Theories of opening and closing of stomata

  The process of turgor change in the guard cells is explained by different theories. Some of them are as follows:

1. Photosynthetic theory:

                           This theory was proposed by Von Mohl in 1956. According to this theory, chloroplast present in guard cells forms osmotically active substances by photosynthesis which increases osmotic pressure and anturgoror pressure due to which endosmosis takes place. This results opening of stomata. This theory is not accepted because in many cases, the chloroplast of guard cells is poorly developed.

2. Starch-sugar inter-conversion theory:

                               According to this theory, the opening and closing of stomata depend upon starch and sugar interconversion. During this time, the pH of the cell increases due to the intake of CO₂ in photosynthesis. At the high pH of the cell, the starch hydrolyzing enzyme becomes active and starch is converted into sugar(glucose). Due to the presence of sugar, the osmotic pressure of the guard cell increases and water enters into the guard cell osmotically. By this process, turgidity of the guard cell takes place, and stomata open. During the night, the sugar is converted into starch, and cells lose water due to which guard cells become flaccid and stomata close.

Fig: Starch sugar interconversion

3. Active K⁺ transport theory :

                                        According to this theory, the turgidity of the guard cell depends upon the entrance of K⁺ into the guard cell. During this time, in the presence of light, malic acid is formed in the guard cell. It dissociates into malate and H⁺ ions. The H⁺ ions are exchanged with K⁺ionsn and form potassium malate in the guard cell. It makes the cell osmotically active and by endosmosis, water enters into the guard cells. This causes the opening of stomata in the daytime. The reverse process takes place during the night.