Editing Stoma (section)

==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 400 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 |url-status=dead }}</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
|url-status=dead
}}</ref>