Stoma
Overview
 

In botany
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...

, a stoma (also stomate; plural stomata) is a pore, found in the leaf and stem epidermis that is used for
gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...

. The pore is bordered by a pair of specialized parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...

 cells known as guard cells
Guard cell
Guard cells are specialized cells located in the Leaf epidermis of plants. Pairs of guard cells surround tiny stomatal airway pores . These tiny holes in the surface of leaves are necessary for gas exchange into and out of the plant; carbon dioxide enters the plant allowing the carbon fixation...

 that are responsible for regulating the size of the opening. The term stoma is also used collectively to refer to an entire stomatal complex, both the pore itself and its accompanying guard cells.
Encyclopedia

In botany
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...

, a stoma (also stomate; plural stomata) is a pore, found in the leaf and stem epidermis that is used for
gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...

. The pore is bordered by a pair of specialized parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...

 cells known as guard cells
Guard cell
Guard cells are specialized cells located in the Leaf epidermis of plants. Pairs of guard cells surround tiny stomatal airway pores . These tiny holes in the surface of leaves are necessary for gas exchange into and out of the plant; carbon dioxide enters the plant allowing the carbon fixation...

 that are responsible for regulating the size of the opening. The term stoma is also used collectively to refer to an entire stomatal complex, both the pore itself and its accompanying guard cells. Air containing carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 enters the plant through these openings where it is used in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 and respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

, respectively. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 produced by photosynthesis in the spongy layer
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

cells (parenchyma cells with pectin) of the leaf interior exits through these same openings. Also, water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 is released into the atmosphere through these pores in a process called transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

.

Stomata are present in the sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

 generation of all land plant groups except liverwort
Marchantiophyta
The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

s. Dicotyledons usually have more stomata on the lower epidermis
Epidermis (botany)
The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...

 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 Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 στόμα, "mouth".

Carbon gain and water loss

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 vapour, which exits the leaf through the stomata (this is known as transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.

Alternative approaches

Ordinarily, carbon dioxide is fixed to ribulose-1,5-bisphosphate
Ribulose-1,5-bisphosphate
Ribulose-1,5-bisphosphate is an organic substance that is involved in photosynthesis. The anion is a double phosphate ester of the ketose called ribulose. Salts of this species can be isolated, but its crucial biological function involves this colourless anion in solution...

 (RuBP) by the enzyme RuBisCO
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...

 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
Photorespiration
Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP has oxygen added to it by the enzyme , instead of carbon dioxide during normal photosynthesis. This is the beginning step of the Calvin-Benson cycle...

. 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, PEPcase (Phosphoenolpyruvate carboxylase
Phosphoenolpyruvate carboxylase
Phosphoenolpyruvate carboxylase is an enzyme in the family of carboxy-lyases that catalyzes the addition of bicarbonate to phosphoenolpyruvate to form the four-carbon compound oxaloacetate:This reaction is used for carbon fixation in so-called "CAM" and "C4" plants where it plays a key role in...

). Retrieving the products of carbon fixation from PEPCase is in 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.

CAM plants

A group of mostly desert plants called "CAM" plants (Crassulacean acid metabolism
Crassulacean acid metabolism
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. The stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide...

, 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
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...

. 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

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
Proton pump
A proton pump is an integral membrane protein that is capable of moving protons across a cell membrane, mitochondrion, or other organelle. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.-Function:...

 drives protons (H+) from the guard cell
Guard cell
Guard cells are specialized cells located in the Leaf epidermis of plants. Pairs of guard cells surround tiny stomatal airway pores . These tiny holes in the surface of leaves are necessary for gas exchange into and out of the plant; carbon dioxide enters the plant allowing the carbon fixation...

s. This means that the cells' electrical potential becomes increasingly negative. The negative potential opens potassium voltage-gated channels and so an uptake of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 ions (K+) 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
Water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...

 inside the cell, which results in the diffusion of water into the cell through osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

. This increases the cell's volume and turgor pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....

. 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 epidermal
Epidermis (botany)
The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...

 cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can move.

When the roots begin to sense a water shortage in the soil, abscisic acid
Abscisic acid
Abscisic acid , also known as abscisin II and dormin, is a plant hormone. ABA functions in many plant developmental processes, including bud dormancy. It is degraded by the enzyme -abscisic acid 8'-hydroxylase.-Function:...

 (ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

 of the cells and cause the concentration of free Ca2+ to increase in the cytosol due to influx from outside the cell and release of Ca2+ from internal stores such as the endoplasmic reticulum and vacuoles. This causes the chloride (Cl-) and inorganic ions to exit the cells. Second, this stops the uptake of any further K+ into the cells and, subsequently, the loss of K+. The loss of these solutes causes an increase in water potential
Water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...

, which results in the diffusion of water back out of the cell by osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

. This makes the cell flaccid, which results in the closing of the stomatal pores.

It is interesting to note that guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.

Inferring stomatal behavior from gas exchange

The degree of stomatal resistance can be determined by measuring leaf gas exchange of a leaf. The transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

 rate is dependent on the diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 resistance provided by the stomatal pores, and also on the humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

 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.
Evaporation (E) can be calculated as;

E = (ei - ea) / P r

where ei and ea = partial pressures of water in the leaf and in the ambient air; P = atmospheric pressure; and r = stomatal resistance.
The inverse of r is conductance to water vapor (g), so the equation can be rearranged to;

E = (ei - ea) g / P

and solved for g;

g = EP / (ei - ea)

The rate of evaporation from a leaf can be determined using a photosynthesis system
Photosynthesis system
Photosynthesis systems are electronic scientific instruments designed for non-destructive measurement of photosynthetic rates in the field. Photosynthesis systems are commonly used in agronomic and environmental research, as well as studies of the global carbon cycle.- How photosynthesis systems...

. 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), stomatal conductance (gs
GS
-Places:* South Georgia and the South Sandwich Islands ** .gs, an Internet country code top-level domain-Science:* g-force , multiples of the acceleration due to gravity* Guanosine synthetase-Technologies:* Glideslope* Ground Speed...

), intrinsic water use efficiency (A/gs), and sub-stomatal CO2 concentration (Ci).[3] These scientific instruments are commonly used by plant physiologists to measure CO2 uptake and thus measure photosynthetic rate.

Evolution

The fossil record has little to say about the evolution of stomata. They may have evolved by the modification of conceptacles from plants' alga-like ancestors.
It is clear, however, that the evolution of stomata must have happened at the same time as the waxy cuticle
Plant cuticle
Plant cuticles are a protective waxy covering produced only by the epidermal cells of leaves, young shoots and all other aerial plant organs without periderm...

 was evolving - these two traits together constituted a major advantage for primitive terrestrial plants.

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
Trichome
Trichomes are fine outgrowths or appendages on plants and certain protists. These are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae.- Algal trichomes :...

s, pavement cells and guard cell
Guard cell
Guard cells are specialized cells located in the Leaf epidermis of plants. Pairs of guard cells surround tiny stomatal airway pores . These tiny holes in the surface of leaves are necessary for gas exchange into and out of the plant; carbon dioxide enters the plant allowing the carbon fixation...

s, all of which are arranged in a non-random, non-predictable fashion.

Production is reliant on interactions between SPCH (speechless), EPF (downregulates stomata), TMM (too many mouths, downregulates stomata) and stomagen (upregulates stomata, inhibits SPCH), ERL and YODA downregulate stomata too.

Stomata positioning is down to CO2 activating EPF1, which activates TMM/ERL which together activate YODA, YODA in turn inhibits SPCH, in turn SPCH activation decreases, allowing asymmetry.

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.

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
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

plant leaves in the experiment, the bacteria released the chemical coronatine, which forced the stomata open again within a few hours.
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