Youstomata (from the greek stoma - mouth) are found in the plant epidermis, mainly in the leaf. These small structures are fundamental to the process ofphotosynthesis, since they are related to the entry and exit of gases, in addition to water vapor.
A stoma is formed by the guard cells and by the ostiole, a small opening. It is this opening that connects the external environment to the interior of the plant's tissues, an intercellular space called the substomatic chamber. By controlling the opening and closing of the ostioles, the plant is able to control the entry of gases and prevent excessive water loss.
The opening and closing of the stoma are determined by changes in the guard cells. The opening happens when these cells become more turgid, whereas the closing occurs when they become more flaccid and withered. This turgor mechanism happens thanks to a osmotic movement.
Stomatal movement occurs through the entry and exit of potassium ions (K+) of the guard cells. Under certain conditions, such as the presence of light or low levels of carbon dioxide, K ions
+ are pumped into the guard cells. As this ion increases, these cells begin to absorb water, thus becoming turgid, and the ostioles immediately open.For stomatal closure, the process is reversed, that is, the guard cells lose the K ions+. When they lose this ion, they also lose water, which makes the cells wither. So, consequently, the stoma closes up.
It is believed that this mechanism, in water stress situations, is mainly controlled by the abscisic acid, a plant hormone known as ABA. This hormone binds to receptors on the membrane of guard cells and promotes the entry of calcium ions (CA2+), which act as messengers by opening ion channels. When these channels are opened, the chlorine ions (Cl-) and malate2- pass from the interior of the cell to the wall and open the K channels+, causing the exit of that ion as well. This process causes the water to come out and stomatal closure occurs. When the ABA is released from the receptor, the ions that left the cytoplasm, along with the water, return to the interior of the cell, making it turgid. At that moment, the stoma opens.
It can be seen, therefore, that the ABA causes the stoma to close, thus ensuring water savings. In environments with water stress, this mechanism ensures the survival of the plant.
In addition to water stress, several factors directly affect the opening and closing of the ostiole, such as carbon dioxide concentration, light, temperature and water loss. It is also worth noting that the stomata also open and close in circadian rhythms, that is, according to daily rhythms.
By Ma. Vanessa dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/biologia/mecanismo-abertura-fechamento-estomato.htm
