Question

Explain the starch - sugar interconversion hypothesis of stomatal opening and closing.

Answer 1

1. Starch - sugar interconversion hypothesis of stomata opening and closing.

(a) During daytime: According to this theory, the $C{O}_2$ released in respiration is utilized in the process of photosynthesis, which makes the medium of the guard cell alkaline. Due to this high pH, the starch produced in the night is converted into sugar in the presence of enzyme phosphorylase. Sugar is soluble in water and consequently increases to the OP of the guard cells. Therefore, the cells become turgid. In this state, the thin outer wall of guard cell stretches outward and opens the stomata.

(b) During the night: During the night, the $C{O}_2$ produced in respiration is not utilized and diffuses into the cytoplasm of guard cells. It makes the medium of the guard cells acidic (low pH). At this low pH, the sugar made during daytime is also converted into starch. Starch being insoluble in water reduces the osmotic pressure of the guard cell. Consequently, water moves from guard cells to the attached subsidiary cells. It makes the guard cells flaccid and therefore, stomata close.