A stoma is a minute pore on the epidermis of parts through which exchange of gases and transpiration take place. Each stoma is surrounded by a pair of kidney-shaped guard cells (in dicot) and dumbbell-shaped guard cells (in monocot). Each guard cell is a modified epidermal cell showing a prominent nucleus, cytoplasm, and plastids. The wall of the guard cell is differentially thickened. The inner wall of each cell facing the stoma is concave and thick and rigid. The outer wall is convex and is thin and elastic. The guard cells are surrounded by a variable number of epidermal cells called subsidiary cells or accessory cells. The average length of stomata is 20 to 28µm and breadth 5 µm
Mechanism of Opening and Closing of Stomata
Opening and closing of stomata occur due to turgor changes in guard cells. Due to endosmosis, an increase in the turgor of the guard cells takes place which finally results in stretching and bulging out of their outer thin walls. This results in the pulling apart of the opposed inner thicker walls creating an opening or pore in guard cells of stomata. When the turgor pressure of guard cells decreases inner walls sag, leading to the closure of space between them. This is due to the loss of water from guard cells, resulting in thicker walls to move closer and finally shut the opening. There are different theories to explant the opening and opening and closing of stomata. Two of them are
- Starch sugar interconvertible and closing of stomata
Starch sugar Hypothesis
This theory was developed by Lioyd (1908) and later expanded by Sayere (1926). According to this theory closing and opening of stomata depends upon the turgidity of guards cells and turgidity always depends upon inter conversation of starch and sugar. During light utilization of CO2 takes place during photosynthesis which decreases CO2 concentration in leaf cells and increases pH. In high PH starch converts into sugar which increases the osmotic pressure of guard cells and withdraws water from surrounding cells. Due to which the turgor pressure of guard cells increases and stomata open. But during the night time accumulation of CO2 in the protoplast of guard cells of stoma takes place due to lack of photosynthesis. It leads to a decrease in PH in guard cells due to the formation of carbonic acid. The low PH converted sugar into starch which decreases the osmotic pressure of guard cells. Then water present in the guard cell is transferred to subsidiary cells. As the turgor pressure of guard cells decreases and stomatal aperture closes.
- Malate Or active potassium iron (k+) transportation hypothesis
This theory was proposed by Fujino (1959). It also called k+ iron transport and hormonal regulation theory. According to this hypothesis, the accumulation of k+ iron in guard cells brings the opening of stomata, and loss of k+ irons causes the closing of stomata. The overall process can be represented as