Radiation and light measurements

Abstract

Measurements of radiation in physiological ecology are of primary importance because of their role in energy balance determinations (see Chapter 7) and in photosynthesis measurements (see Chapter 11). Although less frequently made in physiological ecology, light quality measurements are important with respect to photomorphogenesis, or for understanding the effects of specific wavelengths, such as UV radiation, on physiological processes. Each of these measurements requires a different approach. In energy balance studies we are primarily interested in the energy incident on a leaf or plant, and how much of this energy is absorbed. Photosynthesis, on the other hand, is a photochemical process that is driven by absorption of photons by chlorophyll in the wavelength band from 400 to 700 nm. Here the absorptance of most leaves is quite high (Fig. 6.1). According to Planck’s law, the energy content of a 700 nm photon (171 kJ mol⁻¹) is only 57% of that of a 400 nm photon (299 kJ mol⁻¹). However, photosynthetic rate should be essentially independent of whether it was driven by the absorption of a photon of 400 or 700 nm light, despite the large difference in energy content. Any excess energy in the absorbed photon is dissipated as heat or fluorescence. Thus photosynthetic rate should more closely follow the number of photons absorbed rather than the amount of energy absorbed.