In 1964, scientist Steward introduced the theory of starch-glucose conversion.
(i) Stomatal opening technique
CO2 is used in the photosynthetic process in the guard cells during the day. This decreases the CO2 concentration in the guard cells and increases the pH. Because of this, starch or starch is converted into glucose 6-phosphate under the influence of phosphoglucomutase enzyme. Later, with the help of phosphatase enzyme, glucose 6-phosphate is converted into glucose 1-phosphate. It increases the density of guard cells. Water enters the guard cell by the process of endorespiration. The guard cell swells and bends like a bow and the stomata open.
Starch → Glucose 6-phosphate → Glucose 1-phosphate
(ii) Mechanism of stomatal closure
CO2 is produced during respiration in the guard cells at night. CO2 reacts with water to form carbonic acid. As a result the pH of the guard cell decreases. Because of this, under the influence of phosphatase enzyme, glucose 1-phosphate is converted into glucose 6-phosphate. Glucose 6-phosphate is then converted to starch/starch by the enzyme phosphoglucomutase. It reduces the density of guard cells. In the process of exudation, water escapes from the guard cells. The guard cell loses water and relaxes and the stomata close.
Glucose 1-phosphate → glucose 6-phosphate → starch/starch