Leaf color is fine-tuned on the solar spectra to avoid strand direct solar radiation
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The spectral distributions of light absorption rates by intact leaves are notably different from the incident solar radiation spectra, for reasons that remain elusive. Incident global radiation comprises two main components; direct radiation from the direction of the sun, and diffuse radiation, which is sunlight scattered by molecules, aerosols and clouds. Both irradiance and photon flux density spectra differ between direct and diffuse radiation in their magnitude and profile. However, most research has assumed that the spectra of photosynthetically active radiation (PAR) can be averaged, without considering the radiation classes. We used paired spectroradiometers to sample direct and diffuse solar radiation, and obtained relationships between the PAR spectra and the absorption spectra of photosynthetic pigments and organs. As monomers in solvent, the spectral absorbance of Chl a decreased with the increased spectral irradiance (W m−2 nm−1) of global PAR at noon (R2 = 0.76), and was suitable to avoid strong spectral irradiance (λmax = 480 nm) rather than absorb photon flux density (μmol m−2 s−1 nm−1) efficiently. The spectral absorption of photosystems and the intact thallus and leaves decreased linearly with the increased spectral irradiance of direct PAR at noon (I dir-max), where the wavelength was within the 450–650 nm range (R2 = 0.81). The higher-order structure of photosystems systematically avoided the strong spectral irradiance of I dir-max. However, when whole leaves were considered, leaf anatomical structure and light scattering in leaf tissues made the leaves grey bodies for PAR and enabled high PAR use efficiency. Terrestrial green plants are fine-tuned to spectral dynamics of incident solar radiation and PAR absorption is increased in various structural hierarchies.
KeywordsAbsorption spectra Chlorophyll Photosystem Solar radiation spectra Spectral matching Terrestrial environment
This research was part of a joint study among NIES, JAXA/EORC, and Aerological Observatory of Japan Meteorological Agency (JMA), and received collaboration of various specialists of the institutions. We thank N. Saigusa (NIES), Y. Hirose (NIES), and H. Murakami (JAXA) for the development and operation of the solar measurement system. We also thank Y. Nakajima (JAXA) for calibration using integrating spheres, and T. Sakai (JMA) for the use of inspection equipment with incident angle characters. We also thank the two anonymous reviewers for their insightful comments and suggestions.
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