Diffusion and Osmosis:

Diffusion is the movement of molecules (gases, liquids, or solutes) from regions of higher concentration to lower concentration until they are evenly distributed throughout the space. In plants, diffusion facilitates the exchange of gases, like oxygen and carbon dioxide, and the movement of water and nutrients.

Osmosis is a special type of diffusion where water molecules move from a region of lower solute concentration to a region of higher solute concentration through a semi-permeable membrane. The cell membrane is a semi-permeable membrane that allows the movement of water but restricts solute movement.

Types of Osmosis:

1. • Endoosmosis: The diffusion of water into a cell, causing it to swell.
   • Exoosmosis: The diffusion of water out of a cell, causing it to shrink.

Imbibition: Imbibition is the process by which dry substances absorb water due to their affinity for water (hydrophilic nature). This is an essential initial step in water absorption by plants. The rate of imbibition increases with temperature. Imbibition is more prominent in seeds, especially oily seeds compared to starchy seeds.

Plasmolysis: Plasmolysis refers to the shrinkage of the protoplasm within a plant cell due to the loss of water when the cell is placed in a hypertonic solution. The point at which plasmolysis begins, though not visible, is termed incipient plasmolysis. This process helps understand the effects of external solute concentration on cell turgor and water regulation.

Transpiration:

• Transpiration is the process by which plants lose water in the form of vapor through stomata (small pores) in the leaves, stems, and other aerial parts.
• It helps in the movement of water from the roots to the leaves and contributes to cooling the plant.

Types of Transpiration:

• Stomatal Transpiration: Water loss through stomata, accounting for about 90-95% of total transpiration.
• Cuticular Transpiration: Water loss through the cuticle (waxy layer) of the plant surface, which is less significant.
• Lenticular Transpiration: Water loss through lenticels (small openings in the bark), which occurs in woody plants.

Mechanism of Transpiration:

• Water is absorbed by the roots from the soil.
• It travels through the xylem vessels to the leaves.
• In the leaves, water moves into the intercellular spaces and evaporates through the stomata into the atmosphere.
• This creates a transpiration pull, helping in the upward movement of water and nutrients through the plant.

Factors Affecting Transpiration:

• Light Intensity: Increased light leads to the opening of stomata, increasing transpiration.
• Temperature: Higher temperatures increase the rate of evaporation of water from the plant surface.
• Humidity: Lower humidity (drier air) increases transpiration, as the water vapor gradient between the plant and the surrounding air becomes steeper.
• Wind Speed: Wind moves the moist air away from the leaf surface, increasing transpiration by maintaining a steep water vapor gradient.
• Soil Water Availability: Sufficient water supply ensures that transpiration can occur at its optimal rate. Lack of water reduces transpiration.
• Stomatal Opening: The size of stomatal pores, controlled by guard cells, regulates the rate of transpiration.
• Leaf Surface Area: Larger leaf surface area increases the transpiration rate.
• Plant Type: C4 and CAM plants transpire less due to adaptations to hot and dry environments.

Importance of Transpiration:

• Water Regulation: Transpiration helps maintain water balance within the plant.
• Cooling Effect: The evaporation of water from leaves helps cool the plant, especially in high-temperature conditions.
• Nutrient Transport: Transpiration creates a transpiration stream, facilitating the upward movement of water, minerals, and nutrients from the soil to the plant.
• Turgor Pressure Maintenance: Helps in maintaining turgor pressure within plant cells, which is crucial for structural rigidity.
• Gas Exchange: Stomata, through which transpiration occurs, also allow gas exchange (CO₂ uptake and O₂ release) for photosynthesis.

Transpiration Stream:

• The transpiration stream refers to the continuous movement of water from the soil to the leaves.
• Water is absorbed by roots, transported through the xylem, and evaporated from the leaf surface, creating a pulling force that draws water from the roots to the leaves.

Adaptations in Plants:

• Reduced Stomatal Density: Plants in arid environments (xerophytes) often have fewer stomata to minimize water loss.
• Leaf Modifications: Some plants have thick cuticles or leaf modifications (e.g., needle-like leaves in conifers) to reduce transpiration.
• CAM and C4 Pathways: These plants have adaptations to reduce water loss by opening stomata at night (CAM plants) or minimizing photorespiration (C4 plants).

Guttation vs. Transpiration:

• Guttation is the exudation of liquid water from the plant, usually through specialized structures called hydathodes in the leaf margins, typically occurring when transpiration is low, and root pressure is high.
• Transpiration involves water loss as vapor, while guttation involves the loss of liquid water.

Stomata:

Stomata are specialized epidermal cells found primarily on the lower surface of leaves in terrestrial plants. Approximately 97% of transpiration occurs through the lower surface. Each stoma has two kidney-shaped guard cells that regulate its opening and closing.

Classification of Stomata:

1. Apple and Mulberry type: Stomata on the lower surface only.
2. Potato type: More stomata on the lower surface than the upper surface.
3. Oat type: Stomata are equally distributed on both surfaces.
4. Water Lily type: Stomata only on the upper surface.
5. Potamogeton type: Stomata are either absent or functionless (mainly in submerged aquatic plants).

Factors Regulating Stomatal Movement:

• Guard Cells: The opening and closing of stomata are controlled by the turgidity of guard cells. When guard cells are turgid, stomata open; when guard cells lose turgor, stomata close.
• Hormones:
  • Abscisic Acid (ABA): Stimulates stomatal closure, particularly during water stress.
  • Auxins and Cytokinins: Influence stomatal behavior and plant growth, indirectly affecting transpiration.

Transpiration and Climate:

• High Humidity: Reduces transpiration by decreasing the water vapor gradient between the plant and the atmosphere.
• Low Humidity: Increases transpiration, especially in dry conditions where water vapor tends to diffuse rapidly into the air.

Transpiration Rate Measurement: The rate of transpiration can be measured by methods such as the potometer (measuring the uptake of water by a plant) or the lysimeter (measuring the water loss from a plant).