Leaf Surface Thermography Under Controlled Vapor Pressure Deficit (VPD) Shift
Executive Abstract
This empirical research note investigates whether fine-scale leaf surface temperature deltas (ΔT = Tleaf - Tair) measured via uncooled LWIR thermal optics provide a statistically significant pre-wilting signal of stomatal closure during elevated vapor pressure deficit (VPD) conditions. In controlled benchtop trials on Monstera deliciosa, stomatal transpiration shutdown caused a measurable +1.4°C leaf thermal elevation 180 minutes before cellular turgor loss was visually apparent.
1. Physical Principles & Energy Balance
Foliar transpiration acts as an evaporative heat sink. Under steady-state solar or ambient illumination, the leaf energy balance equation is given by:
Where Rn is net radiation, λE is latent heat flux, rs is stomatal resistance, and ra is boundary layer resistance. As stomata constrict to prevent desiccation, rs increases rapidly, diminishing latent heat dissipation and causing Tleaf to rise toward or above Tair.
2. Experimental Methodology
Sensor Hardware Suite
- FLIR Lepton 3.5 uncooled thermal imaging core (160x120 resolution)
- Bosch BME688 MOX gas & VOC sensor
- Sensirion SHT41 high-precision humidity/temperature reference
- ESP32-S3 microcontroller executing local telemetry alignment
Environmental Conditions
- Specimen: Monstera deliciosa (3-year indoor specimen)
- Ambient Temperature: 23.5°C ± 0.4°C
- Relative Humidity Shift: 65% → 28% over 4 hours
- Calculated VPD Range: 1.2 kPa → 2.8 kPa
3. Empirical Telemetry Data
Logged time-series observations detailing atmospheric VPD, leaf surface temperature delta, and gas resistance shift:
Interactive Telemetry Time-Series
Leaf Temperature Delta (ΔT) vs Vapor Pressure Deficit (VPD) Shift
At VPD 2.8 kPa, leaf thermal delta reached +1.2°C.
| Elapsed Time | VPD (kPa) | Leaf ΔT (°C) | BME688 Gas R (kΩ) | Inferred Physiological State |
|---|---|---|---|---|
| 00:00 | 1.2 kPa | -1.8°C | 480 kΩ | Optimal Transpiration |
| 01:00 | 1.6 kPa | -1.4°C | 495 kΩ | Optimal Transpiration |
| 02:00 | 2.1 kPa | -0.6°C | 520 kΩ | Stomatal Constriction Begins |
| 03:00 | 2.6 kPa | +0.4°C | 565 kΩ | Transpiration Shutdown |
| 04:00 | 2.8 kPa | +1.2°C | 590 kΩ | Pre-Wilting Stress Signal |
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