Temperature and Energy Budgets

Light absorption causes molecules to attain states that are simply too improbable to be reached by collisions based on thermal energy. Transitions from an excited state to another one at a lower energy or to the ground state can be radiationless, releasing energy as heat that is eventually shared by the surrounding molecules. The surface temperature of an object indicates both the wavelengths where radiation from it will be maximal and the total energy radiated. The temperature of an object reflects the net result of all the ways that energy can enter or exit from it. This chapter examines these various ways, especially for leaves. The temperatures of leaves and more massive plant parts can then be predicted, based on the ambient environmental conditions. An approach is indicated in the chapter to the study of plant physiology and physiological ecology rather than providing a compendium of facts suitable for all situations. Nevertheless, certain basic features should become clear. For instance, CO 2 uptake during photosynthesis is accompanied by a water efflux through the stomata. This water loss during transpiration cools a leaf. In addition, energy influxes are balanced against effluxes by changes in leaf temperature, which affects the amount of radiation emitted by a leaf as well as the heat conducted to the surrounding air. Another generality is that the temperatures of small leaves tend to be closer to those of the air than do the temperatures of large leaves. To appreciate the relative contributions of the various factors, the chapter uses representative values for leaf and environmental parameters to describe the gas fluxes and the energy balance of leaves.