What is the function of stomata in plant tissue?

The stomata are structures or small pores (ostioles) that are located in the epidermis (external tissue) of plants, often on the underside of the leaves, that is, in the lower part of the leaf, and that allow the exchange of gases between the plant and the environment. Plant epidermal cells are normally thick and elongated, and their continuity is interrupted by stomata, which can also be anywhere on the plant except the root.

The stomatal apparatus

Each stoma is in turn composed of other structures and cell types to form the stomatal apparatus. In addition to the pore , through which gases such as CO ₂ , water vapor, and oxygen enter and exit, the anatomy of the stoma is made up of two guard cells (also known as guard cells) surrounded by auxiliary cells ( also known as annex or companion cells). The pore and the set of occlusive and auxiliary cells make up the substomatal cavity (respiratory cavity of the stoma).

Stomata are responsible for many important physiological processes in plants. _{Through the stomata, CO 2} enters , which is used as raw material to produce carbohydrates in the process of photosynthesis, where oxygen is generated as a by-product that is released into the atmosphere. They also have an important role in the plant’s transpiration: when they open, a water potential is created which, in turn, promotes the absorption of water by the roots and its subsequent translocation to the rest of the plant’s organs.  

Stomatal classification

There are several ways to classify stomata: by the type of plants where they are found, according to their location in the plant epidermis, by the origin of the cells that make up the stomatal apparatus, and according to the presence or absence of cells. attached.

By the type of plants where they are found . Stomata of monocot and dicot plants differ in the shape of the guard cells. The stomata of monocot plants have bell-shaped guard cells, while in dicot plants the two guard cells surrounding the stomata are bean-shaped.

According to the location of the stomata in the epidermis. This classification depends on the particular distribution of the stomata in the plant:

• Amphistomatic . In monocots, it is of the amphistomatic type, that is, they are present in the upper (bundle or adaxial face of the leaf) and lower (underside or abaxial face) epidermis of the leaves.
• Hypostomatic . In dicotyledonous plants the distribution is of the hypostomatic type, present in the lower epidermis (abaxial face) of the leaf.
• Epistomatic . There is a third category of stomata distribution, the epistomatic that occurs in those plants that have them distributed in the adaxial epidermis, generally found in aquatic plants with floating leaves, such as the water lily.

According to the origin of all the cells that make up the stomatal apparatus. It is another way of grouping and classifying stomata:

• Mesogenic stoma : Guard cells and annex cells originate from the same cell by 3 successive divisions. This type of stoma is not found in monocots. 
• perigenous stoma . The mother cell originates only the occlusive cells, the annexed cells originate from other protodermal cells (cells that, when differentiated, originate the epidermis). This type of stoma is present in species from all groups of vascular plants.
• Mesoperigene stoma : The mother cell gives rise to the occlusive cells and an annexed cell, while the rest originate from other protodermal cells. This type of stoma has been found in all groups of vascular plants.

According to the presence or absence of attached cells, the stomata are classified as:

• Anomocytic stoma : has no annex or auxiliary cells.
• Anisocitic stoma : They have three annexed cells of different size. 
• Paracytic stoma – Has two annex cells arranged with their long axis parallel to the long axis of the guard cells.
• Diacitic stoma : they have two attached cells arranged with their longitudinal axis perpendicular to the longitudinal axis of the guard cells.
• Tetracytic stoma – has 4 annex (auxiliary) cells around the guard cells.
• Cyclocytic stoma – Numerous annexed (auxiliary) cells, arranged in one or two circles around the guard cells.
• Helicocytic stoma – with several annex (auxiliary) cells arranged in a spiral around the two guard cells.

The position of the stoma is also of botanical interest, depending on the type of plant species they can be located protruding from the epidermis, at the level of the epidermis or sunk in special cavities, which depends on the type of plant and the place where it develops. In mesophytic plants (those that require access to water or environments with non-extreme temperatures) the stomata are generally at the same level as the other cells. In hygrophytic plants (aquatic plants) the stomata are raised above the other cells (which favor transpiration). Xerophytic plants (from dry environments) have small, narrow and sunken stomata, and in large quantities to favor gas exchange when the water supply is favourable,

Environmental conditions and the mechanism of stomatal movements

Under optimal conditions, the stomata are open, allowing gas exchange with the atmosphere. However, it should be noted that the mechanism of stomatal movements depends on changes in the turgor pressure of the guard cells and adjacent (companion) epidermal cells. These changes in shape when the pore opens or closes are possible by a mechanism that transforms the starch content in the cell into sugar; once the cells contain a high concentration of sugars and potassium salts, due to the osmosis process, the water coming from the attached cells infiltrates and consequently the occlusive cells swell, that is, they increase in size. If, on the other hand, the guard cells lose water, the cell walls move closer to each other in the center, closing the opening or pore.

The stomatal pore opening and closing mechanism responds to variations in certain environmental and internal factors, particularly light, CO ₂ concentration , leaf water potential, and temperature. Humidity is an example of an environmental condition that regulates the opening or closing of stomata. When humidity conditions are optimal, the stomata are open, but when the humidity levels in the air around plant leaves decrease, either due to rising temperatures or wind, the stomata close to prevent transpiration and excessive water loss. This mechanism of plants allows them to respond quickly to changes in the environment.

Functions of the stomatal apparatus

The stomata fulfill very important functions in the plant world, because through them the plants take in the CO ₂ found in the atmosphere and expel the oxygen during the process of photosynthesis; on the contrary, in the process of respiration they take in oxygen and expel CO ₂ .

The water that is lost from the plant occurs through the process of stomatal transpiration, which dominates the control of the plant’s water potential. A regulation mechanism possessed by higher plants is to keep the stomata closed when water is scarce, even when they are in the presence of sunlight. The stomata are closed to prevent the loss of water through transpiration, since it comes out as water vapor. For this to happen, the cells lose water, become flaccid and the pore closes; On the other hand, when the cells are full of water and turgid, the thin wall yields thanks to a complicated mechanism that involves sugars, phytohormones, K + ions ^and Ca ²⁺ ions , and the pore opens allowing gas exchange.

On the other hand, when there are low concentrations of CO ₂ in the mesophyll (the tissue found between the upper epidermis and the underside of the leaves), the guard cells produce the opening of the stoma. Guard cells have the ability to capture and integrate multiple internal (chemical) and external (environmental) stimuli such as light, CO ₂ concentration , and temperature, which are the dominant environmental signals for stomatal movement control.

Key aspects of stomas

• Among the factors that control the opening and closing of the stomata are the concentration of CO ₂ inside the leaves, atmospheric humidity, the water potential of the leaf, temperature and wind.
• Stomata are important in gas exchange, both in the photosynthesis process and in respiration and transpiration (efficient use of water in the plant).
• The stomata are responsible for the loss of water during transpiration under different environmental conditions, and it is regulated through the mechanism of action of the occlusive cells with opening and closing movements of the stomata, thus adjusting the water supply. Environmental factors trigger the hormonal signals that direct this type of physiological process in the plant.
• The distribution of stomata in the epidermis is variable and depends on the plant species. The environmental conditions clearly influence the distribution of the stomata, for example a species in environmental conditions with high solar radiation or luminosity will have the greatest number of stomata on the upper side of the leaves.

font s

Metcalfe, C.R. and L. Chalk. 1979. Anatomy of dicots . Vol. 1. Clarendon Press Oxford.

Roth, Ingrid. 1976. Anatomy of higher plants . Editions of the library, Caracas, UCV- Editions of the library.