Leaf-to-air Temperature Difference Explained.

Introduction.

Leaves frequently operate at temperatures that differ from the surrounding air. This difference reflects the balance between absorbed radiation, evaporative cooling and heat exchange with the atmosphere. Measuring leaf to air temperature difference (LATD) therefore helps researchers investigate how plants respond to changing water availability and atmospheric conditions.

The Implexx LT-1T leaf temperature sensor provides repeated measurements from an individual leaf. Paired with a suitable air temperature sensor, it allows researchers to follow LATD through drought development, heat events and recovery after irrigation.

What is LATD.

LATD is the difference between leaf temperature and a defined reference air temperature. Here, the sign convention is leaf temperature minus air temperature:

Both temperatures are measured simultaneously, usually in degrees Celsius. For example, a leaf at 28 °C in air at 30 °C has a LATD of −2 °C.

LATD	Temperature relationship
Negative	The leaf is cooler than the reference air
Zero	The leaf and reference air have the same temperature
Positive	The leaf is warmer than the reference air

These categories describe thermal conditions. Their physiological interpretation requires information about radiation, humidity, air movement and the leaf itself. A positive LATD can occur in actively transpiring vegetation; Still et al. (2022), for example, found that canopy leaves across several forest ecosystems were warmer than air during much of the daytime.

The science behind leaf heating and cooling.

A leaf absorbs shortwave solar radiation and exchanges longwave radiation with its surroundings. It also exchanges sensible heat with the air and loses latent heat as water evaporates during transpiration. At approximate steady state, neglecting heat storage and small biochemical energy terms, the leaf energy balance can be written as:

Here, Rₙ is net radiation, H is sensible heat exchange and λE is latent heat loss through transpiration, all expressed as energy fluxes per unit leaf area. E is the mass flux of transpired water and λ is the latent heat of vaporisation (Jones, 1999a).

When transpiration dissipates a greater share of the available energy, less sensible heat loss is required and leaf temperature generally falls. If a leaf becomes cooler than the surrounding air, sensible heat flows towards the leaf and can help supply the energy for evaporation. Conversely, reduced evaporative cooling can increase leaf temperature. This energy balance underpins the interpretation of leaf temperature measurements.

During rapid changes in sunlight or wind, heat storage can be significant. Transient temperature responses should therefore be distinguished from the steady state assumed by many models (Muller et al., 2023).

Why LATD matters in plant physiology.

Stomatal regulation and water use.

Stomata regulate much of the water vapour leaving a leaf. Under comparable environmental conditions, stomatal closure often reduces evaporative cooling and shifts LATD towards more positive values. Following this response over time can help researchers investigate stomatal sensitivity to soil drying, atmospheric demand or an experimental treatment.

Species can differ in how they balance water conservation and leaf cooling. Fauset et al. (2018) documented contrasting thermal and water use responses among three co-occurring Atlantic forest tree species, illustrating why LATD should be interpreted in relation to species physiology.

Heat exposure and photosynthesis.

Leaf temperature determines the thermal conditions experienced by photosynthetic tissues. A weather station can therefore underestimate or overestimate the temperature experienced by the leaf. Retaining both absolute leaf temperature and LATD helps researchers distinguish thermal exposure from the degree of leaf cooling relative to air.

Cooling also has a water cost, and its relationship with carbon uptake can change during extreme heat. In Eucalyptus parramattensis, Drake et al. (2018) observed sustained transpiration during an experimental heatwave even as midday canopy photosynthesis approached zero. Temperature and water flux measurements together can reveal responses that would be missed by either measurement alone.

Thermal traits and environmental responses.

Leaf dimensions and exposure affect thermal behaviour. Effective leaf width influences the boundary layer through which heat is exchanged, while leaf angle and canopy position influence radiation loading. Leigh et al. (2017) showed that effective leaf width predicted thermal response time and LATD across 68 Proteaceae species. These physical differences should be considered when comparing species or treatments.

LATD and leaf-to-air vapour pressure deficit.

Leaf temperature also affects the vapour pressure gradient driving transpiration. Assuming saturated air within the leaf, leaf to air vapour pressure deficit is:

Here, the first term is saturation vapour pressure at leaf temperature. The second is the actual vapour pressure of the reference air, obtained from air temperature and relative humidity.

For an illustrative calculation, air at 30 °C and 50% relative humidity has an atmospheric VPD of approximately 2.12 kPa. If the leaf is at 35 °C, its leaf to air vapour pressure deficit is approximately 3.50 kPa. Using air temperature for both sides of the calculation would substantially underestimate that gradient. The feedback between leaf warming, vapour pressure deficit and transpiration is explored by Konrad et al. (2021).

Factors affecting LATD.

Several processes can produce similar changes in LATD. Stronger radiation can warm a leaf even while transpiration remains high. Wind alters heat and water vapour exchange across the boundary layer. Changes in humidity alter evaporative demand, while soil water availability and plant hydraulic function influence the capacity to sustain water loss.

Subtracting air temperature removes one source of variation, but radiation, humidity and wind still affect the result. Comparisons are strongest when leaf exposure is matched and relevant environmental variables are recorded. Raw LATD also differs from a normalised crop water stress index, which requires appropriate reference conditions or baselines (Jones, 1999b).

Night-time cooling requires separate interpretation because longwave radiation can cool a leaf below air temperature when transpiration is low. Rainfall, dew and irrigation wetting can also alter temperature through evaporation from the leaf surface.

Measuring LATD with the Implexx leaf temperature sensor.

The Implexx LT-1T uses a small, glass-encapsulated thermistor held against the leaf by a lightweight stainless steel clip. The thermistor has a contact area of approximately 1 mm². Its fine leads help minimise conductive heat exchange with the sensor assembly. This design supports repeated measurements at a defined location on a leaf.

The LT-1T measures absolute leaf temperature. LATD is calculated by pairing this measurement with a separate, synchronised air temperature channel. The sensor provides SDI-12 output for compatible data loggers and operates from a 5–24 VDC supply. The manual specifies a measurement range of −5 to +50 °C. Verify that expected leaf temperatures fall within this range (Implexx, LT-1T manual).

Contact measurements follow the same leaf over time without requiring an infrared emissivity setting. Thermal cameras provide complementary spatial information across leaves or canopies, with their own requirements for emissivity and background radiation corrections (Muller et al., 2021). A single contact point should be interpreted as a local measurement, particularly on large leaves with substantial temperature gradients.

Designing an LATD experiment.

Select comparable leaves. Use leaves of comparable age, orientation and canopy position. Define sunlit and shaded groups where relevant. Replicate across independent plants; multiple leaves on one plant and repeated readings over time form a nested sampling structure.
Install the LT-1T carefully. Place the thermistor against the lower, shaded side of the leaf, following the installation guidance. Secure the cable to reduce movement without changing the natural leaf angle. Inspect contact during leaf growth, curling or senescence.
Measure representative air temperature. Use a radiation-shielded, adequately ventilated air temperature sensor at representative leaf height. Position it outside the immediate leaf boundary layer and avoid shading the measured leaf or obstructing airflow. Report the location and shielding, because the air reference defines the resulting LATD.
Synchronise and check the temperature channels. Check both channels against a common temperature reference before deployment. Log simultaneous measurements and retain the original leaf and air temperatures. Select a sampling interval that resolves the processes of interest, allowing for the thermal response of the leaf and sensors.
Record the conditions needed for interpretation. Measure humidity, radiation, wind and soil water status where relevant. Flag wet leaves, lost contact and periods outside the sensor range. Compare treatments over matched conditions or account for environmental differences in the analysis.

Instrument calibration describes only part of the measurement uncertainty. Thermal contact, radiation exposure, heat conduction and the air temperature reference can also affect LATD. Experimental work on contact thermometry has demonstrated the importance of wire properties and adequate leaf contact (Tarnopolsky and Seginer, 1999).

Combining LATD with other physiological measurements.

Combining the LT-1T with an Implexx sap flow sensor allows researchers to examine leaf thermal responses alongside water transport through the stem. Leaf water potential, soil moisture and dendrometer measurements can add information about water availability and changes in plant hydration.

Leaf transpiration and stem sap flow may respond at different times because internal water storage can buffer water use. These lags should be considered when relating LATD to stem measurements (Goldstein et al., 1998). Porometer or gas exchange measurements provide an additional check where the research question concerns stomatal conductance.

Estimating stomatal conductance quantitatively from temperature requires additional energy balance information or a validated reference surface approach. Jones (1999a) demonstrated such an approach using thermal measurements and reference surfaces, with conductance estimates compared against porometer measurements.

Using LATD in scientific research.

LATD provides a measurable link between the leaf energy balance and plant water use. The Implexx LT-1T supports experiments that follow this link on individual leaves through changing environmental conditions. With a suitable air temperature reference, replication and complementary physiological measurements, researchers can investigate when plants maintain evaporative cooling, when leaf temperature rises, and how those responses relate to water conservation and thermal exposure.