Question

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

Answer 1

Hint: The hypothesis is regarding the role of two types of molecules, starch, and sugar in changing osmotic pressure in the guard cells which in turn leads to the opening or closing of the stomata.

Complete answer:
According to the starch-sugar interconversion hypothesis of stomatal opening and closing, the events happening during daytime and night time have been explained below –
During the daytime, CO₂ is released when plants perform respiration and the released CO₂ is utilized in the process of photosynthesis. Due to intake of CO₂, the medium inside the guard cells become alkaline with high pH value. Under the effect of this high pH value, the starch that was produced in the night through dark reaction is converted back into sugar in the presence of phosphorylase enzyme. We know that sugar is soluble in water and thus, as the sugar content in the guard cells increases, it increases the osmotic pressure of the guard cells and the cells become turgid. Due to this turgidity, the outer wall of the guard cells which is relatively thin stretches outward and the stomata open.
During the night, since photosynthesis is not occurring, the CO₂ does not diffuse into the guard cells and the medium remains acidic. Due to the low pH there is no conversion of starch into the sugar and being insoluble in water, the starch does not increase the osmotic pressure as sugar does and the osmotic pressure in the guard cells decreases that leads to the outward movement of water causing guard cells to be flaccid and thus, stomata close.

Note: This hypothesis is mainly based on the concept of osmotic pressure. Out of the two molecules, since one i.e. the sugar is soluble in water hence it will increase osmotic pressure and the other will decrease. The change in osmotic pressure, in turn, affects the movement of water in and out of the guard cells.