7 - Temperature and Energy Budgets

Abstract

The temperature of an object reflects the net result of all the ways that energy can enter or leave it. The chapter examines these various ways, especially for leaves. One will then be able to predict the temperatures of leaves, as well as more massive plant parts, based on the ambient environmental conditions. One can also appreciate the consequences of certain adaptations of plants to their environment and can identify the experimental data needed for future refinements of our calculations. It should be emphasized at the outset that individual plants as well as environmental conditions vary tremendously. Thus the chapter indicates an approach to the study of plant physiology and physiological ecology rather than provide a compendium of facts suitable for all situations. Nevertheless, certain basic features should become clear. For instance, CO2 uptake during photosynthesis is accompanied by a water efflux through the stomata. This water loss during transpiration cools a leaf. Also, energy influxes are balanced against effluxes by changes in leaf temperature, which affect the amount of radiation emitted by the 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.