Question

Explain stomatal opening and closing mechanism with reference to starch sugar interconversion theory.

Answer 1

Opening and closing of the stoma is controlled by the turgor of guard cells. During day time, guard cells become turgid due to endosmosis. Thus, turgor pressure is exerted on the thin walls of guard cells. Being elastic and thin, the lateral walls are stretched out. Due to the kidney or dumbbell-like shape, the inner thick walls are pulled apart to open (widen) the stoma. During night time, guard cells become flaccid due to exosmosis. Flaccidity closes the stoma almost completely. Endosmosis and exosmosis occur due to diurnal changes in the osmotic potential of guard cells.

According to the starch-sugar interconversion theory (Steward 1964), during the daytime, the enzyme phosphorylase converts starch to sugar, thus increasing the osmotic potential of guard cells, closing the entry of water, and thereby the guard cells are stretched, and the stoma widens. The reverse reaction occurs at night, bringing about the closure of the stoma.