Question

Explain the osmotic theory of opening and closing of stomata.

Answer 1

The opening and closing of stomata depend upon the turgidity of guard cells. When the guard cells are turgid, stomata are open and when they are flaccid, stomata are closed. When the osmotic concentration of guard cells is more, the water enters into them from their adjacent cells by endosmosis which results into the increase in their turgor pressure. It exerts pressure on its outer thin wall due to which it expands too much. By pulling of the outer wall outside, the inner thick wall also pulled outwards and stomata become open. When the turgidity and turgor pressure decreases in the guard cells they become flaccid and their wall revert back to their original position, thus closing the stomatal pore.
A number of theories have been put forth from time to time to explain the mechanism of opening and closing of stomata. Some important theories are as follows:
(1) Photosynthetic theory: According to Von Mohl, during the day time, sugar on accumulating in the guard cells through the process of photosynthesis whereby the concentration of these cells increases and they become turgid and after absorbing water from their adjacent cells. Hence, stomata are open. During night, the process reverses. As the photosynthesis does not take place during the night, sugar is not synthesized and the already synthesized sugar is either utilized or converted into starch during the night due to which the concentration of cells is decreased, whereby the water goes into the adjacent cells, therefore, stomata are closed. This theory is not accepted because the chloroplasts of guard cells are either not capable of performing photosynthesis or they perform very little photosynthesis due to which the required concentration cannot be obtained in the guard cells.
(2) Lioyd theory: According to this theory, enzyme phosphorylase is found in guard cells which convert soluble sugars into insoluble starch at night. Therefore, the osmotic pressure of guard cells does not increase. Hence, the stomata are closed. But, during the day time, this enzyme converts insoluble starch into soluble sugar due to which the osmotic pressure of guard cell increases and stomata are open.