Stomatal control and water loss

What are Stomata?

• Stomata (singular: stoma) are tiny pores found predominantly on the underside of leaves, though they can also be present on stems.
• Their primary function is to facilitate gas exchange between the plant's internal tissues and the atmosphere. This includes:
  • Uptake of carbon dioxide (CO2) for photosynthesis.
  • Release of oxygen (O2), a byproduct of photosynthesis.
  • Release of water vapor (H2O), a process known as transpiration.

Structure of Stomata

• Each stoma is surrounded by two specialized cells called guard cells.
• Guard cells are kidney-bean shaped and contain chloroplasts, allowing them to photosynthesize.
• The inner walls of guard cells (facing the pore) are thicker than their outer walls.
• Changes in the turgor pressure within guard cells cause the stomata to open or close.

What is Transpiration?

• Transpiration is the process by which water vapor is lost from the plant, primarily through the stomata, to the atmosphere.
• It is essentially the evaporation of water from the mesophyll cells inside the leaf, followed by the diffusion of water vapor out of the stomata.
• Transpiration is an inevitable consequence of the need for plants to open their stomata for carbon dioxide uptake.

How Guard Cells Control Stomata

• The opening and closing of stomata are regulated by changes in the turgor pressure within the guard cells.

Stomatal Opening:

1. Light intensity: In the presence of light, guard cells photosynthesize.
2. Potassium ion (K+) uptake: Photosynthesis leads to the active transport of K+ ions into the guard cells from surrounding epidermal cells.
3. Water potential: The influx of K+ ions lowers the water potential inside the guard cells.
4. Osmosis: Water then moves by osmosis from adjacent epidermal cells into the guard cells.
5. Turgor pressure increases: The guard cells become turgid (swollen).
6. Cell wall differential thickness: Due to the thicker inner walls and thinner outer walls, the guard cells bow outwards when turgid, causing the stoma to open.

Stomatal Closing:

1. Absence of light/Water stress: In darkness or when the plant experiences water stress, K+ ions are actively pumped out of the guard cells.
2. Water potential increases: The loss of K+ ions increases the water potential inside the guard cells.
3. Osmosis: Water moves by osmosis out of the guard cells into the surrounding epidermal cells.
4. Turgor pressure decreases: The guard cells become flaccid (limp).
5. Stoma closes: The guard cells lose their bowed shape and move closer together, closing the stoma.

Factors Affecting Stomatal Movement

• Light intensity: Stomata generally open in light and close in darkness (to conserve water when photosynthesis isn't occurring).
• Carbon dioxide concentration: Low CO2 levels inside the leaf (due to high photosynthetic rates) tend to cause stomata to open. High CO2 levels cause them to close.
• Water availability: When the plant is under water stress, the hormone abscisic acid (ABA) is produced, which signals guard cells to close the stomata to conserve water, even in the light.

The rate at which water is lost from a plant through transpiration is influenced by several environmental factors:

• Temperature:

  • Higher temperatures increase the kinetic energy of water molecules, leading to a faster rate of evaporation from the mesophyll cells and a faster diffusion of water vapor out of the stomata.
  • Warm air can hold more water vapor, increasing the water potential gradient between the leaf and the atmosphere.

• Humidity:

  • Lower humidity (drier air) means a steeper water potential gradient between the inside of the leaf (saturated with water vapor) and the outside air.
  • This steeper gradient increases the rate of diffusion of water vapor out of the stomata, thus increasing transpiration.
  • High humidity reduces the gradient, slowing down transpiration.

• Wind speed:

  • Increased wind speed blows away the layer of humid air (boundary layer) that accumulates around the leaf surface.
  • Removing this humid air maintains a steep water potential gradient between the leaf and the atmosphere, thus increasing the rate of transpiration.
  • Still air allows a humid boundary layer to build up, reducing the gradient and slowing transpiration.

• Light intensity:

  • Higher light intensity generally causes stomata to open wider to allow more CO2 for photosynthesis.
  • With more open stomata, there are more pores for water vapor to escape, increasing transpiration.
  • Light also increases leaf temperature, which indirectly increases transpiration.

• Water availability (soil moisture):

  • If the soil is dry and the plant cannot absorb enough water, the guard cells will lose turgor and the stomata will close to conserve water.
  • This directly reduces the rate of transpiration, even if other environmental factors might otherwise promote it.

Importance of Transpiration

1. Transpiration Stream (Transpirational Pull):

   • The loss of water vapor from the leaves creates a 'pull' or tension that draws water up from the roots through the xylem vessels.
   • This continuous column of water, maintained by cohesion (attraction between water molecules) and adhesion (attraction between water and xylem walls), is known as the transpiration stream.
   • It is the primary mechanism for water transport in plants.

2. Transport of Mineral Salts:

   • Mineral ions absorbed by the roots are dissolved in the water within the xylem.
   • The transpiration stream carries these essential mineral nutrients from the roots to all parts of the plant where they are needed for growth and metabolic processes.

3. Cooling Effect:

   • As water evaporates from the leaf surface, it absorbs latent heat of vaporization from the leaf.
   • This evaporative cooling helps to prevent the leaf from overheating, especially in direct sunlight, which could otherwise damage enzymes and cellular structures.

Consequences of Excessive Water Loss

• Wilting: If the rate of water loss through transpiration exceeds the rate of water absorption by the roots, the plant cells lose turgor pressure. This causes the plant to become flaccid and droop, a condition known as wilting.
• Reduced Photosynthesis: Wilting often leads to stomatal closure to conserve water. While this reduces water loss, it also limits the uptake of CO2, thereby reducing the rate of photosynthesis.
• Death: Prolonged and severe water loss can lead to irreversible damage to plant tissues and ultimately, the death of the plant.