E3 Thermal System

Topic

A leaf's energy balance is the sum of all heat and radiation fluxes entering and leaving the leaf at any given moment; this balance determines the resulting leaf temperature: when incoming fluxes exceed outgoing ones, leaf temperature rises, and when outgoing fluxes exceed incoming ones, leaf temperature falls.

The leaf's energy balance begins with the absorption of solar and thermal radiation: the leaf absorbs both the direct solar radiation received from the sun and the thermal radiation emitted by the surrounding air, soil, and objects; all this absorbed radiation enters the energy balance as energy available to heat the leaf.

To avoid accumulating all that absorbed energy as a continuous rise in leaf temperature, the leaf dissipates part of it through convection: the surrounding air carries away heat via direct contact with the leaf surface, and the greater the difference between leaf temperature and air temperature, the more heat the leaf releases through convection.

The leaf also dissipates energy through transpiration: by evaporating water through its stomata, the leaf releases the energy consumed by that evaporation as latent heat; the more water the leaf evaporates during transpiration, the more energy it dissipates this way, leaving less energy available to raise its temperature.

In addition to convection and transpiration, the leaf dissipates energy by re-emitting it as infrared radiation toward the surrounding air and objects; the higher the leaf temperature, the more energy the leaf releases through this infrared re-emission, acting as an additional brake that intensifies as the leaf temperature rises. The leaf temperature resulting from this balance—between absorbed solar and thermal radiation and energy dissipated via convection, transpiration, and infrared re-emission—determines the rates of all photosynthetic enzymatic reactions within the leaf, the stomatal opening through which the leaf regulates its own transpiration, and the vulnerability of leaf tissue to thermal damage; thus, the leaf's energy balance links heat physics with photosynthetic biochemistry and the physiology of heat and frost stress, because it is the very leaf temperature resulting from this energy balance that—depending on whether it deviates from the optimal enzymatic range due to excess or deficit—manifests as heat or frost stress.

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