4.2: Transpiration

• Process of Transpiration:

  • Transpiration occurs as water vaporizes from the surfaces of plant leaves and enters the surrounding atmosphere.
  • Stomata, which are tiny pores found on the surface of leaves, regulate the exchange of gases and water vapor between the plant and the environment.
  • During transpiration, water evaporates from the moist surfaces inside the leaf and diffuses out through the stomata into the air.
• Factors Influencing Transpiration:

  • Transpiration rates are influenced by environmental factors such as temperature, humidity, wind speed, and light intensity.
  • Higher temperatures and low humidity levels increase transpiration rates, as they promote faster evaporation of water from the leaf surfaces.
  • Wind speed enhances transpiration by removing water vapor from the vicinity of the leaves, creating a gradient that facilitates further water loss.
• Functions of Transpiration:

  • Transpiration plays several essential roles in plants, including:
  • Facilitating the transport of water and minerals from the roots to the leaves through the xylem vessels.
  • Cooling the plant by dissipating excess heat through the process of evaporative cooling.
  • Maintaining turgor pressure within plant cells, which is essential for structural support and cell expansion.
• Consequences of Excessive Transpiration:

  • Excessive transpiration can lead to water stress in plants, especially under conditions of water scarcity or high temperatures.
  • When water loss through transpiration exceeds water uptake by the roots, plants may exhibit symptoms of wilting, where their leaves become limp and droopy due to loss of turgor pressure.
  • Wilting is a protective mechanism employed by plants to reduce water loss and prevent dehydration during periods of water deficit.
• Entry into Leaves:

  • Water enters leaves through the xylem vessels, which transport water from the roots to the aerial parts of the plant.
  • Upon reaching the leaf, water moves out of the xylem vessels and into the leaf cells, primarily through specialized cells known as mesophyll cells.
• Utilization of Water:

  • Inside the leaf, water is utilized by chloroplast-containing cells for photosynthesis, the process by which plants convert light energy into chemical energy to produce glucose.
  • While some water is used during photosynthesis, the majority of it does not remain in the cells.
• Evaporation and Transpiration:

  • Liquid water that enters the leaf cells eventually moves into the cellulose cell walls and transitions into water vapor through the process of evaporation.
  • The water vapor then diffuses into the air spaces between the leaf cells.
  • These air spaces are connected to the external environment through small openings called stomata, which are primarily located on the underside of the leaf.
  • Water vapor can pass through the stomata and into the surrounding atmosphere.
• Transpiration:

  • The loss of water vapor from the aerial parts of plants, particularly from leaves, is termed transpiration.
  • Transpiration serves several important functions, including the regulation of internal water balance, cooling of the plant, and facilitation of nutrient uptake from the soil.
  • It is a crucial component of the plant's water transport system, ensuring the movement of water and nutrients from the roots to the leaves.
• Plants transport sugars through a process known as translocation
• Photosynthesis occurs in the leaves, where sugars are made
• Sugars made in the leaves are transported to the rest of the plant for energy through translocation
• Translocation is mainly achieved by phloem cells arranged end to end to form phloem tubes
• Phloem cells have small pores between adjacent cells that enable the movement of cell sap, a liquid mixture of water and sugar
• Sugars made in the leaves can be transported long distances through multiple cells via the phloem tubes
• Sugars transported through phloem can be used directly for energy or stored for later use
• Phloem can transport substances in either direction, up or down the plant
• Xylem tubes transport water and mineral ions from the roots to the leaves
• Xylem tubes are made up of dead xylem cells with no ends between them, forming one long hollow tube
• Xylem tubes are strengthened with lignin
• Water transported by xylem is used in photosynthesis in the leaves
• Water movement through a plant is driven by the evaporation of water from the leaves
• Evaporation of water from the leaves is called transpiration
• The movement of water molecules is known as the transpiration stream
• The rate of transpiration can vary and is influenced by light intensity, temperature, air flow, and humidity
• Factors influencing the rate of transpiration:

  • Light intensity: Brighter light leads to more photosynthesis, more open stomata, and higher transpiration rate
  • Temperature: Warmer temperatures increase the rate of transpiration due to higher energy water particles
  • Air flow: High airflow increases the rate of transpiration by quickly removing water molecules from the leaf
  • Humidity: More humid air decreases the rate of transpiration by reducing the concentration gradient for water diffusion
• Structural Support: Water provides turgidity to plant cells, helping them maintain their shape and rigidity. Sufficient water content in plant cells ensures that the plant remains upright and supported.
• Transport of Nutrients: Water acts as a solvent, facilitating the transport of dissolved mineral nutrients from the soil through the plant's vascular system, particularly through the xylem vessels.
• Cooling: Water evaporates from the surfaces of leaves through transpiration, dissipating heat and cooling the plant. This is particularly crucial for plants growing in hot environments to prevent overheating.
• Photosynthesis: Water is a crucial reactant in the process of photosynthesis, providing electrons during the light-dependent reactions and contributing to the synthesis of glucose, the primary energy source for the plant.