Inbotany, astoma (pl.:stomata, fromGreekστόμα, "mouth"), also called astomate (pl.:stomates), is a pore found in theepidermis of leaves, stems, and other organs, that controls the rate ofgas exchange between the internal air spaces of the leaf and the atmosphere. The pore is bordered by a pair of specializedparenchyma cells known asguard 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.[1] Air, containingoxygen, which is used inrespiration, andcarbon dioxide, which is used inphotosynthesis, passes through stomata by gaseousdiffusion.Water vapour diffuses through the stomata into the atmosphere as part of a process calledtranspiration.
Stomata are present in thesporophyte generation of the vast majority ofland plants, with the exception ofliverworts, as well as somemosses andhornworts. In vascular plants the number, size and distribution of stomata varies widely.Dicotyledons usually have more stomata on the lower surface of the leaves than the upper surface.Monocotyledons such asonion,oat andmaize may have about the same number of stomata on both leaf surfaces.[2]: 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.[3] Leaves with stomata on both the upper and lower leaf surfaces are calledamphistomatous leaves; leaves with stomata only on the lower surface arehypostomatous, and leaves with stomata only on the upper surface are epistomatous orhyperstomatous.[3] Size varies across species, with end-to-end lengths ranging from 10 to 80μm and width ranging from a few to 50 μm.[4]
Carbon dioxide, a key reactant inphotosynthesis, 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 theleaf are saturated withwater vapour, which exits the leaf through the stomata in a process known astranspiration. Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.[5]
Ordinarily, carbon dioxide is fixed toribulose 1,5-bisphosphate (RuBP) by the enzymeRuBisCO inmesophyll 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 calledphotorespiration. 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.
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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.
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), aproton pump drivesprotons (H+) 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 ofpotassium 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 ionmalate is produced in guard cells. This increase in solute concentration lowers thewater potential inside the cell, which results in the diffusion of water into the cell throughosmosis. This increases the cell's volume andturgor pressure. Then, because of rings of cellulosemicrofibrils 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 surroundingepidermal cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can diffuse.[6]
When the roots begin to sense a water shortage in the soil,abscisic acid (ABA) is released.[7] ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of thecytosol 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.[8] This causes the chloride (Cl−) and organic 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 inwater potential, which results in the diffusion of water back out of the cell byosmosis. This makes the cellplasmolysed, 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.[9][10]
The degree of stomatal resistance can be determined by measuring leaf gas exchange of a leaf. Thetranspiration rate is dependent on thediffusion resistance provided by the stomatal pores and also on thehumidity 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, andozone.[11] Evaporation (E) can be calculated as[12]
whereei andea are the partial pressures of water in the leaf and in the ambient air respectively,P is atmospheric pressure, andr is stomatal resistance.The inverse ofr is conductance to water vapor (g), so the equation can be rearranged to[12]
and solved forg:[12]
Photosynthetic CO2 assimilation (A) can be calculated from
whereCa andCi are the atmospheric and sub-stomatal partial pressures of CO2 respectively[clarification needed]. The rate of evaporation from a leaf can be determined using aphotosynthesis system. These scientific instruments measure the amount of water vapour leaving the leaf and the vapor pressure of the ambient air. Photosynthetic systems may calculatewater use efficiency (A/E),g, intrinsic water use efficiency (A/g), andCi. These scientific instruments are commonly used by plant physiologists to measure CO2 uptake and thus measure photosynthetic rate.[13][14]
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.[15] They may have evolved by the modification ofconceptacles from plants' alga-like ancestors.[16]However, the evolution of stomata must have happened at the same time as the waxycuticle was evolving – these two traits together constituted a major advantage for early terrestrial plants.[citation needed]
There are three major epidermal cell types which all ultimately derive from the outermost (L1) tissue layer of theshoot apical meristem, called protodermal cells:trichomes,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.[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 manysignal transduction components such asEPF (Epidermal Patterning Factor),ERL (ERecta Like) andYODA (a putativeMAP 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 theSPCH (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]
Mostangiosperm trees have stomata only on their lower leaf surface.Poplars andwillows 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.White ash andwhite birch leaves had fewer stomata but larger in size. On the other handsugar maple andsilver maple had small stomata that were more numerous.[22]
Different classifications of stoma types exist. One that is widely used is based on the types thatJulien 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 fordicots:
Inmonocots, several different types of stomata occur such as:
Inferns, four different types are distinguished:
A catalogue of leaf epidermis prints showing stomata from a wide range of species can be found in Wikimedia commonshttps://commons.wikimedia.org/wiki/Category:Leaf_epidermis_and_stomata_prints
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 ofwax. 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] andDrimys winteri which is a species of plant found in thecloud forest.[27]
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 toArabidopsis plant leaves can release the chemicalcoronatine, which induce the stomata to reopen.[29]
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 withblue light being almost 10 times as effective asred light in causing stomatal response. Research suggests this is because the light response of stomata to blue light is independent of other leaf components likechlorophyll. Guard cellprotoplasts swell under blue light provided there is sufficient availability ofpotassium.[31] 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 daysucrose plays a larger role in regulating stomatal opening.[32]Zeaxanthin in guard cells acts as a blue light photoreceptor which mediates the stomatal opening.[33] The effect of blue light on guard cells is reversed by green light, which isomerizes zeaxanthin.[33]
Stomatal density and aperture (length of stomata) varies under a number of environmental factors such as atmospheric CO2 concentration, light intensity, air temperature and photoperiod (daytime duration).[34][35]
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 impacttranspiration 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 causingturgor loss, termed hydropassive closure. Hydroactive closure is contrasted as the whole leaf affected by drought stress, believed to be most likely triggered byabscisic acid.[38]
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 thatgenotypes of today’s plants have diverged from their pre-industrial relatives.[30]
The geneHIC (high carbon dioxide) encodes a negative regulator for the development of stomata in plants.[39] Research into theHIC gene usingArabidopsis thaliana found no increase of stomatal development in the dominantallele, but in the ‘wild type’recessive allele showed a large increase, both in response to rising CO2 levels in the atmosphere.[39] These studies imply the plants response to changing CO2 levels is largely controlled by genetics.
The CO2fertiliser effect has been greatly overestimated duringFree-Air Carbon dioxide Enrichment (FACE) experiments where results show increased CO2 levels in the atmosphere enhances photosynthesis, reduce transpiration, and increasewater use efficiency (WUE).[36] Increasedbiomass is one of the effects with simulations from experiments predicting a 5–20% increase in crop yields at 550 ppm of CO2.[40] Rates of leaf photosynthesis were shown to increase by 30–50% inC3 plants, and 10–25% inC4 under doubled CO2 levels.[40] The existence of afeedback mechanism results aphenotypic plasticity in response to [CO2]atm that may have been an adaptive trait in the evolution of plant respiration and function.[30][35]
Predicting how stomata perform during adaptation is useful for understanding the productivity of plant systems for both natural andagricultural systems.[34] 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.[36]
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