Answer
ABA
Step-by-step explanation:
It is believed that this mechanism, in situations of hydric stress, is controlled mainly by abscisic acid, a plant hormone known as ABA. This hormone binds to receptors in the membrane of the guard cells and promotes the entry of calcium ions (CA2+), which function as messengers when opening ionic channels. When these channels are opened, the chlorine(Cl-) and malate2- ions pass from the inside of the cell to the wall and open the K+ channels, causing the exit of this ion as well. This process causes the water to come out and the stomach closure occurs. When the ABA is released from the receiver, the ions that came out of the cytoplasm, together with the water, return to the inside of the cell, making it turgid. At this moment, the stomach opens.
Therefore, it is perceived that the ABA causes the stomatum to close, thus guaranteeing water savings. In environments with water stress, this mechanism guarantees the survival of the plant.
In addition to water stress, several factors directly affect the opening and closing of the ostyolus, such as the concentration of carbon dioxide, light, temperature and water loss. It is also worth mentioning that the stomata also open and close at circadian rhythms, that is, according to daily rhythms.