around RuBisCO occurs.

There is also evidence of inducible {{C4}} photosynthesis by non-kranz aquatic macrophyte Hydrilla verticillata under warm conditions, although the mechanism by which {{CO2}} leakage from around RuBisCO is minimised is currently uncertain.{{cite journal | vauthors = Holaday AS, Bowes G | title = C(4) Acid Metabolism and Dark CO(2) Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata) | journal = Plant Physiology | volume = 65 | issue = 2 | pages = 331–5 | date = February 1980 | pmid = 16661184 | pmc = 440321 | doi = 10.1104/pp.65.2.331 }}

In {{C3}} plants, the first step in the light-independent reactions of photosynthesis is the fixation of {{CO2}} by the enzyme RuBisCO to form 3-phosphoglycerate. However, RuBisCo has a dual carboxylase and oxygenase activity. Oxygenation results in part of the substrate being oxidized rather than carboxylated, resulting in loss of substrate and consumption of energy, in what is known as photorespiration. Oxygenation and carboxylation are competitive, meaning that the rate of the reactions depends on the relative concentration of oxygen and {{CO2}}.

In order to reduce the rate of photorespiration, {{C4}} plants increase the concentration of {{CO2}} around RuBisCO. To do so two partially isolated compartments differentiate within leaves, the mesophyll and the bundle sheath. Instead of direct fixation by RuBisCO, {{CO2}} is initially incorporated into a four-carbon organic acid (either malate or aspartate) in the mesophyll. The organic acids then diffuse through plasmodesmata into the bundle sheath cells. There, they are decarboxylated creating a {{CO2}}-rich environment. The chloroplasts of the bundle sheath cells convert this {{CO2}} into carbohydrates by the conventional {{C3}} pathway.

There is large variability in the biochemical features of C4 assimilation, and it is generally grouped in three subtypes, differentiated by the main enzyme used for decarboxylation ( NADP-malic enzyme, NADP-ME; NAD-malic enzyme, NAD-ME; and PEP carboxykinase, PEPCK). Since PEPCK is often recruited atop NADP-ME or NAD-ME it was proposed to classify the biochemical variability in two subtypes. For instance, maize and sugarcane use a combination of NADP-ME and PEPCK, millet uses preferentially NAD-ME and Megathyrsus maximus, uses preferentially PEPCK.

File:C4 photosynthesis NADP-ME type en.svgThe first step in the NADP-ME type {{C4}} pathway is the conversion of pyruvate (Pyr) to phosphoenolpyruvate (PEP), by the enzyme Pyruvate phosphate dikinase (PPDK). This reaction requires inorganic phosphate and ATP plus pyruvate, producing PEP, AMP, and inorganic pyrophosphate (PP_i). The next step is the carboxylation of PEP by the PEP carboxylase enzyme (PEPC) producing oxaloacetate. Both of these steps occur in the mesophyll cells:

:pyruvate + P_i + ATP → PEP + AMP + PP_i

:PEP + {{CO2}} → oxaloacetate

PEPC has a low K_M for {{chem|HCO|3|-|link=Bicarbonate}} — and, hence, high affinity, and is not confounded by O₂ thus it will work even at low concentrations of {{CO2}}.

The product is usually converted to malate (M), which diffuses to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated by the NADP-malic enzyme (NADP-ME) to produce {{CO2}} and pyruvate. The {{CO2}} is fixed by RuBisCo to produce phosphoglycerate (PGA) while the pyruvate is transported back to the mesophyll cell, together with about half of the phosphoglycerate (PGA). This PGA is chemically reduced in the mesophyll and diffuses back to the bundle sheath where it enters the conversion phase of the Calvin cycle. For each {{CO2}} molecule exported to the bundle sheath the malate shuttle transfers two electrons, and therefore reduces the demand of reducing power in the bundle sheath.

File:C4 photosynthesis NAD-ME type.svg

Here, the OAA produced by PEPC is transaminated by aspartate aminotransferase to aspartate (ASP) which is the metabolite diffusing to the bundle sheath. In the bundle sheath ASP is transaminated again to OAA and then undergoes a futile reduction and oxidative decarboxylation to release {{CO2}}. The resulting Pyruvate is transaminated to alanine, diffusing to the mesophyll. Alanine is finally transaminated to pyruvate (PYR) which can be regenerated to PEP by PPDK in the mesophyll chloroplasts. This cycle bypasses the reaction of malate dehydrogenase in the mesophyll and therefore does not transfer reducing equivalents to the bundle sheath.

File:C4 photosynthesis PEPCK type.svg

In this variant the OAA produced by aspartate aminotransferase in the bundle sheath is decarboxylated to PEP by PEPCK. The fate of PEP is still debated. The simplest explanation is that PEP would diffuse back to the mesophyll to serve as a substrate for PEPC. Because PEPCK uses only one ATP molecule, the regeneration of PEP through PEPCK would theoretically increase photosynthetic efficiency of this subtype, however this has never been measured. An increase in relative expression of PEPCK has been observed under low light, and it has been proposed to play a role in facilitating balancing energy requirements between mesophyll and bundle sheath.

While in {{C3}} photosynthesis each chloroplast is capable of completing light reactions and dark reactions, {{C4}} chloroplasts differentiate in two populations, contained in the mesophyll and bundle sheath cells. The division of the photosynthetic work between two types of chloroplasts results inevitably in a prolific exchange of intermediates between them. The fluxes are large and can be up to ten times the rate of gross assimilation.{{cite journal | vauthors = Bellasio C | title = A generalized stoichiometric model of C3, C2, C2+C4, and C4 photosynthetic metabolism | journal = Journal of Experimental Botany | volume = 68 | issue = 2 | pages = 269–282 | date = January 2017 | pmid = 27535993 | pmc = 5853385 | doi = 10.1093/jxb/erw303 | url = }} The type of metabolite exchanged and the overall rate will depend on the subtype. To reduce product inhibition of photosynthetic enzymes (for instance PECP) concentration gradients need to be as low as possible. This requires increasing the conductance of metabolites between mesophyll and bundle sheath, but this would also increase the retro-diffusion of {{CO2}} out of the bundle sheath, resulting in an inherent and inevitable trade off in the optimisation of the {{CO2}} concentrating mechanism.

To meet the NADPH and ATP demands in the mesophyll and bundle sheath, light needs to be harvested and shared between two distinct electron transfer chains. ATP may be produced in the bundle sheath mainly through cyclic electron flow around Photosystem I, or in the mesophyll mainly through linear electron flow depending on the light available in the bundle sheath or in the mesophyll. The relative requirement of ATP and NADPH in each type of cells will depend on the photosynthetic subtype. The apportioning of excitation energy between the two cell types will influence the availability of ATP and NADPH in the mesophyll and bundle sheath. For instance, green light is not strongly adsorbed by mesophyll cells and can preferentially excite bundle sheath cells, or vice versa for blue light.{{Citation| vauthors = Evans JR, Vogelmann TC, von Caemmerer S |title=Balancing light capture with distributed metabolic demand during C4 photosynthesis|date=2008-03-01|url=https://www.worldscientific.com/doi/abs/10.1142/9789812709523_0008|work=Charting New Pathways to C4 Rice|pages=127–143|publisher=WORLD SCIENTIFIC|doi=10.1142/9789812709523_0008|isbn=978-981-270-951-6|access-date=2020-10-12 |url-access=subscription}} Because bundle sheaths are surrounded by mesophyll, light harvesting in the mesophyll will reduce the light available to reach bundle sheath cells. Also, the bundle sheath size limits the amount of light that can be harvested.{{cite journal | vauthors = Bellasio C, Lundgren MR | title = Anatomical constraints to C4 evolution: light harvesting capacity in the bundle sheath | journal = The New Phytologist | volume = 212 | issue = 2 | pages = 485–96 | date = October 2016 | pmid = 27375085 | doi = 10.1111/nph.14063 | doi-access = free }}

Different formulations of efficiency are possible depending on which outputs and inputs are considered. For instance, average quantum efficiency is the ratio between gross assimilation and either absorbed or incident light intensity. Large variability of measured quantum efficiency is reported in the literature between plants grown in different conditions and classified in different subtypes but the underpinnings are still unclear. One of the components of quantum efficiency is the efficiency of dark reactions, biochemical efficiency, which is generally expressed in reciprocal terms as ATP cost of gross assimilation (ATP/GA).

In {{C3}} photosynthesis ATP/GA depends mainly on {{CO2}} and O₂ concentration at the carboxylating sites of RuBisCO. When {{CO2}} concentration is high and O₂ concentration is low photorespiration is suppressed and {{C3}} assimilation is fast and efficient, with ATP/GA approaching the theoretical minimum of 3.

In {{C4}} photosynthesis {{CO2}} concentration at the RuBisCO carboxylating sites is mainly the result of the operation of the {{CO2}} concentrating mechanisms, which cost circa an additional 2 ATP/GA but makes efficiency relatively insensitive of external {{CO2}} concentration in a broad range of conditions.

Biochemical efficiency depends mainly on the speed of {{CO2}} delivery to the bundle sheath, and will generally decrease under low light when PEP carboxylation rate decreases, lowering the ratio of {{CO2}}/O₂ concentration at the carboxylating sites of RuBisCO. The key parameter defining how much efficiency will decrease under low light is bundle sheath conductance. Plants with higher bundle sheath conductance will be facilitated in the exchange of metabolites between the mesophyll and bundle sheath and will be capable of high rates of assimilation under high light. However, they will also have high rates of {{CO2}} retro-diffusion from the bundle sheath (called leakage) which will increase photorespiration and decrease biochemical efficiency under dim light. This represents an inherent and inevitable trade off in the operation of {{C4}} photosynthesis. {{C4}} plants have an outstanding capacity to attune bundle sheath conductance. Interestingly, bundle sheath conductance is downregulated in plants grown under low light{{cite journal | vauthors = Bellasio C, Griffiths H | title = Acclimation to low light by C4 maize: implications for bundle sheath leakiness | journal = Plant, Cell & Environment | volume = 37 | issue = 5 | pages = 1046–58 | date = May 2014 | pmid = 24004447 | doi = 10.1111/pce.12194 | doi-access = free | bibcode = 2014PCEnv..37.1046B }} and in plants grown under high light subsequently transferred to low light as it occurs in crop canopies where older leaves are shaded by new growth.{{cite journal | vauthors = Bellasio C, Griffiths H | title = Acclimation of C4 metabolism to low light in mature maize leaves could limit energetic losses during progressive shading in a crop canopy | journal = Journal of Experimental Botany | volume = 65 | issue = 13 | pages = 3725–36 | date = July 2014 | pmid = 24591058 | pmc = 4085954 | doi = 10.1093/jxb/eru052 | url = }}

{{Further information|Evolutionary history of plants#Evolution of photosynthetic pathways}}

{{C4}} plants have a competitive advantage over plants possessing the more common {{C3}} carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or {{CO2}} limitation. When grown in the same environment, at 30 °C, {{C3}} grasses lose approximately 833 molecules of water per {{CO2}} molecule that is fixed, whereas {{C4}} grasses lose only 277. This increased water use efficiency of {{C4}} grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.{{cite book|last1=Sage|first1=Rowan|first2=Russell|last2=Monson | name-list-style = vanc |title={{C4}} Plant Biology|year=1999|pages=228–229|chapter=7|publisher=Elsevier |isbn=978-0-12-614440-6|chapter-url=https://books.google.com/books?id=H7Wv9ZImW-QC&pg=PA228}}

{{C4}} carbon fixation has evolved in at least 62 independent occasions in 19 different families of plants, making it a prime example of convergent evolution.{{cite journal| author=Sage RF, Christin PA, Edwards EJ| title=The C(4) plant lineages of planet Earth. | journal=J Exp Bot | year= 2011 | volume= 62 | issue= 9 | pages= 3155–69 | pmid=21414957 | doi=10.1093/jxb/err048 | pmc= | url=https://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=21414957 }} {{Cite journal|last=Sage|first=Rowan F. | name-list-style = vanc |date=2004-02-01|title=The evolution of {{C4}} photosynthesis|journal=New Phytologist|language=en|volume=161|issue=2|pages=341–370|doi=10.1111/j.1469-8137.2004.00974.x|pmid=33873498 |issn=1469-8137|doi-access=free}} This convergence may have been facilitated by the fact that many potential evolutionary pathways to a {{C4}} phenotype exist, many of which involve initial evolutionary steps not directly related to photosynthesis.{{cite journal | vauthors = Williams BP, Johnston IG, Covshoff S, Hibberd JM | title = Phenotypic landscape inference reveals multiple evolutionary paths to C4 photosynthesis | journal = eLife | volume = 2 | pages = e00961 | date = September 2013 | pmid = 24082995 | pmc = 3786385 | doi = 10.7554/eLife.00961 | doi-access = free }} {{C4}} plants arose around {{Ma|35|}} during the Oligocene (precisely when is difficult to determine) and were becoming ecologically significant in the early Miocene around {{Ma|21|}}.[https://www.discovermagazine.com/the-sciences/wooded-grasslands-flourished-in-africa-21-million-years-ago-new-research Wooded Grasslands Flourished in Africa 21 Million Years Ago – New Research Forces a Rethink of Ape Evolution] {{C4}} metabolism in grasses originated when their habitat migrated from the shady forest undercanopy to more open environments,{{cite journal | vauthors = Edwards EJ, Smith SA | title = Phylogenetic analyses reveal the shady history of C4 grasses | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 6 | pages = 2532–7 | date = February 2010 | pmid = 20142480 | pmc = 2823882 | doi = 10.1073/pnas.0909672107 | bibcode = 2010PNAS..107.2532E | doi-access = free }} where the high sunlight gave it an advantage over the {{C3}} pathway. Drought was not necessary for its innovation; rather, the increased parsimony in water use was a byproduct of the pathway and allowed {{C4}} plants to more readily colonize arid environments.{{cite journal | vauthors = Osborne CP, Freckleton RP | title = Ecological selection pressures for C4 photosynthesis in the grasses | journal = Proceedings. Biological Sciences | volume = 276 | issue = 1663 | pages = 1753–60 | date = May 2009 | pmid = 19324795 | pmc = 2674487 | doi = 10.1098/rspb.2008.1762 }}

Today, {{C4}} plants represent about 5% of Earth's plant biomass and 3% of its known plant species.{{cite journal | vauthors = Bond WJ, Woodward FI, Midgley GF | title = The global distribution of ecosystems in a world without fire | journal = The New Phytologist | volume = 165 | issue = 2 | pages = 525–37 | date = February 2005 | pmid = 15720663 | doi = 10.1111/j.1469-8137.2004.01252.x | bibcode = 2005NewPh.165..525B | s2cid = 4954178 }} Despite this scarcity, they account for about 23% of terrestrial carbon fixation.{{cite journal | vauthors = Osborne CP, Beerling DJ | title = Nature's green revolution: the remarkable evolutionary rise of C4 plants | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1465 | pages = 173–94 | date = January 2006 | pmid = 16553316 | pmc = 1626541 | doi = 10.1098/rstb.2005.1737 | author-link2 = David Beerling }}{{cite journal | vauthors = Kellogg EA | title = C4 photosynthesis | journal = Current Biology | volume = 23 | issue = 14 | pages = R594-9 | date = July 2013 | pmid = 23885869 | doi = 10.1016/j.cub.2013.04.066 | author-link = Elizabeth Anne Kellogg | doi-access = free }} Increasing the proportion of {{C4}} plants on earth could assist biosequestration of {{CO2}} and represent an important climate change avoidance strategy. Present-day {{C4}} plants are concentrated in the tropics and subtropics (below latitudes of 45 degrees) where the high air temperature increases rates of photorespiration in {{C3}} plants.

{{see also|List of C4 plants|l1=List of {{C4}} plants}}

About 8,100 plant species use {{C4}} carbon fixation, which represents about 3% of all terrestrial species of plants.{{cite journal | vauthors = Sage RF | title = A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame | journal = Journal of Experimental Botany | volume = 67 | issue = 14 | pages = 4039–56 | date = July 2016 | pmid = 27053721 | doi = 10.1093/jxb/erw156 | doi-access = free }} All these 8,100 species are angiosperms. {{C4}} carbon fixation is more common in monocots compared with dicots, with 40% of monocots using the {{C4}} pathway{{Clarify|reason=Numbers don't agree: monocots include 60,000 species, 40% of that far exceeds 8,100|date=January 2023}}, compared with only 4.5% of dicots. Despite this, only three families of monocots use {{C4}} carbon fixation compared to 15 dicot families. Of the monocot clades containing {{C4}} plants, the grass (Poaceae) species use the {{C4}} photosynthetic pathway most. 46% of grasses are {{C4}} and together account for 61% of {{C4}} species. {{C4}} has arisen independently in the grass family some twenty or more times, in various subfamilies, tribes, and genera,{{cite journal | vauthors = Grass Phylogeny Working Group II | title = New grass phylogeny resolves deep evolutionary relationships and discovers C4 origins | journal = The New Phytologist | volume = 193 | issue = 2 | pages = 304–12 | date = January 2012 | pmid = 22115274 | doi = 10.1111/j.1469-8137.2011.03972.x | hdl-access = free | hdl = 2262/73271 }} {{open access}} including the Andropogoneae tribe which contains the food crops maize, sugar cane, and sorghum. Teff is also {{C4}}, as are various kinds of millet. {{Cite book| title= Encyclopedia of Food Grains| last= Bultosa|first=G.|publisher=Academic Press| year=2016| isbn= 9780123947864| editor-last= Wrigley| editor-first=Colin W. |edition=2nd| location= Kidlington, Oxford, UK| pages= 209 ff| chapter=Teff: Overview| oclc=939553708|quote=Teff is a C4 self-pollinated tetraploid cereal plant with a chromosome number of 2n=4x=20.|editor-last2=Corke|editor-first2=Harold|editor-last3= Seetharaman|editor-first3= Koushik| editor-last4=Faubion|editor-first4= Jonathan|chapter-url= https://books.google.com/books?id=ce7tBgAAQBAJ&pg=PA209}}{{cite book|last1=Sage |first1=Rowan|first2=Russell |last2=Monson | name-list-style = vanc |title={{C4}} Plant Biology|year=1999 |pages=551–580 |chapter=16 |publisher=Elsevier |isbn=978-0-12-614440-6 |chapter-url=https://books.google.com/books?id=H7Wv9ZImW-QC&pg=PA551}}{{cite journal | vauthors = Zhu XG, Long SP, Ort DR | title = What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? | journal = Current Opinion in Biotechnology | volume = 19 | issue = 2 | pages = 153–9 | date = April 2008 | pmid = 18374559 | doi = 10.1016/j.copbio.2008.02.004 | url = https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=36097&content=PDF | access-date = 29 December 2018 | archive-date = 1 April 2019 | archive-url = https://web.archive.org/web/20190401014953/https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=36097&content=PDF | url-status = dead | url-access = subscription }} Of the dicot clades containing {{C4}} species, the order Caryophyllales contains the most species. Of the families in the Caryophyllales, the Chenopodiaceae use {{C4}} carbon fixation the most, with 550 out of 1,400 species using it. About 250 of the 1,000 species of the related Amaranthaceae also use {{C4}}.{{cite journal| vauthors = Kadereit G, Borsch T, Weising K, Freitag H |title=Phylogeny of Amaranthaceae and Chenopodiaceae and the Evolution of {{C4}} Photosynthesis|year=2003|journal=International Journal of Plant Sciences|volume=164|issue=6|pages=959–986|doi=10.1086/378649|bibcode=2003IJPlS.164..959K |s2cid=83564261}}

Members of the sedge family Cyperaceae, and members of numerous families of eudicots – including Asteraceae (the daisy family), Brassicaceae (the cabbage family), and Euphorbiaceae (the spurge family) – also use {{C4}}.

No large trees (above 15 m in height) use {{C4}},{{cite journal |last1=Sage |first1=R.F. |date=May 2001 |title=Environmental and Evolutionary Preconditions for the Origin and Diversification of the C4 Photosynthetic Syndrome |journal=Plant Biology |volume=3|issue=3 |pages=202–213 |doi=10.1055/s-2001-15206|bibcode=2001PlBio...3..202S }} however a number of small trees or shrubs smaller than 10 m exist which do: six species of Euphorbiaceae all native to Hawaii and two species of Amaranthaceae growing in deserts of the Middle-East and Asia.{{cite journal |last1=Young |first1=Sophie N R |last2=Sack |first2=Lawren |last3=Sporck-Koehler |first3=Margaret J |last4=Lundgren |first4=Marjorie R |date=6 August 2020 |title=Why is C4 photosynthesis so rare in trees? |url=https://academic.oup.com/jxb/article/71/16/4629/5837361 |journal=Journal of Experimental Botany |volume=71 |issue=16 |pages=4629–4638 |doi=10.1093/jxb/eraa234|pmid=32409834 |pmc=7410182 }}

==Genetic engineering {{C4}} rice project==

Given the advantages of {{C4}}, a group of scientists from institutions around the world are working on the {{C4}} Rice Project to produce a strain of rice, naturally a {{C3}} plant, that uses the {{C4}} pathway by studying the {{C4}} plants maize and Brachypodium.{{cite journal | vauthors = Slewinski TL, Anderson AA, Zhang C, Turgeon R | title = Scarecrow plays a role in establishing Kranz anatomy in maize leaves | journal = Plant & Cell Physiology | volume = 53 | issue = 12 | pages = 2030–7 | date = December 2012 | pmid = 23128603 | doi = 10.1093/pcp/pcs147 | doi-access = free }} As rice is the world's most important human food—it is the staple food for more than half the planet—having rice that is more efficient at converting sunlight into grain could have significant global benefits towards improving food security. The team claims {{C4}} rice could produce up to 50% more grain—and be able to do it with less water and nutrients.{{cite news|url=https://www.theguardian.com/science/2012/jan/24/scientists-boost-rice-crop-yield|title=Researchers aim to flick the high-carbon switch on rice |author=Gilles van Kote |work=The Guardian |date=2012-01-24 |access-date=2012-11-10}}{{cite journal | vauthors = von Caemmerer S, Quick WP, Furbank RT | title = The development of C₄ rice: current progress and future challenges | journal = Science | volume = 336 | issue = 6089 | pages = 1671–2 | date = June 2012 | pmid = 22745421 | doi = 10.1126/science.1220177 | s2cid = 24534351 | bibcode = 2012Sci...336.1671V }}{{cite journal | vauthors = Hibberd JM, Sheehy JE, Langdale JA | title = Using C4 photosynthesis to increase the yield of rice-rationale and feasibility | journal = Current Opinion in Plant Biology | volume = 11 | issue = 2 | pages = 228–31 | date = April 2008 | pmid = 18203653 | doi = 10.1016/j.pbi.2007.11.002 | author-link1 = Julian Hibberd | author-link3 = Jane A. Langdale }}

The researchers have already identified genes needed for {{C4}} photosynthesis in rice and are now looking towards developing a prototype {{C4}} rice plant. In 2012, the Government of the United Kingdom along with the Bill & Melinda Gates Foundation provided US$14 million over three years towards the {{C4}} Rice Project at the International Rice Research Institute.{{cite news|url=http://www.thenews.com.pk/Todays-News-3-141210-C4-rice-project-gets-financial-boost|title={{C4}} rice project gets financial boost|first=Munawan|last=Hasan|name-list-style=vanc|publisher=The News|date=2012-11-06|access-date=2012-11-10|archive-date=2012-11-10|archive-url=https://web.archive.org/web/20121110132715/http://www.thenews.com.pk/Todays-News-3-141210-C4-rice-project-gets-financial-boost|url-status=dead}} In 2019, the Bill & Melinda Gates Foundation granted another US$15 million to the Oxford-University-led C4 Rice Project. The goal of the 5-year project is to have experimental field plots up and running in Taiwan by 2024.{{cite web | url = https://www.ox.ac.uk/news/2019-12-03-rice-feed-world-given-funding-boost | date = 3 December 2019 | title = Rice to feed the world given a funding boost | publisher = University of Oxford | access-date = 29 January 2022}}

C₂ photosynthesis, an intermediate step between {{C3}} and Kranz {{C4}}, may be preferred over {{C4}} for rice conversion. The simpler system is less optimized for high light and high temperature conditions than {{C4}}, but has the advantage of requiring fewer steps of genetic engineering and performing better than {{C3}} under all temperatures and light levels.{{cite journal |last1=Bellasio |first1=Chandra |last2=Farquhar |first2=Graham D. |title=A leaf-level biochemical model simulating the introduction of C 2 and C 4 photosynthesis in C 3 rice: gains, losses and metabolite fluxes |journal=New Phytologist |date=July 2019 |volume=223 |issue=1 |pages=150–166 |doi=10.1111/nph.15787|pmid=30859576 |s2cid=75139004 |doi-access=free |hdl=1885/159508 |hdl-access=free }} In 2021, the UK Government provided £1.2 million on studying C₂ engineering.{{cite web |title=Enhancing crops with C2 photosynthesis |url=https://gtr.ukri.org/projects?ref=MR%2FT043970%2F1 |website=GtR}}

• C₂ photosynthesis
• CAM photosynthesis
• {{C3}} photosynthesis

{{Reflist|30em}}

• [http://www.khanacademy.org/video/c-4-photosynthesis?playlist=Biology Khan Academy, video lecture]

{{Poaceae-navbox}}

{{Authority control}}

Category:Photosynthesis