Remote Sensing of Photosynthesis with Chlorophyll Fluorescence and Photochemical Reflectance Index

Abstract

The monitoring of photosynthetic activity stands as a central objective for the upcoming generation of satellite missions. Two promising avenues, Sun-Induced Chlorophyll Fluorescence (SIF) and Photochemical Reflectance Index (PRI), have emerged for remotely sensing leaf physiology, thereby advancing our comprehension of plant–climate interactions. Over the past few decades, substantial progress has been made in measurement techniques, retrieval algorithms, and modeling biochemical and radiative transfer processes related to SIF and PRI. However, it is essential to acknowledge that SIF and PRI observations are subject to influences such as canopy structure, soil background, and sun-observer geometry, in addition to leaf physiological status. Consequently, the key challenges in SIF and PRI remote sensing involve downscaling canopy SIF and PRI to leaf-level values and decoupling structural and physiological information.   Drawing a comparison between the radiative transfer processes involved in SIF and reflectance, a physical relationship between reflectance and scattering of far-red SIF has been established, giving rise to the R2F (Reflectance-to-Fluorescence) theory. Based on this theory, a Fluorescence Correction Vegetation Index (FCVI) has been formulated. The R2F theory and FCVI index have been successfully applied to interpret SIF observations across various scenarios, offering a theoretical foundation for the conversion of SIF across different scales—from canopy to leaf and photosynthetic systems.   Furthermore, an analysis of the radiative transfer processes determining PRI signals reveals that the deviation between canopy- and leaf-level PRI primarily stems from the soil background. Both canopy structure and sun-observer geometry indirectly affect canopy PRI by altering the contribution of soil to canopy reflectance. The structural and angular variation in canopy PRI can be elucidated by considering the soil background. In order to mitigate the effects of soil background, as well as associated structural and angular effects, a soil-adjusted canopy PRI (SacPRI) is proposed. This adjustment involves subtracting the soil contribution, estimated using red reflectance, from the original canopy reflectance. Field and numerical experiments confirm that compared with the raw canopy PRI, SacPRI more closely aligns with the PRI observed in sunlit leaves.   In summary, the proposed R2F relationship and the soil-adjusted canopy PRI offer valuable tools for enhancing the accuracy of photosynthesis monitoring through SIF and PRI, enabling more robust assessments across various scales and environmental conditions.