Stoma
In botany, a stoma (pl.: stomata, from Greek στόμα, "mouth"), also called a stomate (pl.: stomates), is a pore found in the 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 parenchyma cells known as guard cells 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.
Stomata are present in the
Function
CO2 gain and water loss
Alternative approaches
Ordinarily, carbon dioxide is fixed to
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
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
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
When the roots begin to sense a water shortage in the soil,
Guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.[9][10]
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.[11] Evaporation (E) can be calculated as[12]
where ei and ea 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[12]
and solved for g:[12]
Photosynthetic CO2 assimilation (A) can be calculated from
where Ca and Ci are the atmospheric and sub-stomatal partial pressures of CO2 respectively[
Evolution
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.
Development
There are three major epidermal cell types which all ultimately derive from the outermost (L1) tissue layer of the
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.[17] Cell division is inhibited in some cells so there is always at least one cell between stomata.[18]
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).[18] Mutations in any one of the genes which encode these factors may alter the development of stomata in the epidermis.[18] For example, a mutation in one gene causes more stomata that are clustered together, hence is called Too Many Mouths (TMM).[17] Whereas, disruption of the SPCH (SPeecCHless) gene prevents stomatal development all together.[18] 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.[18][19] 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.[20]
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.[21]
Most
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,[23] 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.[24] They distinguish for dicots:
- 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.
- 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.
- 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 Chenopodiaceae, and Cucurbitaceae. It is sometimes called ranunculaceous type.
- 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.
- 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.
- 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 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. This type can be found in monocot families including Poaceae and Cyperaceae.[25]
- 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.
- 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 Asclepiadaceae.
In ferns, four different types are distinguished:
- 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.
- pericytic stomata have two guard cells that are entirely encircled by one continuous subsidiary cell (like a donut).
- desmocytic stomata have two guard cells that are entirely encircled by one subsidiary cell that has not merged its ends (like a sausage).
- 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).
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 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 oleander, conifers, Hakea[26] and Drimys winteri which is a species of plant found in the cloud forest.[27]
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.[28] However, pathogenic bacteria applied to Arabidopsis plant leaves can release the chemical coronatine, which induce the stomata to reopen. [29]
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.[30]
Stomata are responsive to light with
Decreasing stomatal density is one way plants have responded to the increase in concentration of atmospheric CO2 ([CO2]atm).[36] Although changes in [CO2]atm 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.[30][37]
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
Future adaptations during climate change
It is expected that [CO2]atm will reach 500–1000 ppm by 2100.[30] 96% of the past 400,000 years experienced below 280 ppm CO2. From this figure, it is highly probable that genotypes of today’s plants have diverged from their pre-industrial relatives.[30]
The gene HIC (high carbon dioxide) encodes a negative regulator for the development of stomata in plants.
Agricultural implications
The CO2 fertiliser effect has been greatly overestimated during
Predicting how stomata perform during adaptation is useful for understanding the productivity of plant systems for both natural and
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