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Plant Water Transport Lab

Investigate how plants lose water through transpiration and transport it from roots to leaves. Vary environmental conditions, collect data on transpiration rates and water uptake, and discover the relationships driving plant water balance.

Guided Experiment: Environmental Factors and Transpiration

How do you predict transpiration rate will change when you increase temperature, decrease humidity, or increase wind speed independently? Which factor do you think has the greatest effect?

Write your hypothesis in the Lab Report panel, then click Next.

Controls

Leaf Area50 cm²
Stomatal Density200 /mm²
Stomatal Aperture8.0 μm
Temperature25.0 °C
Humidity50 %
Light Intensity1000 μmol/m²/s
Wind Speed1.5 m/s

Results

E=gs⋅VPDPatm=3.073 mmol/m2/sE = g_s \cdot \frac{\mathrm{VPD}}{P_{\mathrm{atm}}} = 3.073 \text{ mmol/m}^2\text{/s}
Conductance (gs)
0.9930 mol/m²/s
VPD
1.584 kPa
Transpiration Rate
3.073 mmol/m²/s
Water Uptake
0.997 mL/h
Conditions
Temperature25.0°CHumidity50%Leaf Area50 cm²Aperture8.0 μm

Temperature Response

Transpiration rate vs temperature at current stomatal and humidity settings

Transpiration Rate

Stomatal Aperture Response

Transpiration rate vs aperture width at current environmental conditions

Transpiration Rate

Data Table

(0 rows)
#TrialLeaf Area(cm²)Stomatal Density(/mm²)Temperature(°C)Humidity(%)Light(μmol/m²/s)Transpiration Rate(mmol/m²/s)Water Uptake(mL/h)
0 / 500
0 / 500
0 / 500

Reference Guide

Transpiration

Transpiration is the evaporation of water from plant leaves through stomata. It accounts for about 99% of the water a plant absorbs, creating the driving force for water transport.

E=gs⋅VPDPatmE = g_s \cdot \frac{\mathrm{VPD}}{P_{\mathrm{atm}}}

The transpiration rate depends on stomatal conductance and the vapor pressure deficit between the leaf air spaces and the atmosphere.

Vapor Pressure Deficit

VPD is the difference between how much moisture the air can hold and how much it currently holds. The saturation vapor pressure follows the Magnus formula.

es=0.611×exp⁡ ⁣(17.27 TT+237.3)e_s = 0.611 \times \exp\!\left(\frac{17.27\,T}{T + 237.3}\right)

Because saturation vapor pressure increases exponentially with temperature, hot environments can have very high VPD, driving rapid transpiration.

Stomatal Control

Stomata are microscopic pores in the leaf epidermis, each bordered by two guard cells. When guard cells swell with water (become turgid), the stoma opens.

  • Light triggers stomatal opening in most plants, enabling CO₂ uptake for photosynthesis.
  • Water stress and the hormone ABA cause stomata to close, conserving water at the cost of reduced photosynthesis.
  • Stomatal density varies widely: 50-500 per mm², mainly on the lower leaf surface.

Cohesion-Tension Theory

Water moves through xylem vessels under tension (negative pressure) created by transpiration at the leaf surface. This pull is transmitted through the continuous water column due to cohesion between water molecules.

Ψleaf=Ψsoil−ρgh\Psi_{\text{leaf}} = \Psi_{\text{soil}} - \rho g h

The water potential gradient from soil to leaf drives the entire soil-plant-atmosphere continuum. Each 10 m of height requires about -0.1 MPa just to overcome gravity.