Stoma

A stoma or stomata ( pl. stomata; Greek στόμα stoma " mouth ", " mouth ", " opening ") is a pore in the epidermis of plants. The stoma is usually formed of two bean- shaped cells called guard cells. Counting the cells that lie around the guard cells around ( accessory cells ), just adds one speaks of the stomatal apparatus or stomatal complex. While the epidermal cells do not contain chloroplasts, are located in the differentiated guard cells chloroplasts.

The stomata of the plant to regulate the gas exchanges with the ambient air. In general, it involves the delivery of oxygen and water ( transpiration), as well as the absorption of carbon dioxide. Regardless of the type of plant and adaptation to specific site conditions takes place the opening and closing of the stomata by the same mechanism.

The stomata are located on most plants in the lower epidermis of the leaves of grasses on both leaf surfaces and floating leaf plants only on the leaf surface. Stomata are also found in the epidermis of shoot axes and petals, but never on roots.

Importance

The gas exchange with the ambient air is particularly important for the supply of CO2. Carbon dioxide is absorbed by the plants by physical processes (gas exchange) from the air. If the CO2 uptake exclusively via the cell walls, these would have to be extremely thin in order to ensure an adequate supply of the plant. However, this would have reduced stability, as well as an increased water loss. About the separation of the intercellular space in the Journal of the dry outside air through the stomata the plant gets control of their water resources.

Further meaning: Through the pores evaporation takes ( stomatal transpiration) instead that generates a transpiration, is transported by the water from the roots to the leaves. With the water minerals are absorbed from the soil and distributed in the plant. In addition, the evaporation cools the leaves, they do not overheat in strong sunlight and the specific temperature optimum of the enzymes in the leaf tissues is not exceeded.

Transpiration alone on the surface of the stomata, which account for only 1-2 % of the total leaf surface is up to 2 /3 of the evaporation, ie the zero-resistance evaporation, an equally large water surface. Studies have shown that in many small holes in the same surface more water evaporates. The reason is the so -called " edge effect ": molecules at the edge of a stoma can also diffuse to the side while the hinder each other in the middle.

The proportion of the cuticular transpiration is very low, at hygrophytes ( plants in humid areas ) with tender leaves less than 10 % of the evaporation of a free water surface, in trees less than 0.5 %, and even cacti only 0.05%.

Structure of the stomatal apparatus

The outermost layer of cells of a leaf, the epidermis, is usually a single layer of usually chlorophyll-free cells. The epidermis is the outside through the cuticle, a nearly impermeable layer of cutin with a superimposed layer of wax separated. The leaf tissue between the epidermal layers, the mesophyll consists of palisade tissue in which mainly photosynthesis takes place, which also photosynthetically active spongy tissue and veins. The sponge fabric is saturated with water vapor and eases the diffusion due to its increased surface area. Between his cells are intercellular spaces, also called respiratory caves, with its air of the gas exchange takes place. They open into the stomata.

The gap opening apparatus consists of two guard cells, usually bean-shaped cells, which adhere to the two ends together. Between them is an intercellular space, the pore, which represents the connection between the outside air and air pocket.

In some plants, the two guard cells of specialized epidermal cells, are the subsidiary cells ( in the pictures light blue), surrounding, which are indirectly involved in opening and closing of the gap opening. In addition to the cells can often be seen leucoplasts.

The guard cells contain chloroplasts, so they can perform photosynthesis. The opening width of the pore is variable, sunlight and sufficient water supply, they are wide open normally closed at night or in low water.

Types

From the shape of the guard cells, one can distinguish three main types: For the dumbbell-shaped graminaceous type of grasses and the kidney-shaped Helleborus type of inputs and dicots still the Mniumtyp the Moose joins. Sometimes you separate them still the Xerophytentyp, which can be found on needle leaves and a single-celled stomatal apparatus, which occurs in a few mosses and ferns.

Abbreviations: E = epidermal cell M = epidermal Meristemoide SzMz = guard cell mother cell

Assignment criteria are the number and arrangement of the subsidiary cells:

Leaf types on the location of the Stomata

Depending on the location of the stomata can distinguish three leaf types.

• Hypostomatisch: stomata are just on the underside of leaves (often )
• Epistomatisch: stomata are only on the leaf surface (rare) example with water lilies
• Amphistomatisch: stomata are on the upper leaf surface and on the underside. e.g. Grasses, needle leaves

Formation

In addition to epidermal cells arise from Meristemoiden ( embryonic cells of the epidermis ) through Stomata are so unequal cell divisions, the number of which varies depending on the type.

The cells of the stomatal apparatus can arise in two different ways:

• Continuous cell divisions a Meristemoide. The last cell division results in the closed -cell mother cell. Example: Brassicaceae - type
• Abjunction the subsidiary cells of adjacent Meristemoiden. Example: graminaceous type

As the last result of the closed -cell parent cell, the two guard cells by drawing in two cell walls in the middle ( aequale pitch, ie two identical daughter cells ) that are separated from each other by dissolving the middle lamella, thus forming the gap opening.

General mechanism

Loss of water leads to a reduction of the internal pressure ( turgor pressure ) of the guard cells. At low turgor their cell walls are in direct contact with each other, so that the central gap is closed. An increase of turgor by water absorption into the cell leads to a deformation due to the special shape and the cell strongly unevenly thickened cell walls. The cells bulge in the central contact area away from each other. The gap opens.

Can illustrate this principle using a bicycle tube. Holding the vacuum hose in hand, he hangs limply down - his inner surfaces touching. If it is inflated, the moving apart of both sides can be seen. Thus, the central space corresponding to the central gap of the stoma increased.

Molecular Mechanisms

The opening width of the stomata is controlled by light intensity, light quality, water supply and CO2 concentration. Here also the phytohormones auxin and abscisic acid play a role.

If the slit opening is extended, the membrane potential falls and potassium ions to flow into the interior of the closed cells. As a charge balancing anions chloride also flow, predominantly inward and malate ions are synthesized. Due to the increased ion concentration of water over aquaporins now first enters into the cytosol and then into the vacuole ( osmosis).

In the dark, the guard cell to a negative membrane potential of -55 mV. When a light stimulus is reinforced this negative voltage to -110 mV ( hyperpolarization ). This hyperpolarization is effected by a controlled light- ATPase, the proton with ATP consumption pumped from inside to outside. The necessary ATP could come from the photosynthesis of the guard cells, which have usually the only epidermal cells chloroplasts.

The hyperpolarization is necessary because in the cytosol, the K concentration higher ( about 100 mM ) than in the apoplast (about 1 mM ) - the source of potassium ions - is. The potassium channels open from -100 mV, and the potassium ions are able to diffuse to their concentration gradient, but with the potential gradient from the outside to the inside.

Trigger for opening of potassium channels, an increased proton concentration (ie, a pH reduction ) in the apoplast. However, the protons released from not only the opening, but they also make it easier by moving the threshold potential to open up to more positive values ​​.

In the dark, the ATPase cease to function, the membrane potential rises again to -55 mV and the potassium ions flow again according to its concentration gradient to the outside. Simultaneously, the chloride ions to migrate to the outside, water now flows back to the outside, the turgor pressure decreases and the opening gap is closed.

In KST1 ( potassium channel of Solanum tuberosum) with an inward rectifier pH sensor is available. In the corresponding channel of Arabidopsis higher proton concentration in the apoplast to open the channel are needed.

Example C4 plant: When sunlight is bound in the guard cells carbon dioxide to phosphoenolpyruvate ( PEP) and is to malate ( malic acid ). This dissociated, there are H ions released. The H ions are transported through ion pumps in the membrane of energy costs ( breakdown of ATP to ADP, and P) in the neighboring cells. This creates a negative charge in the closed cells, are attracted by the positive K ions. They diffuse into the cell, thus increasing the osmotic value. This water diffuses from neighboring cells in the guard cells, these expand to twice their volume and thus give free the stomata. As long as the sun shines on the guard cell, the stoma is kept open by these reactions, the stronger the sun, the blazing are the guard cells and the further the gap opening is open. Can according to the light intensity, so no longer find all responses in full instead, the osmotic value ( turgor ) of the guard cells decreases and they become limp - the stomata close. Instead takes off the water supply of the plant, reduces total osmotic pressure ( turgor ) of the plant cells, less water diffuses into the guard cells and the stomata close also. By the narrowing of the stoma, the plant transpires less, they will dry out more slowly.