Photoprotection dynamics observed at leaf level from fast temporal reflectance changes

Vegetation dynamically reacts to the available photosynthetically active radiation (PAR) by adjusting the photosynthetic apparatus to either a light harvesting or a photoprotective modus. When activating the photoprotection mechanism, either minor or major pigment-protein interactions may occur at the leaf level, resulting in different light absorption and consequently reflectance intensities. The reflectance changes were measured during sudden illumination transients designed to provoke fast adaptation to high irradiance. Different spectral reflectance change features were observed during different stages of photoprotection activation, extending over part of the visible spectral range (i.e. 490-650 nm). Due to this multiple wavelength reflectance modification, which affects also the reference band at 570 nm, the commonly used Photochemical Reflectance Index (PRI) is unable to trace and quantify such strong photoprotection mechanism. To quantify the entire photoprotection with a required accuracy, the spectral changes in the full visible range must be characterized.