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{{Short description|In plants, a variable pore between paired guard cells}}
{{distinguish|Stroma (disambiguation){{!}}Stroma}}
{{for|natural and surgically created body openings|Stoma (medicine)}}
[[File:Tomato leaf stomate 1-color.jpg|thumb|Stoma in a [[tomato]] leaf shown via colorized [[scanning electron microscope]] image]]
[[File:HPIM0188-ligusterblad.jpg|thumb|A stoma in horizontal cross section]]
[[File:LeafUndersideWithStomata.jpg|thumb|The underside of a leaf. In this species (''[[Tradescantia zebrina]]''), the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigments.]]

In [[botany]], a '''stoma''' ({{plural form}}: '''stomata''', from [[Greek language|Greek]] ''στόμα'', "mouth"), also called a '''stomate''' ({{plural form}}: '''stomates'''), is a pore found in the [[Epidermis (botany)|epidermis]] of leaves, stems, and other organs, that controls the rate of [[gas exchange]] between the internal air spaces of the leaf and the atmosphere. The pore is bordered by a pair of specialized [[Ground tissue#Parenchyma|parenchyma]] cells known as [[guard cell]]s that regulate the size of the stomatal opening.

The term is usually used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, which is referred to as the stomatal aperture.<ref name="Esau">{{cite book | last=Esau | first=K. | year=1977 | title=Anatomy of Seed Plants | url=https://archive.org/details/anatomyofseedpla00esau | url-access=registration | publisher=Wiley and Sons | page=[https://archive.org/details/anatomyofseedpla00esau/page/88 88] | isbn=978-0-471-24520-9 }}</ref> Air, containing [[oxygen]], which is used in [[cellular respiration|respiration]], and [[carbon dioxide]], which is used in [[photosynthesis]], passes through stomata by gaseous [[diffusion]]. [[Water vapour]] diffuses through the stomata into the atmosphere as part of a process called [[transpiration]].

Stomata are present in the [[sporophyte]] generation of the vast majority of [[land plants]], with the exception of [[Marchantiophyta|liverworts]], as well as some [[mosses]] and [[hornworts]]. In vascular plants the number, size and distribution of stomata varies widely. [[Dicotyledon]]s usually have more stomata on the lower surface of the leaves than the upper surface. [[Monocotyledon]]s such as [[onion]], [[oat]] and [[maize]] may have about the same number of stomata on both leaf surfaces.<ref name=Weyers>{{cite book |first1=J. D. B. |last1=Weyers |first2=H. |last2=Meidner |date=1990 |title=Methods in stomatal research |publisher=Longman Group UK Ltd. |isbn=978-0-582-03483-9 }}</ref>{{rp|5}} In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely. Most tree species have stomata only on the lower leaf surface.<ref name="Willmer-1996">{{Cite book|title=Stomata|last1=Willmer|first1=Colin|last2=Fricker|first2=Mark|publisher=Springer|year=1996|isbn=978-94-010-4256-7|page=16|language=en|doi=10.1007/978-94-011-0579-8|s2cid=224833888 }}</ref> Leaves with stomata on both the upper and lower leaf surfaces are called ''amphistomatous'' leaves; leaves with stomata only on the lower surface are ''hypostomatous'', and leaves with stomata only on the upper surface are epistomatous or ''hyperstomatous''.<ref name="Willmer-1996" /> Size varies across species, with end-to-end lengths ranging from 10 to 80 [[micrometre|μm]] and width ranging from a few to 50&nbsp;μm.<ref name="Fricker">{{cite book | last1=Fricker | first1=M. | last2=Willmer | first2=C. | title=Stomata | publisher=Springer Netherlands | year=2012 | isbn=978-94-011-0579-8 | url=https://books.google.com/books?id=9sjoCAAAQBAJ&pg=PA18 | access-date=15 June 2016 | page=18}}</ref>

==Function==
[[File:Stoma with Accompanying Guard Cells.jpg|thumb|upright=1.1|Electron micrograph of a stoma from a [[bok choy]] (''Brassica chinensis'') leaf]]

===CO<sub>2</sub> gain and water loss===
[[Carbon dioxide]], a key reactant in [[photosynthesis]], is present in the atmosphere at a concentration of about 425 ppm. Most plants require the stomata to be open during daytime. The air spaces in the [[leaf]] are saturated with [[water vapour]], which exits the leaf through the stomata in a process known as [[transpiration]]. Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.<ref>Debbie Swarthout and C.Michael Hogan. 2010. [http://www.eoearth.org/article/Stomata ''Stomata''. Encyclopedia of Earth]. National Council for Science and the Environment, Washington DC</ref>

===Alternative approaches===
Ordinarily, carbon dioxide is fixed to [[ribulose 1,5-bisphosphate]] (RuBP) by the enzyme [[RuBisCO]] in [[mesophyll]] cells exposed directly to the air spaces inside the leaf. This exacerbates the transpiration problem for two reasons: first, RuBisCo has a relatively low affinity for carbon dioxide, and second, it fixes oxygen to RuBP, wasting energy and carbon in a process called [[photorespiration]]. For both of these reasons, RuBisCo needs high carbon dioxide concentrations, which means wide stomatal apertures and, as a consequence, high water loss.

Narrower stomatal apertures can be used in conjunction with an intermediary molecule with a high carbon dioxide affinity, [[phosphoenolpyruvate carboxylase]] (PEPcase). Retrieving the products of carbon fixation from PEPCase is an energy-intensive process, however. As a result, the PEPCase alternative is preferable only where water is limiting but light is plentiful, or where high temperatures increase the solubility of oxygen relative to that of carbon dioxide, magnifying RuBisCo's oxygenation problem.

===C.A.M. plants===
[[File:Differences in Stomata Opening Throughout the Day for C3 plants and CAM plants (1).svg|thumb|C3 and C4 plants(1) stomata stay open all day and close at night. CAM plants(2) stomata open during the morning and close slightly at noon and then open again in the evening.]]

A group of mostly desert plants called "C.A.M." plants ([[crassulacean acid metabolism]], after the family Crassulaceae, which includes the species in which the CAM process was first discovered) open their stomata at night (when water evaporates more slowly from leaves for a given degree of stomatal opening), use PEPcase to fix carbon dioxide and store the products in large vacuoles. The following day, they close their stomata and release the carbon dioxide fixed the previous night into the presence of RuBisCO. This saturates RuBisCO with carbon dioxide, allowing minimal photorespiration. This approach, however, is severely limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is severely limited.

===Opening and closing===
{{further|Guard cell}}
[[File:Opening and Closing of Stoma.svg|upright=1.9|thumb|Opening and closing of stoma]]

However, most plants do not have CAM and must therefore open and close their stomata during the daytime, in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity), a [[proton pump]] drives [[proton]]s (H<sup>+</sup>) from the guard cells. This means that the cells' [[electrical potential]] becomes increasingly negative. The negative potential opens potassium voltage-gated channels and so an uptake of [[potassium]] ions (K<sup>+</sup>) occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. In some cases, chloride ions enter, while in other plants the organic ion [[malate]] is produced in guard cells. This increase in solute concentration lowers the [[water potential]] inside the cell, which results in the diffusion of water into the cell through [[osmosis]]. This increases the cell's volume and [[osmotic pressure|turgor pressure]]. Then, because of rings of cellulose [[microfibrils]] that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held firmly in place by surrounding [[epidermis (botany)|epidermal]] cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can diffuse.<ref>{{cite journal
|journal=Annals of Botany
|volume=89
|issue=1
|date=January 2002
|pages=23–29
|title=Structure and Development of Stomata on the Primary Root of ''Ceratonia siliqua'' L.
|author=N. S. CHRISTODOULAKIS |author2=J. MENTI |author3=B. GALATIS
|pmid=12096815 | doi = 10.1093/aob/mcf002
|pmc=4233769}}</ref>

When the roots begin to sense a water shortage in the soil, [[abscisic acid]] (ABA) is released.<ref>{{cite journal
|journal=Plant Physiology
|volume=102
|issue=2
|year=1993
|pages=497–502
|title=Sensitivity of Stomata to Abscisic Acid (An Effect of the Mesophyll)
|author=C. L. Trejo
|author2=W. J. Davies |author3=LdMP. Ruiz
|pmid=12231838
|pmc=158804 
|doi=10.1104/pp.102.2.497}}</ref> ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the [[cytosol]] of the cells and cause the concentration of free Ca<sup>2+</sup> to increase in the cytosol due to influx from outside the cell and release of Ca<sup>2+</sup> from internal stores such as the endoplasmic reticulum and vacuoles.<ref>{{cite journal
|journal=Journal of Experimental Botany
|volume=52
|issue=363
|pages=1959–1967
|date=October 2001
|title=The role of ion channels in light-dependent stomatal opening
|author=Petra Dietrich
|author2=Dale Sanders |author3=Rainer Hedrich
 |pmid=11559731
|doi=10.1093/jexbot/52.363.1959 |doi-access=free
}}</ref> This causes the chloride (Cl<sup>−</sup>) and organic ions to exit the cells. Second, this stops the uptake of any further K<sup>+</sup> into the cells and, subsequently, the loss of K<sup>+</sup>. The loss of these solutes causes an increase in [[water potential]], which results in the diffusion of water back out of the cell by [[osmosis]]. This makes the cell [[plasmolysed]], which results in the closing of the stomatal pores.

Guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.<ref>{{cite web |url=http://6e.plantphys.net/essay10.01.html |title=Guard Cell Photosynthesis |access-date=2015-10-04 }}</ref><ref>{{cite journal
|title=The Guard Cell Chloroplast: A Perspective for the Twenty-First Century
|author=Eduardo Zeiger
|author2=Lawrence D. Talbott |author3=Silvia Frechilla |author4=Alaka Srivastava |author5= Jianxin Zhu
|journal=New Phytologist
|volume=153
|issue=3 Special Issue: Stomata
|date=March 2002
|pages=415–424
|doi=10.1046/j.0028-646X.2001.NPH328.doc.x
|pmid=33863211
|bibcode=2002NewPh.153..415Z
}}</ref>

===Inferring stomatal behavior from gas exchange===
The degree of stomatal resistance can be determined by measuring leaf gas exchange of a leaf. The [[transpiration]] rate is dependent on the [[diffusion]] resistance provided by the stomatal pores and also on the [[humidity]] gradient between the leaf's internal air spaces and the outside air. Stomatal resistance (or its inverse, [[stomatal conductance]]) can therefore be calculated from the transpiration rate and humidity gradient. This allows scientists to investigate how stomata respond to changes in environmental conditions, such as light intensity and concentrations of gases such as water vapor, carbon dioxide, and [[ozone]].<ref>{{cite journal |first=Michael |last=Hopkin |title=Carbon sinks threatened by increasing ozone |journal=Nature |volume=448 |pages=396–397 |date=2007-07-26 |bibcode=2007Natur.448..396H |doi=10.1038/448396b |issue=7152 |pmid=17653153|doi-access=free }}</ref> Evaporation (''E'') can be calculated as<ref name=calculations>{{cite web |url=http://4e.plantphys.net/article.php?ch=9&id=134 |title=Calculating Important Parameters in Leaf Gas Exchange |work=Plant Physiology Online |publisher=Sinauer |access-date=2013-02-24 |archive-date=2008-06-16 |archive-url=https://web.archive.org/web/20080616013217/http://4e.plantphys.net/article.php?ch=9&id=134 }}</ref>

: <math>E = \frac{e_\text{i} - e_\text{a}}{Pr},</math>

where ''e''<sub>i</sub> and ''e''<sub>a</sub> are the partial pressures of water in the leaf and in the ambient air respectively, ''P'' is atmospheric pressure, and ''r'' is stomatal resistance.
The inverse of ''r'' is conductance to water vapor (''g''), so the equation can be rearranged to<ref name=calculations/>

: <math>E = (e_\text{i} - e_\text{a})g / P</math>

and solved for ''g'':<ref name=calculations/>

: <math>g = \frac{EP}{e_\text{i} - e_\text{a}}.</math>

Photosynthetic CO<sub>2</sub> assimilation (''A'') can be calculated from

: <math>A = \frac{(C_\text{a} - C_\text{i})g}{1.6P},</math>

where ''C''<sub>a</sub> and ''C''<sub>i</sub> are the atmospheric and sub-stomatal partial pressures of CO<sub>2</sub> respectively{{clarify|reason=what is "1.6"?|date=March 2023}}. The rate of evaporation from a leaf can be determined using a [[photosynthesis system]]. These scientific instruments measure the amount of water vapour leaving the leaf and the vapor pressure of the ambient air. Photosynthetic systems may calculate [[water use efficiency]] (''A''/''E''), ''g'', intrinsic water use efficiency (''A''/''g''), and ''C''<sub>i</sub>. These scientific instruments are commonly used by plant physiologists to measure CO<sub>2</sub> uptake and thus measure photosynthetic rate.<ref>{{cite journal
|journal=Photosynthesis Research
|volume=9
|issue=3
|date=January 1986
|pages=345–357
|title=A system for measuring leaf gas exchange based on regulating vapour pressure difference
|author=Waichi Agata
|author2=Yoshinobu Kawamitsu
|author3=Susumu Hakoyama
|author4=Yasuo Shima
|doi=10.1007/BF00029799
|issn=1573-5079
|pmid=24442366
|bibcode=1986PhoRe...9..345A
|s2cid=28367821
}}</ref><ref>{{citation
|title=Portable Gas Exchange Fluorescence System GFS-3000. Handbook of Operation
|date=March 20, 2013
|url=http://www.walz.com/downloads/manuals/gfs-3000/gfs-3000_Manual_8a.pdf
|access-date=October 20, 2014
|archive-date=December 15, 2017
|archive-url=https://web.archive.org/web/20171215121153/http://www.walz.com/downloads/manuals/gfs-3000/gfs-3000_Manual_8a.pdf
}}</ref>

==Evolution==
[[File:Tomato stoma observed through immersion oil.gif|thumb|Tomato stoma observed through immersion oil]]
There is little evidence of the evolution of stomata in the fossil record, but they had appeared in land plants by the middle of the Silurian period.<ref>{{Cite journal
 | last1 = D. Edwards | first1 = H. Kerp
 | last2 = Hass | first2 = H.
 | year = 1998
 | title = Stomata in early land plants: an anatomical and ecophysiological approach
 | journal = Journal of Experimental Botany
 | volume = 49
 | pages = 255–278
 | doi = 10.1093/jxb/49.Special_Issue.255
 | issue = Special Issue
| doi-access = free
 }}</ref> They may have evolved by the modification of [[conceptacles]] from plants' alga-like ancestors.<ref>{{cite book
 | last = Krassilov | first = Valentin A.
 | chapter = Macroevolutionary events and the origin of higher taxa
 | pages=265–289
 | chapter-url=https://books.google.com/books?id=tJeZC885-OcC&pg=PA265
 | isbn = 978-1-4020-1693-6
 | editor-first = Solomon P.
 | editor-last= Wasser
 | year = 2004
 | publisher = Kluwer Acad. Publ.
 | location = Dordrecht
 | title = Evolutionary theory and processes : modern horizons: papers in honour of Eviatar Nevo }}</ref>
However, the evolution of stomata must have happened at the same time as the waxy [[Plant cuticle|cuticle]] was evolving&nbsp;– these two traits together constituted a major advantage for early terrestrial plants.{{citation needed|date=May 2017}}

==Development==
There are three major epidermal cell types which all ultimately derive from the outermost (L1) tissue layer of the [[shoot apical meristem]], called protodermal cells: [[trichome]]s, [[pavement cells]] and guard cells, all of which are arranged in a non-random fashion.

An asymmetrical cell division occurs in protodermal cells resulting in one large cell that is fated to become a pavement cell and a smaller cell called a meristemoid that will eventually differentiate into the guard cells that surround a stoma. This meristemoid then divides asymmetrically one to three times before differentiating into a guard mother cell. The guard mother cell then makes one symmetrical division, which forms a pair of guard cells.<ref name="Bergmann-2004">{{Cite journal|last1=Bergmann|first1=Dominique C.; Lukowitz, Wolfgang; Somerville, Chris R.|last2=Lukowitz|first2=W|last3=Somerville|first3=CR|date=4 July 2004|title=Stomatal Development and Pattern Controlled by a MAPKK Kinase|url=http://www.sciencemag.org/cgi/content/ful/304/5676/1494/DC1|journal=Science|volume=304|issue=5676|pages=1494–1497|doi=10.1126/science.1096014|pmid=15178800|bibcode=2004Sci...304.1494B|s2cid=32009729|url-access=subscription}}</ref> Cell division is inhibited in some cells so there is always at least one cell between stomata.<ref name="Pillitteri-2013">{{Cite journal|last1=Pillitteri|first1=Lynn Jo|last2=Dong|first2=Juan|date=2013-06-06|title=Stomatal Development in Arabidopsis|journal=The Arabidopsis Book|volume=11|article-number=e0162|doi=10.1199/tab.0162|issn=1543-8120|pmc=3711358|pmid=23864836}}</ref>

Stomatal patterning is controlled by the interaction of many [[signal transduction]] components such as ''EPF'' (Epidermal Patterning Factor), ''ERL'' (ERecta Like) and ''YODA'' (a putative [[MAP kinase kinase kinase]]).<ref name="Pillitteri-2013" /> Mutations in any one of the genes which encode these factors may alter the development of stomata in the epidermis.<ref name="Pillitteri-2013" /> For example, a mutation in one gene causes more stomata that are clustered together, hence is called Too Many Mouths (''TMM'').<ref name="Bergmann-2004" /> Whereas, disruption of the ''SPCH'' (SPeecCHless) gene prevents stomatal development all together.<ref name="Pillitteri-2013" /> &nbsp;Inhibition of stomatal production can occur by the activation of EPF1, which activates TMM/ERL, which together activate YODA. YODA inhibits SPCH, causing SPCH activity to decrease, preventing asymmetrical cell division that initiates stomata formation.<ref name="Pillitteri-2013" /><ref>{{Cite journal|last1=Casson|first1=Stuart A|last2=Hetherington|first2=Alistair M|date=2010-02-01|title=Environmental regulation of stomatal development|journal=Current Opinion in Plant Biology|volume=13|issue=1|pages=90–95|doi=10.1016/j.pbi.2009.08.005|pmid=19781980|bibcode=2010COPB...13...90C }}</ref> Stomatal development is also coordinated by the cellular peptide signal called stomagen, which signals the activation of the SPCH, resulting in increased number of stomata.<ref>{{Cite journal|last1=Sugano|first1=Shigeo S.|last2=Shimada|first2=Tomoo|last3=Imai|first3=Yu|last4=Okawa|first4=Katsuya|last5=Tamai|first5=Atsushi|last6=Mori|first6=Masashi|last7=Hara-Nishimura|first7=Ikuko|date=2010-01-14|title=Stomagen positively regulates stomatal density in Arabidopsis|journal=Nature|language=en|volume=463|issue=7278|pages=241–244|doi=10.1038/nature08682|pmid=20010603|issn=0028-0836|bibcode=2010Natur.463..241S|hdl=2433/91250|s2cid=4302041|hdl-access=free}}</ref>

Environmental and hormonal factors can affect stomatal development. Light increases stomatal development in plants; while, plants grown in the dark have a lower amount of stomata. [[Auxin]] represses stomatal development by affecting their development at the receptor level like the ERL and TMM receptors. However, a low concentration of auxin allows for equal division of a guard mother cell and increases the chance of producing guard cells.<ref>{{cite journal | last1 = Balcerowicz | first1 = M. | last2 = Ranjan | first2 = A. | last3 = Rupprecht | first3 = L. | last4 = Fiene | first4 = G. | last5 = Hoecker | first5 = U. | year = 2014 | title = Auxin represses stomatal development in dark-grown seedling via Aux/IAA proteins | journal = Development | volume = 141 | issue = 16| pages = 3165–3176 | doi=10.1242/dev.109181| pmid = 25063454 | doi-access = free }}</ref>

Most [[angiosperm trees]] have stomata only on their lower leaf surface. [[Populus|Poplars]] and [[willows]] have them on both surfaces. When leaves develop stomata on both leaf surfaces, the stomata on the lower surface tend to be larger and more numerous, but there can be a great degree of variation in size and frequency about species and genotypes. [[Fraxinus americana|White ash]] and [[white birch]] leaves had fewer stomata but larger in size. On the other hand [[sugar maple]] and [[silver maple]] had small stomata that were more numerous.<ref>{{cite journal |last1=Pallardy |first1=Stephen |title=Physiology of Woody Plants |journal=Journal of Applied Ecology |date=1983 |volume=20 |issue=1 |page=14|doi=10.2307/2403413 |jstor=2403413 |bibcode=1983JApEc..20..352J }}</ref>

== Types ==
Different classifications of stoma types exist. One that is widely used is based on the types that [[Julien Joseph Vesque]] introduced in 1889, was further developed by Metcalfe and Chalk,<ref>{{cite book|first1= C.R.|last1= Metcalfe|first2= L.|last2= Chalk|year= 1950|title= Anatomy of Dicotyledons|volume= 1: Leaves, Stem, and Wood in relation to Taxonomy, with notes on economic Uses}}</ref> and later complemented by other authors. It is based on the size, shape and arrangement of the subsidiary cells that surround the two guard cells.<ref>{{cite journal|first= W.R.F.|last= van Cotthem|year= 1970|title= A Classification of Stomatal Types|journal= [[Botanical Journal of the Linnean Society]]|volume= 63|issue= 3|pages= 235–246|doi=10.1111/j.1095-8339.1970.tb02321.x|bibcode= 1970BJLS...63..235C|doi-access= }}</ref>
They distinguish for [[Dicotyledon|dicots]]: 
* '''{{vanchor|actinocytic}}''' (meaning ''star-celled'') stomata have guard cells that are surrounded by at least five radiating cells forming a star-like circle. This is a rare type that can for instance be found in the family [[Ebenaceae]].
* '''{{vanchor|anisocytic}}''' (meaning ''unequal celled'') stomata have guard cells between two larger subsidiary cells and one distinctly smaller one. This type of stomata can be found in more than thirty dicot families, including [[Brassicaceae]], [[Solanaceae]], and [[Crassulaceae]]. It is sometimes called ''cruciferous type''.
* '''{{vanchor|anomocytic}}''' (meaning ''irregular celled'') stomata have guard cells that are surrounded by cells that have the same size, shape and arrangement as the rest of the epidermis cells. This type of stomata can be found in more than hundred dicot families such as [[Apocynaceae]], [[Boraginaceae]], [[Chenopodiaceae]], and [[Cucurbitaceae]]. It is sometimes called ''ranunculaceous type''.
* '''{{vanchor|diacytic}}''' (meaning ''cross-celled'') stomata have guard cells surrounded by two subsidiary cells, that each encircle one end of the opening and contact each other opposite to the middle of the opening. This type of stomata can be found in more than ten dicot families such as [[Caryophyllaceae]] and [[Acanthaceae]]. It is sometimes called ''caryophyllaceous type''.
* '''{{vanchor|hemiparacytic}}''' stomata are bordered by just one subsidiary cell that differs from the surrounding epidermis cells, its length parallel to the stoma opening. This type occurs for instance in the [[Molluginaceae]] and [[Aizoaceae]].
* '''{{vanchor|paracytic}}''' (meaning ''parallel celled'') stomata have one or more subsidiary cells parallel to the opening between the guard cells. These subsidiary cells may reach beyond the guard cells or not. This type of stomata can be found in more than hundred dicot families such as [[Rubiaceae]], [[Convolvulaceae]] and [[Fabaceae]]. It is sometimes called ''rubiaceous type''.

In [[Monocotyledon|monocots]], several different types of stomata occur such as: 
* '''gramineous or graminoid''' (meaning ''grass-like'') stomata have two guard cells surrounded by two lens-shaped subsidiary cells. The guard cells are narrower in the middle and bulbous on each end. This middle section is strongly thickened. The axis of the subsidiary cells are parallel stoma opening.<ref>{{Cite journal |last=Nunes |first=Tiago D. G. |last2=Zhang |first2=Dan |last3=Raissig |first3=Michael T. |date=10 February 2020 |title=Form, development and function of grass stomata |url=https://onlinelibrary.wiley.com/doi/10.1111/tpj.14552 |journal=The Plant Journal |language=en |volume=101 |issue=4 |pages=780–799 |doi=10.1111/tpj.14552 |issn=0960-7412|doi-access=free }}</ref> This type can be found in monocot families including [[Poaceae]] and [[Cyperaceae]].<ref>{{cite journal|last1=Nunes|first1=Tiago D. G.|last2=Zhang|first2=Dan|last3=Raissig|first3=Michael T.|date=February 2020|title=Form, development and function of grass stomata|journal=The Plant Journal|volume=101|issue=4|pages=780–799|doi=10.1111/tpj.14552|pmid=31571301|doi-access=free|bibcode=2020PlJ...101..780N }}</ref>
* '''{{vanchor|hexacytic}}''' (meaning ''six-celled'') stomata have six subsidiary cells around both guard cells, one at either end of the opening of the stoma, one adjoining each guard cell, and one between that last subsidiary cell and the standard epidermis cells. This type can be found in some monocot families.
* '''{{vanchor|tetracytic}}''' (meaning ''four-celled'') stomata have four subsidiary cells, one on either end of the opening, and one next to each guard cell. This type occurs in many monocot families, but also can be found in some dicots, such as ''[[Tilia]]'' and several [[Asclepiadaceae]].

In [[fern]]s, four different types are distinguished:
* '''{{vanchor|hypocytic}}''' stomata have two guard cells in one layer with only ordinary epidermis cells, but with two subsidiary cells on the outer surface of the epidermis, arranged parallel to the guard cells, with a pore between them, overlying the stoma opening.
* '''{{vanchor|pericytic}}''' stomata have two guard cells that are entirely encircled by one continuous subsidiary cell (like a donut).
* '''{{vanchor|desmocytic}}''' stomata have two guard cells that are entirely encircled by one subsidiary cell that has not merged its ends (like a sausage).
* '''{{vanchor|polocytic}}''' stomata have two guard cells that are largely encircled by one subsidiary cell, but also contact ordinary epidermis cells (like a U or horseshoe).
A catalogue of leaf epidermis prints showing stomata from a wide range of species can be found in Wikimedia commons https://commons.wikimedia.org/wiki/Category:Leaf_epidermis_and_stomata_prints

== Stomatal crypts ==
Stomatal crypts are sunken areas of the leaf epidermis which form a chamber-like structure that contains one or more stomata and sometimes trichomes or accumulations of [[epicuticular wax|wax]]. Stomatal crypts can be an adaption to drought and dry climate conditions when the stomatal crypts are very pronounced. However, dry climates are not the only places where they can be found. The following plants are examples of species with stomatal crypts or antechambers: [[Nerium|''Nerium oleander'']], conifers, ''Hakea''<ref name=Jordan-2008>{{cite journal|title=The evolutionary relations of sunken, covered, and encrypted stomata to dry habitats in Proteaceae |first1=Gregory J |last1=Jordan |first2=Peter H |last2=Weston |first3=Raymond J |last3=Carpenter |first4=Rebecca A. |last4=Dillon |first5=Timothy J. |last5=Brodribb |date=2008  |journal=American Journal of Botany |volume=95 |issue=5 |pages=521–530|doi=10.3732/ajb.2007333 |pmid=21632378 |bibcode=2008AmJB...95..521J }}</ref> and ''[[Drimys winteri]]'' which is a species of plant found in the [[cloud forest]].<ref>{{cite journal|author1=Roth-Nebelsick, A.|author2=Hassiotou, F.|author3=Veneklaas, E. J|year=2009|title=Stomatal crypts have small effects on transpiration: A numerical model analysis. |journal=Plant Physiology|volume=151|issue=4|pages=2018–2027|doi=10.1104/pp.109.146969|pmid=19864375|pmc=2785996 |bibcode=2009PlanP.151.2018R }}</ref>

==Stomata as pathogenic pathways==
Stomata are holes in the leaf by which pathogens can enter unchallenged. However, stomata can sense the presence of some, if not all, pathogens.<ref name=Underwood2006>{{cite journal
|journal=Cell |volume=126 |pages=969–980 |date=2006|title=Plant Stomata in innate immunity against bacterial invasion |author1=Maeli Melotto |author2=William Underwood |author3=Jessica Koczan |author4=Kinya Nomura |author5=Sheng Yang He |doi=10.1016/j.cell.2006.06.054| pmid=16959575 |issue=5| s2cid=13612107 |doi-access=free }}</ref> However, pathogenic bacteria applied to [[Arabidopsis thaliana|''Arabidopsis'']] plant leaves can release the chemical [[coronatine]], which induce the stomata to reopen.
<ref name=SchulzeLefert2006>{{cite journal |first1=P |last1=Schulze-Lefert |first2=S |last2= Robatzek
|date=2006 |title=Plant pathogens trick guard cells into opening the gates |journal=Cell |volume=126 |issue=5 |pages=831–834 |doi=10.1016/j.cell.2006.08.020 |pmid=16959560 |bibcode=2006Cell..126..831S |hdl=11858/00-001M-0000-0012-394E-B |hdl-access=free }}</ref>

==Stomata and climate change==
===Response of stomata to environmental factors===
[[Photosynthesis]], plant water transport ([[xylem]]) and gas exchange are regulated by stomatal function which is important in the functioning of plants.<ref name="Rico-2013">{{cite journal | last1 = Rico | first1 = C | last2 = Pittermann | first2 = J | last3 = Polley | first3 = HW | last4 = Aspinwall | first4 = MJ | last5 = Fay | first5 = PA | year = 2013 | title = The effect of subambient to elevated atmospheric CO2 concentration on vascular function in Helianthus annuus: implications for plant response to climate change | journal = New Phytologist | volume = 199 | issue = 4| pages = 956–965 | doi=10.1111/nph.12339| pmid = 23731256 | doi-access = free | bibcode = 2013NewPh.199..956R }}</ref>

Stomata are responsive to light with [[Visible spectrum|blue light]] being almost 10 times as effective as [[Red|red light]] in causing stomatal response. Research suggests this is because the light response of stomata to blue light is independent of other leaf components like [[chlorophyll]]. Guard cell [[protoplasts]] swell under blue light provided there is sufficient availability of [[potassium]].<ref>{{cite book |last=McDonald |first=Maurice S. |title=Photobiology of Higher Plants |publisher=Wiley |date=2003 |page=293 |isbn=978-0-470-85523-2}}</ref> Multiple studies have found support that increasing potassium concentrations may increase stomatal opening in the mornings, before the photosynthesis process starts, but that later in the day [[sucrose]] plays a larger role in regulating stomatal opening.<ref>{{cite book |editor-first1=Konrad |editor-last1=Mengel |editor-first2=Ernest A. |editor-last2=Kirkby |editor-first3=Harald |editor-last3=Kosegarten |editor-first4=Thomas |editor-last4=Appel |title=Principles of Plant Nutrition |publisher=Springer |date=2001 |page=205 |isbn=978-94-010-1009-2 |doi=10.1007/978-94-010-1009-2|s2cid=9332099 }}</ref> [[Zeaxanthin]] in guard cells acts as a blue light photoreceptor which mediates the stomatal opening.<ref name="KochharKaur2020">{{cite book |last1=Kochhar |first1=S. L. |last2=Gujral |first2=Sukhbir Kaur |title=Plant Physiology: Theory and Applications |date=2020 |publisher=Cambridge University Press |isbn=978-1-108-48639-2 |pages=75–99 |edition=2 |doi=10.1017/9781108486392.006 |chapter=Transpiration}}</ref> The effect of blue light on guard cells is reversed by green light, which isomerizes zeaxanthin.<ref name="KochharKaur2020"/>

[[Stomatal density]] and aperture (length of stomata) varies under a number of environmental factors such as atmospheric CO<sub>2</sub> concentration, light intensity, air temperature and photoperiod (daytime duration).
<ref name="Buckley-2013">{{cite journal | last1 = Buckley | first1 = TN | last2 = Mott | first2 = KA | year = 2013 | title = Modelling stomatal conductance in response to environmental factors | journal = Plant, Cell and Environment | volume = 36 | issue = 9| pages = 1691–1699 | doi=10.1111/pce.12140| pmid = 23730938 | doi-access = free | bibcode = 2013PCEnv..36.1691B }}</ref><ref name="Rogiers-2011">{{cite journal | last1 = Rogiers | first1 = SY | last2 = Hardie | first2 = WJ | last3 = Smith | first3 = JP | year = 2011 | title = Stomatal density of grapevine leaves (Vitis Vinifera L.) responds to soil temperature and atmospheric carbon dioxide | journal = Australian Journal of Grape and Wine Research | volume = 17 | issue = 2| pages = 147–152 | doi=10.1111/j.1755-0238.2011.00124.x| doi-access =  }}</ref>

Decreasing stomatal density is one way plants have responded to the increase in concentration of atmospheric CO<sub>2</sub> ([CO<sub>2</sub>]<sub>atm</sub>).<ref name="Ceccarelli-2010">{{cite journal | last1 = Ceccarelli | first1 = S | last2 = Grando | first2 = S | last3 = Maatougui | first3 = M | last4 = Michael | first4 = M | last5 = Slash | first5 = M | last6 = Haghparast | first6 = R | last7 = Rahmanian | first7 = M | last8 = Taheri | first8 = A | last9 = Al-Yassin | first9 = A | last10 = Benbelkacem | first10 = A | last11 = Labdi | first11 = M | last12 = Mimoun | first12 = H | last13 = Nachit | first13 = M | year = 2010 | title = Plant breeding and climate changes | journal = The Journal of Agricultural Science | volume = 148 | issue = 6| pages = 627–637 | doi=10.1017/s0021859610000651| s2cid = 86237270 | doi-access = free }}</ref> Although changes in [CO<sub>2</sub>]<sub>atm</sub> response is the least understood mechanistically, this stomatal response has begun to plateau where it is soon expected to impact [[transpiration]] and photosynthesis processes in plants.<ref name="Rico-2013" /><ref>{{cite journal | last1 = Serna | first1 = L | last2 = Fenoll | first2 = C | year = 2000 | title = Coping with human CO2 emissions | journal = Nature | volume = 408 | issue = 6813| pages = 656–657 | doi=10.1038/35047202| pmid = 11130053 | s2cid = 39010041 }}</ref>

Drought inhibits stomatal opening, but research on soybeans suggests moderate drought does not have a significant effect on stomatal closure of its leaves. There are different mechanisms of stomatal closure. Low humidity stresses guard cells causing [[turgor]] loss, termed hydropassive closure. Hydroactive closure is contrasted as the whole leaf affected by drought stress, believed to be most likely triggered by [[abscisic acid]].<ref>{{cite book |editor-first1=Konrad |editor-last1=Mengel |editor-first2=Ernest A. |editor-last2=Kirkby |editor-first3=Harald |editor-last3=Kosegarten |editor-first4=Thomas |editor-last4=Appel |title=Principles of Plant Nutrition |publisher=Springer |date=2001 |page=223 |isbn=978-94-010-1009-2 |doi=10.1007/978-94-010-1009-2|s2cid=9332099 }}</ref>

===Future adaptations during climate change===
It is expected that [CO<sub>2</sub>]<sub>atm</sub> will reach 500–1000 ppm by 2100.<ref name="Rico-2013" /> 96% of the past 400,000 years experienced below 280 ppm CO<sub>2</sub>. From this figure, it is highly probable that [[genotype]]s of today's plants have diverged from their pre-industrial relatives.<ref name="Rico-2013" />

The gene ''HIC'' (high carbon dioxide) encodes a negative regulator for the development of stomata in plants.<ref name="Gray-2000">{{cite journal | last1 = Gray | first1 = J | last2 = Holroyd | first2 = G | last3 = van der Lee | first3 = F | last4 = Bahrami | first4 = A | last5 = Sijmons | first5 = P | last6 = Woodward | first6 = F | last7 = Schuch | first7 = W | last8 = Hetherington | first8 = A | year = 2000 | title = The ''HIC'' signalling pathway links CO<sub>2</sub> perception to stomatal development | journal = Nature | volume = 408 | issue = 6813| pages = 713–716 | bibcode = 2000Natur.408..713G | doi = 10.1038/35047071 | pmid = 11130071 | s2cid = 83843467 }}</ref> Research into the ''HIC'' gene using'' [[Arabidopsis thaliana]]'' found no increase of stomatal development in the dominant [[allele]], but in the 'wild type' [[recessive allele]] showed a large increase, both in response to rising CO<sub>2</sub> levels in the atmosphere.<ref name="Gray-2000" /> These studies imply the plants response to changing CO<sub>2</sub> levels is largely controlled by genetics.

===Agricultural implications===

The CO<sub>2 </sub>fertiliser effect has been greatly overestimated during [[Free-air concentration enrichment|Free-Air Carbon dioxide Enrichment]] (FACE) experiments where results show increased CO<sub>2</sub> levels in the atmosphere enhances photosynthesis, reduce transpiration, and increase [[Water-use efficiency|water use efficiency]] (WUE).<ref name="Ceccarelli-2010" /> Increased [[biomass]] is one of the effects with simulations from experiments predicting a 5–20% increase in crop yields at 550 ppm of CO<sub>2</sub>.<ref name="Tubiello-2007">{{cite journal | last1 = Tubiello | first1 = FN | last2 = Soussana | first2 = J-F | last3 = Howden | first3 = SM | year = 2007 | title = Crop and pasture response to climate change | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 50| pages = 19686–19690 | doi=10.1073/pnas.0701728104| pmid = 18077401 | pmc = 2148358 | bibcode = 2007PNAS..10419686T | doi-access = free }}</ref> Rates of leaf photosynthesis were shown to increase by 30–50% in [[C3 carbon fixation|C3]] plants, and 10–25% in [[C4 carbon fixation|C4]] under doubled CO<sub>2</sub> levels.<ref name="Tubiello-2007" /> The existence of a [[feedback mechanism]] results a [[phenotypic plasticity]] in response to [CO<sub>2</sub>]<sub>atm</sub> that may have been an adaptive trait in the evolution of plant respiration and function.<ref name="Rico-2013" /><ref name="Rogiers-2011" />

Predicting how stomata perform during adaptation is useful for understanding the productivity of plant systems for both natural and [[agricultural systems]].<ref name="Buckley-2013" /> Plant breeders and farmers are beginning to work together using evolutionary and participatory plant breeding to find the best suited species such as heat and drought resistant crop varieties that could naturally evolve to the change in the face of food security challenges.<ref name="Ceccarelli-2010" />

==References==
{{reflist|30em}}

==External links==
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[[Category:Plant anatomy]]
[[Category:Plant cells]]
[[Category:Plant physiology]]
[[Category:Photosynthesis]]