Stoma
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2008-07-09T16:09:17Z
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robot Adding: [[ja:気孔]]
{{for|surgically created body openings|Stoma (medicine)}}
[[Image:Tomato leaf stomate 1-color.jpg|300px|thumb|Stoma in a [[tomato]] leaf shown via colorized [[scanning electron microscope]] image]]
In [[botany]], a '''stoma''' (also '''stomate'''; plural '''stomata''') is a tiny opening or pore, found mostly on the underside of a [[plant]] [[leaf]] and used for
[[gas]] exchange. The pore is formed by a pair of specialized [[parenchyma]] cells known as [[guard cells]] which are responsible for regulating the size of the opening. Air containing [[carbon dioxide]] enters the plant through these openings where it is used in [[photosynthesis]] and [[Cellular respiration|respiration]]. [[Oxygen]] produced by photosynthesis in the '''[[leaf|spongy layer]]''' cells (parenchyma cells with pectin) of the leaf interior exits through these same openings. Also, [[water vapor]] is released into the atmosphere through these pores in a process called [[transpiration]].
Stomata are present in the [[sporophyte]] generation of all [[land plant]] groups except [[liverwort]]s. [[Dicotyledons]] usually have more stomata on the lower [[epidermis (botany)|epidermis]] than the upper epidermis. [[Monocotyledons]], on the other hand, usually have the same number of stomata on the two epidermes. In plants with floating leaves, stomata may be found only on the upper epidermis; submerged leaves may lack stomata entirely.
The word ''stoma'' derives from [[Ancient Greek|Greek]] [[wikt:στόμα|στόμα]] 'mouth'.
==Function==
===Carbon gain and water loss===
[[Image:Stomata Pengo.jpg|thumb|Stoma of a leaf under a [[Optical microscope|light microscope]]|300px]]
Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 384 ppm (as of March 2008). Most plants require the stomata to be open during daytime. The problem is that the air spaces in the leaf are saturated with water vapor, which exits the leaf through the stomata (this is known as [[transpiration]]). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapor.
===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 carbon/water tradeoff 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 high stomatal apertures and consequently high water loss.
However, plants possess another enzyme that can also fix carbon dioxide: PEP carboxylase or [[PEPCase]]. This enzyme has high carbon dioxide affinity, so a given rate of carbon dioxide fixation can be achieved with less stomatal opening, and hence less water loss. The catch is that the products of carbon fixation by PEPCase must be converted in an energy-intensive process to continue through the carbon reactions of photosynthesis. As a result, the PEPCase alternative is only preferable where water is more limiting but light — which provides the energy in this case — is plentiful, and/or where high temperatures increase the solubility of oxygen relative to that of carbon dioxide, magnifying Rubisco's oxygenation problem.
===CAM plants===
A group of mostly desert plants called "CAM" 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 PEPcarboxylase 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 limiting.
===Opening and closure===
[[Image:Plant stoma guard cells.png|thumb|300px|[[Confocal microscopy]] image of an ''[[Arabidopsis thaliana]]'' stoma showing two guard cells exhibiting [[fluorescence]] from [[green fluorescent protein]] and native [[chlorophyll]] (red)]]
However, most plants do not have the aforementioned facility 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. It is not entirely certain how these responses work. However, the basic mechanism involves regulation of osmotic pressure.
When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity), a [[proton pump]] drives [[protons]] (H<sup>+</sup>) from the guard cells. This means that the cells' [[electrical potential]] becomes increasingly negative, and so an uptake of [[potassium]] ions (K<sup>+</sup>) occurs. This in turn increases the [[osmotic pressure]] inside the cell, drawing in water 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 move.<ref>{{cite journal
|journal=Annals of Botany
|volume=89
|issue=1
|year=2002
|month=January
|pages=23–29
|title=Structure and Development of Stomata on the Primary Root of ''Ceratonia siliqua'' L.
|author=N. S. CHRISTODOULAKIS
|coauthors=J. MENTI and B. GALATIS
|pmid=12096815 | doi = 10.1093/aob/mcf002
}}</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
|month=1993
|pages=497–502
|title=Sensitivity of Stomata to Abscisic Acid (An Effect of the Mesophyll)
|author=C. L. Trejo
|coauthors=W. J. Davies; LdMP. Ruiz
|pmid=12231838 }}</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
|month=Oct
|year=2001
|title=The role of ion channels in light-dependent stomatal opening
|author=Petra Dietrich
|coauthors=Dale Sanders; Rainer Hedrich
|pmid=11559731
|doi=10.1093/jexbot/52.363.1959 }}</ref>. This causes the chloride (Cl<sup>-</sup>) and inorganic ions to exit the cells. Secondly, this stops the uptake of any further K<sup>+</sup> into the cells and subsequentally the loss of K<sup>+</sup>. The loss of these solutes causes a reduction in osmotic pressure, thus making the cell [[flaccid]] and so closing the stomatal pores.
Interestingly, 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://4e.plantphys.net/article.php?ch=&id=265 |title=Guard Cell Photosynthesis |accessdate=2007-04-29 |format= |work= }}</ref><ref>{{cite journal
|title=The Guard Cell Chloroplast: A Perspective for the Twenty-First Century
|author=Eduardo Zeiger
|coauthors=Lawrence D. Talbott; Silvia Frechilla; Alaka Srivastava; Jianxin Zhu
|journal=New Phytologist
|volume=153
|issue=3 Special Issue: Stomata
|month=Mar
|year=2002
|pages=415–424
|doi=10.0000/135457099338021
|doi_brokendate=2008-06-26
}}</ref>
===Inferring stomatal behavior from gas exchange===
Another way to find out whether stomata are open or closed, or more accurately, how open they are, is by measuring leaf gas exchange. A leaf is enclosed in a sealed chamber and air is driven through the chamber. By measuring the concentrations of carbon dioxide and water vapor in the air before and after it flows through the chamber, one can calculate the rate of carbon gain (photosynthesis) and water loss (transpiration) by the leaf.
However, because water loss occurs by diffusion, the transpiration rate depends on two things: the gradient in humidity from the leaf's internal air spaces to the outside air, and the diffusion resistance provided by the stomatal pores. Stomatal resistance (or its inverse, stomatal conductance) can therefore be calculated from the transpiration rate and humidity gradient. (The humidity gradient is the humidity inside the leaf, determined from leaf temperature based on the assumption that the leaf's air spaces are saturated with vapor, minus the humidity of the ambient air, which is measured directly.) This allows scientists to learn how stomata respond to changes in environmental conditions, such as light intensity, humidity, or carbon dioxide concentration.
<!--Removed - Wikipedia is not a how-to:
==Viewing==
[[image:Graticule.png|right|Graticule]]
The easiest way to view stomata on a leaf is to take a [[nail varnish]] impression of it.
# Paint about one square [[centimetre|centimeter]] of the underside of the leaf with transparent nail varnish.(or thin layer of [[PVA|PVA glue]])
# Allow to dry out thoroughly (shouldn't take more than 15 minutes).
# Peel off and place on a [[microscope]] slide.
The stomata leave clearly visible impressions in the nail varnish. A [[graticule slide]] allows for the counting of how many stomata (per unit area) are on the leaf surface, a characteristic of physiological significance
-->
==Development==
There are three major epidermal cell types which all ultimately derive from the L1 tissue layer of the [[shoot apical meristem]], called protodermal cells: [[trichome]]s, [[pavement cells]] and [[guard cell]]s, all of which are arranged in a nonrandom 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 evetually differentiate into the guard cells that surround a stoma. This meristemoid then divides assmetrically 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>{{citation| title=Stomatal Development and Pattern Controlled by a MAPKK Kinase| last=Bergmann| first=Dominique C.; Lukowitz, Wolfgang; Somerville, Chris R.| journal=Science| volume=304| date=4 July 2004| url=http://www.sciencemag.org/cgi/content/ful/304/5676/1494/DC1| pages=1494–1497| doi=10.1126/science.1096014| pmid=15178800}}</ref>
==Stomata as pathogenic pathways==
Stomata are an obvious hole in the leaf by which, as was presumed for a while, pathogens can enter unchallenged. However, it has been recently shown that stomata do in fact sense the presence of some, if not all, pathogens. However, with the virulent bacteria applied to Arabidopsis plant leaves in the experiment, the bacteria released the chemical coronatine, which forced the stomata open again within a few hours.
==References==
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{{Botany}}
[[Category:Photosynthesis]]
[[Category:Plant physiology]]
[[Category:Plant anatomy]]
[[Category:Plant cells]]
[[Category:Greek loanwords]]
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