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 purposed by Von Mohl in 1956. According to this theory , chloroplast present in guard cells are form osmotically active substance by photosynthesis which increase osmotic pressure and yurgor pressure due to which endoodmosis takes place. This result opening of stomata. This theory is not accepted because in many cases, chloroplast of guard cells are poorly developed.

2. Starch-sugar inter-conversion theory:

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

3. Active K⁺ transport theory :

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