Stomata

The main difference between stoma and stomata is that stoma is the pore surrounded by two guard cells. In contrast, stomata are the collection of stomas found inside the lower epidermis of plant leaves.

It has an average length and breadth of 10 to 40 µm and 3 to 10 µm, respectively. Stoma varies in shape and size to adapt to different environmental conditions, ensuring optimal conditions for photosynthesis.

It is made of two guard cells – a pair of specialized, kidney-shaped epidermal cells. The epidermal cells are located in the epidermal tissue, border the guard cells. They are also called accessory cells or subsidiary cells. The guard cells are joined together but are free to separate and form a pore. The stoma opens to the interior into a sub-stomatal cavity that remains connected to the intercellular spaces.

Stomata cover 1 to 2% of leaf area. Land plants typically have thousands of stomata on their leaf surface, most of which are found on the lower portion. It reduces its direct exposure to heat and air. In aquatic plants, stomata are found on the upper leaf surface. The number of stomata depends on the type of plant, its photosynthetic rate, and its habitat. The average number of stomata on the leaf surface is about 300 per square mm. Stomata are found on both upper and lower surfaces of the leaf but are more numerous on the lower surface.

The table below shows examples of stomatal density on the upper and lower surfaces of leaves of some common dicot and monocot plants:

Stomata are classified based on different characteristic features of guard cells and subsidiary cells.

Based on the Structure and Arrangement of the Subsidiary Cells

The arrangement of subsidiary cells is found to be different in dicot and monocot plants. Accordingly, stomata are classified into:

Stoma Found in Dicots

• Actinocytic (star-celled): Stoma with guard cells surrounded by at least five subsidiary cells, forming a star-shaped circle. 
• Anisocytic (unequal-celled): Stoma with guard cells between two large subsidiary cells and a smaller one.
• Anomocytic (irregular-celled): Stoma with guard cells surrounded by subsidiary cells of equal size, shape, and arrangement. 
• Diacytic (cross-celled): Stoma with guard cells surrounded by two subsidiary cells perpendicular to each stoma.
• Hemiparacytic (half-parallel celled): Stoma with guard cells that are bordered by one subsidiary cell.
• Paracytic (parallel celled): Stoma with one or more subsidiary cells parallel to the guard cells’ opening. 

Stoma Found in Monocots

• Gramineous (grass-like): Stoma with two guard cells surrounded by two lens-shaped subsidiary cells.
• Hexacytic (six-celled): Stoma with six subsidiary cells around both guard cells.
• Tetracytic (four-celled): Stoma with four subsidiary cells, one on either end of the opening or one next to each guard cell.

Based on Cell Development

These are of three types:

• Mesogynous Type: Here, both guard cells and subsidiary cells are developed from one mother cell.
• Perigynous Type: Here, guard cells are formed from the primary mother cell, while subsidiary cells from accessory mother cells.
• Mesoperigynous Type: Here, guard cells and one subsidiary cell is formed from the mother cell, while other subsidiary cells develop independently.

Based on Arrangement and Distribution of Stomata

• Apple or Mulberry Type: Found only on the lower surface of leaves.
• Potato Type: Found more on the lower surface and less on its upper surface.
• Oat Type: Found equally distributed on both leaf surfaces.
• Water Lily Type: Found only on the upper surface of the leaf.
• Potamogeton Type: They are absent, or if present, they are vestigial.

1. Light Intensity: During low light intensity, photosynthesis will be less, allowing the stomata to remain closed. It allows plants to conserve water. Among all, blue light is found to be more effective in causing stomatal opening. Blue light allows the movement of potassium ions (K⁺). It stimulates malic acid production due to the conversion of starch to sugar. Stomata remain closed in U-V light and green light.
2. Temperature: Stomata open with the rise in temperature and close at low temperature. Temperature significantly increases the permeability of water in the guard cell, causing them to open.
3. Carbon dioxide Concentration: Stomatal movement is influenced by the concentration of carbon dioxide in the intercellular spaces. At a low concentration of carbon dioxide, the stomata remain open. It is closed at high carbon dioxide concentration. When the carbon dioxide concentration inside the cell is higher than the outside environment, the stomata close entirely. A decrease in carbon dioxide concentration inside the leaf triggers a biochemical pathway involving potassium and chloride transporters that allow stomata to reopen.
4. Water Content: It is responsible for causing changes in the turgidity of the guard cell. Guard cells become flaccid on losing water, and thus, the stomata close. Similarly, the guard cells become turgid on gaining water, allowing stomata to open.
5. Potassium Ion Concentration: Accumulation of potassium ions inside the guard cells causes the stomata to open. The stomata close when potassium ions move out of the guard cell.
6. Abscisic Acid (ABA): It is an organic acid that accumulates inside the cell when the plants experience water deficit or water stress. In the presence of ABA, the stomata remain closed.

The opening and closing of stomata happen by two contrasting mechanisms described below:

When the Stomata Remain Closed

When evaporation from the aerial parts is high, stomata close to prevent plant dehydration under hot and dry conditions. The guard cells actively pump potassium ions out of the cell, causing a difference in solute concentration. This ion efflux pushes the water out of the cell from an area of low solute concentration (inside the guard cell) to an area of high solute concentration (surrounding cells). At this stage, the guard cells become flaccid, making it look like the letter ‘I’. Also, during the night, in the absence of transpiration, the stomata remain closed.

When the Stomata Remains Open

For stomata to open during the daytime, potassium ions are actively transported inside the guard cell, increasing its concentration. This ion influx drives the entry of water inside the cell by osmosis, increasing cell turgidity and opening of stomata. At this stage, the two inflated guard cells look like the letter ‘O’.

Stomata serve two primary purposes:

Gas Exchange: Acts as an entryway for carbon dioxide that acts as one of the raw materials for photosynthesis and also the exit point of oxygen that is a byproduct.

Regulating Transpiration: Stomata play an essential role in transpiration by changing their size based on environmental signals. This change is done with guard cells’ help, which expands and contracts to carry out the process. Thus, the entire process from the absorption of water and minerals through roots, their distribution within the plant body, and the final water release depend on the stomatal activity.