Forces Driving Transpiration

Definition of Transpiration: Transpiration is described as "modulated" evaporation from plant surfaces to the atmosphere. This process is critical for the water cycle and influences plant water status.
Rate of Transpiration: The transpiration rate is reflective of water potential differences both inside and outside the plant.

Concept of Water Potential: Water moves from areas of high water potential to areas of low water potential.
High Water Potential: Inside the leaf, there typically exists a high water potential.
Low Water Potential: Conversely, the atmosphere usually presents a low water potential, creating a gradient that drives the movement of water.

Air Saturation: The air inside leaves is generally very humid, suggesting extreme saturation. This high concentration of moisture leads to significant transpiration.
Concentration Gradient: Water concentration inside a leaf is much higher than that of the surrounding atmosphere, which is a driving factor in transpiration.

Definition of Water Potential Measurement: Water potential (Ψ) measures water concentration and is quantified with the standard that pure water equals a potential of 0.
Example Values:
- Ψ = -167 MPa at a relative humidity of 30% and 30°C.
- Ψ = -2 MPa in different conditions.
Understanding MPa: MPa, or millipascals, serves as a unit of pressure measurement in this context.

Driving Forces: Differences in water potential drive the movement of water within the plant.
Stomatal Functionality: When stomata are open, the difference in water potential (near 0 inside and very low outside) causes evaporation at the leaf surface. This creates a "pull" effect, drawing water from the soil through the plant’s vascular system, enhancing nutrient absorption and maintaining physiological functions.

Impact of Humidity: Little transpiration occurs on foggy days, as higher external humidity leads to a decreased concentration gradient between the leaf and surrounding air.

Relative Humidity Inside Leaves: The relative humidity inside a leaf is often close to 100% (Ψ ≈ 0).
Transpiration from Canopy: It is frequently stated that the amount of water transpiring from the canopy of a vegetated area is equivalent to that which would evaporate from a similarly sized body of water located at the same site.
Critical Analysis of Statement: Students are encouraged to explain what aspects of this assertion are correct and what may be misleading or incorrect, fostering critical thinking in plant-water relations.