There are two hypothesis which may explain the opening and closing of stomata.
1) Starch Sugar Hypothesis
2) Influx of K+ ions
Starch Sugar Hypothesis
It was proposed by German Botanist H. Van Mohl. The guard cell absorbs Carbon dioxide. Some CO2 reacts with water in which it is dissolved to form carbonic acid. In the presence of light energy, carbonic acid in the guard cell is converted into CO2 and water, which are rapidly used in the synthesis of Carbohydrates. The contents of illuminated guard cell is: I) The acid concentration is low i.e. pH is high. II) Sugar concentration is high. As sugar concentration increases in the guard cells, as a result water enters the guard cells. The guard cells become turgid (swollen with water). The thin outer walls bulge out and force the inner wall into a crescent shape. In this way a stoma or pore is formed between each pair of guard cell.
Closing of Stomata
In the dark, most of the sugar molecules are removed by respiration or are converted into insoluble starch. So there is an increase in the acidity of the cell contents. As sugar molecules are removed from the guard cell and the relative concentration of water in the guard cell increases, water molecules diffuse out to the epidermal cells. As the guard cell loses water, it becomes flaccid. In contrast to turgidity the loss of water causes them to become weak limp and soft. This condition is known as flaccidity and the cells are said to be flaccid. The inner thick wall moves together until the pore between them is closed. Closing of stomata prevents I) loss of water vapor II) the entry of carbon-dioxide into the leaf. The CO2 produced during respiration is used for photosynthesis even though the stomata are closed.
Influx of K+ ions
The K+ ions concentration in guard cells increase many times depending upon plants species. K+ ions (shown in red dots in the figure) enter guard cells from the surrounding epidermal cells by active transport. The accumulation of K+ decreases the osmotic potential of guard cells. Water enters the guard cell by osmosis. The guard cells become turgid and are stretched and stomata are opened. The guard cells remain in this condition only so long as the pumping of K+ ions into the cell is continue. So for keeping the stomata open a constant expenditure of energy is required.
In darkness K+ ions move out of the guard cells into surrounding epidermal cells. The water potential of guard increase as a result. Water moves out of the cells. The loss of pressure makes the guard cells change their shape again and stomata closes. Level of CO2 decreases in the spaces inside the leaf and light controls the movement of K+ into and out of guard cells. A low level of CO2 favors opening of the stomata and thus allow an increased CO2 level and increase rate of photosynthesis.
Blue light acidify the environment of the guard cells i.e. Pumps out protons which enable the guard cells to take up K+ following by water uptakes as a result turgidity of guard cell increases. In general stoma are open during day and close at night. This prevents needless loss of water by plants when it is too dark for photosynthesis.