Stomatal Physiology

Introduction to Stomata: Stomata are microscopic pores found on the aerial parts of plants, primarily on leaves, stems, and flowers, that regulate the exchange of gases and water vapor between the plant and the atmosphere. The functioning of stomata is essential for the plant’s physiological processes such as photosynthesis, transpiration, and gas exchange.

Structure of Stomata:

• Guard Cells:
  • Guard cells are specialized epidermal cells that surround the stomatal pore. They play a key role in controlling the opening and closing of the stomata.
  • They are kidney-shaped in dicots and dumbbell-shaped in monocots.
  • Guard cells contain chloroplasts which allow them to carry out some photosynthesis.
  • The inner walls of the guard cells are thicker than the outer walls, and this asymmetry helps in stomatal movement.

• Stomatal Pore: The stomatal pore is the space between two guard cells through which gases (CO₂ and O₂) and water vapor diffuse in and out of the plant.
• Subsidiary Cells: Surrounding the guard cells are subsidiary cells that assist in the function and regulation of the stomatal aperture.
• Epidermal Cells: Epidermal cells are the cells that make up the rest of the leaf or stem surface and protect the plant. They are located around the stomata.

Functions of Stomata:

• Gas Exchange:
  • Stomata regulate the exchange of gases essential for photosynthesis and respiration.
    • CO₂ enters the leaf for photosynthesis.
    • O₂ produced during photosynthesis is released through the stomata.
    • Water vapor is lost during transpiration.

• Transpiration:
  • Transpiration is the process of water vapor loss from the plant, primarily through the stomata.
    • Transpiration serves to cool the plant, maintain turgor pressure, and transport water and nutrients from the roots to the leaves.

• Water Regulation:
  • Stomata help the plant regulate water loss. By controlling the size of the stomatal pore, the plant can prevent excessive water loss during dry conditions and open the stomata when more water vapor is needed.

Mechanism of Stomatal Movement:

The opening and closing of stomata are controlled by changes in the turgor pressure of the guard cells. The guard cells change shape in response to environmental cues and the plant’s physiological needs. The movement of water into and out of the guard cells controls the stomatal aperture.

• Opening of Stomata:
  • When guard cells take up water by osmosis, they become turgid (swollen) and the pore opens.
  • The inner walls of the guard cells are thicker than the outer walls. As the guard cells swell, they curve outward, and the stomatal pore opens.
  • Light stimulates the opening of stomata (particularly blue light).
  • Low CO₂ concentration in the leaf also promotes stomatal opening.

• Closing of Stomata:
  • When guard cells lose water, their turgor pressure decreases, causing them to become flaccid, which leads to the closing of the stomatal pore.
  • Abscisic acid (ABA), a stress hormone, induces the closure of stomata during water stress conditions.
  • High CO₂ concentration and darkness also promote the closing of stomata.

Factors Influencing Stomatal Movement:

Several environmental and internal factors influence the stomatal movement:

• Light:
  • Light is a major factor that stimulates the opening of stomata. The blue light wavelengths activate phototropin receptors in guard cells, leading to the uptake of ions and water, causing the stomatal pore to open.
  • In the absence of light (e.g., during the night), stomata tend to close to conserve water.

• Carbon Dioxide (CO₂):
  • Low CO₂ concentration inside the leaf stimulates stomatal opening to allow more CO₂ for photosynthesis.
  • High CO₂ concentration leads to stomatal closure as less CO₂ is required for photosynthesis.

• Water Availability:
  • High soil moisture leads to increased water absorption by roots, resulting in turgid guard cells and stomatal opening.
  • During drought stress or water scarcity, abscisic acid (ABA) is produced, causing the guard cells to lose water, resulting in stomatal closure to conserve water.

• Temperature:
  • High temperatures increase the rate of transpiration. Initially, stomata open to release water vapor. However, if temperatures rise too much, stomata close to prevent excessive water loss.
  • Excessive heat can cause damage to guard cells, affecting their ability to regulate stomatal aperture.

• Humidity:
  • High humidity reduces the gradient for water vapor diffusion between the leaf and the atmosphere, leading to a decrease in transpiration. This can influence stomatal opening, as plants do not need to lose water in such conditions.
  • Low humidity increases the rate of transpiration, leading to the opening of stomata to maintain water balance.

• Wind:
  • Wind can increase transpiration by removing moisture from the leaf surface, encouraging stomata to open more to replace the lost water.
  • However, strong winds may dry out the plant and lead to stomatal closure.

Hormonal Regulation of Stomatal Movement:

• Abscisic Acid (ABA): ABA is the primary plant hormone involved in stomatal closure, especially during water stress conditions. ABA signals guard cells to release water, leading to stomatal closure.
• Auxins, Cytokinins, and Gibberellins: These hormones have a less direct effect on stomatal movement, but they may influence stomatal function by regulating overall plant growth and responses to light and stress.

Stomatal Density:

Stomatal density refers to the number of stomata per unit area of leaf surface. It varies among plant species and environmental conditions.

• High Stomatal Density: Common in plants growing in humid environments with abundant water. Found in plants with high transpiration rates and those needing efficient gas exchange for photosynthesis.

• Low Stomatal Density: Common in plants growing in arid or water-limited environments where water conservation is crucial. These plants have fewer stomata to reduce water loss through transpiration.

Ecological and Evolutionary Significance of Stomata:

• Adaptations to Drought: In plants from arid regions (xerophytes), stomata may be fewer and may remain closed during the day to minimize water loss. Some xerophytes may have sunken stomata to trap moisture around the pore, reducing water loss.
• Adaptations to High Moisture Environments: Plants in high-humidity environments (hydrophytes) may have numerous stomata on their leaves, especially on the upper surface, to facilitate gas exchange and oxygen release in an aquatic environment.
• CAM (Crassulacean Acid Metabolism) Plants: CAM plants (e.g., cacti and succulents) open their stomata at night to prevent water loss during the hot daytime, a crucial adaptation in arid regions.

Role of Stomata in Photosynthesis and Transpiration:

• Photosynthesis: Stomata provide CO₂ for photosynthesis. CO₂ is absorbed through the stomatal pore and enters the leaf cells, where it is used in the Calvin cycle to form glucose.
• Transpiration: Transpiration plays an essential role in nutrient transport from roots to leaves through the xylem. It also helps in cooling the plant, preventing overheating from the sun.

Stomatal Conductance:

• Stomatal conductance is a measure of the ease with which gases, primarily CO₂, can diffuse through the stomata. It depends on the size of the stomatal pore and the density of stomata.
• Measurement: Stomatal conductance is usually measured using a Porometer, a device that determines the rate of water vapor exchange through the stomata.