Transpiration and Guttation



Transpiration : 

  • Transpiration stands as the plant's hydraulic masterpiece, involving the evaporation of water from specialized pores—stomata—chiefly located on leaves. 
  • These pores, more than mere openings, serve dual functions by facilitating gas exchange (CO2 and O2) alongside their role in transpiration. 
  • The pivotal players in stomatal dynamics are the guard cells, which control pore openings based on their turgor pressure.



  • Microfibrils within guard cell walls are akin to structural choreographers, influencing the orientation and responsiveness of guard cells. 
  • Leaves exhibit either dorsiventral (dicots) or isobilateral (monocots) arrangements of stomata, creating distinct patterns in their distribution. 
  • Dorsiventral leaves, characteristic of dicotyledonous plants, exhibit a clear structural bias with distinct upper (adaxial) and lower (abaxial) surfaces. 
  • Stomata, pores crucial for gas exchange and transpiration, are typically more concentrated on the lower surface. This arrangement reduces water loss from the upper surface and shields stomata from direct sunlight. 
  • Isobilateral leaves, prevalent in monocotyledonous plants, showcase a symmetrical design with no marked upper or lower surface. 
  • Stomata are relatively evenly distributed on both surfaces, emphasizing an equal exposure to environmental factors. This symmetry aids in uniform photosynthetic activity and transpiration. 
  •  Influential Factors in Transpiration:

- External Parameters: Temperature, light intensity, humidity, and wind speed act as external conductors, orchestrating the plant's water release. 

- Endogenous Variables: The plant's intrinsic factors, encompassing stomatal density, aperture width, overall water status, and canopy structure, intricately modulate transpiration dynamics. 

  • Sap Ascent and Water Properties:

- Cohesion and Adhesion Forces: The ascent of sap is governed by water's intrinsic properties—cohesion (molecular attraction between water molecules) and adhesion (interaction with vessel surfaces). 

- Capillary Action: Narrow tracheary elements—tracheids and vessel elements—coupled with capillarity, underpin the seamless upward movement of water within the plant. 

  • Tensile Strength – Water's Resilience:

- Water acts like a strong, resilient thread resisting being pulled apart. 

- Within plants, water demonstrates strength, resisting gravity's pull through narrow tubes. 

  • Fueling Photosynthesis – Xylem's Water Contribution:

- Water is essential for the plant's food-making process. 

- Water travels from roots to leaves, supporting the plant's food production. 

  • Transpiration isn't merely a water disposal mechanism; it intricately aligns with the plant's photosynthetic demands, ensuring a constant water supply to leaf cells. 
  • Stomatal evaporation initiates a molecular pull, establishing a dynamic equilibrium between leaf interiors and the atmosphere. 
  • Transpiration and Photosynthesis – a Compromise. Transpiration in plants serves multiple vital functions:

- Transpiration Pull: Initiates absorption and transport of water in plants.

 - Water Supply for Photosynthesis: Provides the essential ingredient for the plant's food-making process. 

- Mineral Transport: Conveys minerals from the soil to various plant parts. 

- Leaf Cooling: Acts as a natural air conditioner, reducing leaf temperatures by 10 to 15 degrees.

 - Structural Support: Maintains cell turgidity, preserving the plant's shape and structure. 

For a thriving, photosynthesizing plant, water is a must. However, the demand for water during photosynthesis poses a challenge, as transpiration rapidly depletes available water. Rainforests owe their humidity to the continuous water cycle – from roots to leaves, into the atmosphere, and back to the soil.

  • C4 Photosynthesis – Nature's Water-Saving Strategy:

The evolution of the C4 photosynthetic system represents a strategic move to balance CO2 availability and water conservation. C4 plants exhibit twice the efficiency of C3 plants in carbon fixation (sugar production). Remarkably, a C4 plant loses only half as much water as a C3 plant while fixing the same amount of CO2.




  • As ions actively traverse from the soil into the roots' vascular tissues, water obediently follows its potential gradient, elevating pressure within the xylem. This positive force, aptly termed root pressure, can exert itself, nudging water to modest heights in the stem. 
  • To observe root pressure, engage in a simple experiment. Choose a small, soft-stemmed plant on a moisture-laden day. Swiftly cut the stem horizontally near the base in the early morning. Witness the emergence of solution droplets, evidence of positive root pressure. By attaching a rubber tube to the cut stem, you can measure the exudation rate and analyze its composition. 
  • Root pressure's effects are also noticeable during low evaporation periods, particularly at night and early morning. In this serene period, excess water collects in droplets around vein openings near the tips of grass blades and leaves – a phenomenon known as guttation.  
  • A pore, bears especially on a leaf, which exudes drops of water is called hydathode. 
  • However, it's crucial to recognize that root pressure, while intriguing, plays a supplementary role in water transport, offering a modest nudge. Tall trees primarily rely on the triumphant force of transpiratory pull to meet their extensive water transport needs. Root pressure, though a player, doesn't steal the limelight in the grand water transport performance orchestrated by transpiration.