D3 Thermometry and Plant Energy Balance

Topic

A plant's energy balance is the sum of all heat and radiation fluxes entering and leaving the leaf, with leaf temperature serving as the variable that integrates this balance into a single value: when incoming heat fluxes exceed outgoing ones, leaf temperature rises, and when outgoing fluxes exceed incoming ones, it falls. A suite of measurement tools allows for the separate quantification of each flux comprising this energy balance, as well as the resulting leaf temperature itself.

The most direct method for measuring leaf temperature is using contact thermocouples; these attach to the leaf surface, ensuring that any temperature change at that specific point is immediately recorded as a measurable electrical signal.

Unlike contact thermocouples, which provide the temperature at only a single point, thermal infrared cameras capture the infrared radiation emitted by the entire leaf surface. They use this data to generate comprehensive leaf temperature maps, revealing variations across different areas of the same leaf or between different leaves on the plant.

To fully close the energy balance—beyond simply determining the resulting leaf temperature—sensors are required to measure the individual heat and radiation fluxes entering and leaving the leaf: the radiation absorbed by the leaf, the heat exchanged with the air via convection, and the heat released through water evaporation during transpiration. By summing these fluxes, heat and radiation sensors make it possible to verify that the energy balance closes—confirming that no significant heat flux has gone unaccounted for.

Within the plant's energy balance, an additional fraction of heat originates not from radiation or convection but from the plant's own metabolism; a plant calorimeter is the tool used to measure this metabolic heat production separately from the other fluxes in the energy balance. Since leaf temperature integrates into a single value all the fluxes measured by contact thermocouples, thermal infrared cameras, heat flux and radiation sensors, and the plant calorimeter, this temperature allows for the diagnosis—without the need to measure each flux individually—of both leaf thermal stress and stomatal status: a leaf temperature that deviates significantly from the optimal range indicates thermal stress, whereas a leaf temperature higher than that of the surrounding air indicates stomatal closure; this is because, as stomata close, the leaf ceases to cool via transpiration, and the energy balance—previously balanced in part by latent heat—shifts to result in a rise in leaf temperature.

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