A stoma or a cleft opening ( pl. Stomata ; Greek στόμα stóma "mouth", "mouth", "opening") is a pore in the epidermis of plants . The stoma is usually made up of two bean-shaped cells called guard cells. If one also counts the cells that are around the guard cells (secondary cells), one speaks of the stomata or stomatal complex. While the epidermal cells do not contain any chloroplasts , there are chloroplasts in the differentiated guard cells.
The stomata regulate the gas exchange between the plant and the surrounding air. In general, this involves the release of oxygen and water ( transpiration ), as well as the absorption of carbon dioxide. Regardless of the type of plant and the adaptation to special site conditions, the opening and closing of the stomata occurs according to the same mechanism.
In most plants, the stomata are located in the lower epidermis of the leaves , in grasses on both leaf sides and in floating leaf plants only on the upper side of the leaf. Stomata are also found in the epidermis of stem axes and petals, but never on roots.
The gas exchange with the surrounding air is particularly important for the supply of CO 2 . Plants absorb carbon dioxide from the air through physical processes ( gas exchange ). If the CO 2 absorption were to take place exclusively through the cell walls, these would have to be extremely thin in order to ensure an adequate supply for the plant. However, this would result in reduced stability and increased water loss. The stomata gives the plant control over its water balance through the separation of the intercellular space in the leaf from the dry outside air.
Further meaning: evaporation (stomatal perspiration) takes place through the pores , which creates a perspiration suction through which water is transported 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.
The transpiration alone over the area of the stomata, which only make up one to two percent of the total leaf surface, is up to two thirds of the evaporation , i.e. the resistance-free evaporation, of an equally large water surface. Studies have shown that more water evaporates from many small openings with the same surface area. The reason is the so-called "edge effect": Molecules on the edge of a stoma can also diffuse to the side, while those in the middle obstruct each other.
The proportion of cuticular transpiration is very low, with hygrophytes (plants in humid areas) with delicate leaves less than ten percent of the evaporation of a free water surface, with trees less than 0.5 percent and with cacti only 0.05 percent.
Structure of the stomata apparatus
The outermost cell layer of a leaf, the epidermis , is usually a single layer of cells that are usually free of chlorophyll . The epidermis is delimited from the outside by the cuticle , a nearly water-impermeable layer of cutin with a layer of wax on top. The leaf tissue lying between the epidermal layers, the mesophyll , consists of the palisade tissue , in which photosynthesis mainly takes place, the also photosynthetically active sponge tissue and the leaf veins . The sponge fabric is saturated with water vapor and facilitates diffusion due to its increased surface. Between its cells there are intercellular spaces , also called respiratory cavities, with whose air the gas exchange takes place. They flow into the stomata.
The stomata consists of two guard cells, usually bean-shaped cells that adhere to each other at both ends. Between them is an intercellular gap, the porus, which is the connection between the outside air and the breathing cavity.
In some plants, the two guard cells are surrounded by specialized epidermal cells, the secondary cells (light blue in the figures), which are indirectly involved in opening and closing the stomata. Leukoplasts can often be seen in the adjoining cells .
The guard cells contain chloroplasts, so they can carry out photosynthesis. The opening width of the porus is variable, with sunlight and sufficient water supply they are usually wide open, at night or closed when there is a lack of water.
The shape of the guard cells can distinguish three main types: To the dumbbell Gramineae type in grasses and the kidney-shaped Helleborus type with inputs and dicotyledons still the joined Mniumtyp the Moose . Sometimes one separates the xerophyte type , which can be found on needle leaves , and a unicellular stomata which occurs in a few mosses and ferns .
|Caryophyllene type||Rhoeo spathacea Tradescantia type|
E = epidermal cell
M = epidermal meristemoids
SzMz = guard cell mother cell
Allocation criteria are the number and arrangement of the secondary cells:
|Brassicaceae type||anisocytic (gr. an = not, isos = equal)|
|Ranunculaceae type||anomocytic: without secondary cells|
|cyclocytic: (Greek kyklos = circle)|
|Caryophyllene||diacytic (Gr. diá = between) 2 secondary cells offset by 90 ° to the guard cells|
|Rubiaceae type||paracytisch (gr. pará = next to) 2 secondary cells parallel to the guard cells|
|tetracyic (gr. tetra = four)|
Leaf types according to the location of the stomata
Depending on the position of the stomata, one can differentiate between three leaf types.
- Hypostomatic: stomata are only on the underside of the leaf (often)
- Epistomatic: stomata are only on the upper side of the leaf (rarely) z. B. with water lilies
- Amphistomatic: stomata lie on the upper and lower side of the leaf. z. B. grasses , needle leaves
The cells of the stomata can arise in two different ways:
- Continuous cell division of a meristemoid. The last cell division results in the guard cell parent cell. Example: Brassicaceae type
- Separation of the secondary cells from neighboring meristemoids. Example: Gramineae type
Finally, the two guard cells are created from the guard cell mother cell by pulling in two cell walls in the middle (equal division, so two daughter cells of the same size are created), which are separated from each other by dissolving the central lamella and thus form the gap opening.
Loss of water leads to a reduction in the internal pressure ( turgor pressure ) of the guard cells. When the turgor is low, their cell walls are in direct contact with each other, which closes the central gap. An increase in the turgor due to the absorption of water into the cell leads to a deformation due to the special cell shape and the unevenly thickened cell walls. The cells bulge away from each other in the central contact area. The gap opens.
This principle can be illustrated using a bicycle tube. If you hold the evacuated hose in your hand, it hangs limply - its inner surfaces are touching. If it is inflated, the divergence of both sides can be seen. The central space, which corresponds to the central gap of the stoma, is thus enlarged.
If the gap opening is to be widened, the membrane potential drops and potassium ions flow into the interior of the guard cells. Anions, predominantly chloride, also flow inwards to balance the charge, and malate ions are synthesized. Due to the increased ion concentration, water flows through aquaporins first into the cytosol and then into the vacuole ( osmosis ).
In the dark, the guard cell has a negative membrane potential of -55 mV. With a light stimulus, this negative voltage increases to –110 mV ( hyperpolarization ). This hyperpolarization occurs through a light-controlled ATPase , which pumps protons from the inside to the outside while consuming ATP. The ATP required for this could come from photosynthesis in the guard cells, which are usually the only epidermal cells to have chloroplasts.
The hyperpolarization is necessary because the K + concentration in the cytosol is higher (approx. 100 mM) than in the apoplast (approx. 1 mM) - the source of the potassium ions. The potassium channels open from −100 mV, and the potassium ions can diffuse against their concentration gradient, but with the potential gradient from outside to inside.
The trigger for the opening of these potassium channels is an increased proton concentration ( i.e. a decrease in pH ) in the apoplast. The protons not only trigger the opening, but also facilitate it by shifting the threshold potential for opening to more positive values.
In the dark, the ATPase stops working, the membrane potential rises again to -55 mV and the potassium ions flow outwards again according to their concentration gradient. At the same time, the chloride ions also migrate outwards, water now flows outwards again, the turgor pressure drops and the gap opening closes.
At KST1 (potassium channel from Solanum tuberosum ) there is an inward rectifier with a pH sensor. With the corresponding channel of Arabidopsis , higher proton concentrations in the apoplast are required to open the channel.
Example C4 plant : When exposed to sunlight, carbon dioxide is bound to phosphoenolpyruvate (PEP) in the guard cells and becomes malic acid . This dissociates to form malate and H + ions are released. The H + ions are transported to the neighboring cells by ion pumps in the membrane with energy expenditure (splitting of ATP into ADP and P). This creates a negative charge in the guard cells, by the positive K + - ions are attracted. They diffuse into the cell interior and thus increase the osmotic value . As a result, water diffuses from the neighboring cells into the guard cells, these expand up to twice their volume and thus free the stomata. As long as the sun shines on the guard cell, these reactions keep the stoma open; the stronger the sun, the more plump the guard cells are and the wider the gap is open. If the light intensity decreases, all reactions no longer take place in full, the osmotic value (turgor pressure) of the guard cells decreases and they become limp - the stomata close. If instead the water supply to the plant decreases, the overall osmotic pressure (turgor) of the plant cells decreases, less water diffuses into the guard cells and the stomata also close. By narrowing the stomata, the plant transpires less and it dries out more slowly.
- Strasburger: Textbook of Botany , 29th edition, revised by v. Denffer et al., Gustav Fischer, Stuttgart 1967, pp. 87-90 (C 1. a) stomata ), pp. 217-218, p. 244, p. 361.
- G. Czihak, H. Langer, H. Ziegler (Eds.): Biology. A textbook , 6th unaltered edition, Springer, Berlin - Heidelberg - New York 1996, pp. 420-424.
- Ulrich Kutschera: Short textbook of plant physiology , Quelle & Meyer, Wiesbaden 1995, p. 383.
- Gerhard Richter: Metabolic Physiology of Plants , Georg Thieme, Stuttgart 1976, p. 295.
- Rainer Franz Hedrich: About the metabolism of guard cells in the light and in the dark. Univ., Diss., Göttingen 1985.