Establishing the Photochemical Reflectance Index (PRI) as a reliable proxy for Non-Photochemical Quenching (NPQ)

On a global scale, the scientific community is to date unable to close the CO2 budget, with implications for policymakers in regard to limiting global warming to internationally agreed levels. Recently, remote sensing of Solar-Induced Chlorophyll Fluorescence (SIF) has emerged as a promising proxy for Gross Primary Productivity (GPP). The upcoming Fluorescence Explorer (FLEX) mission by the European Space Agency (ESA) is anticipated to offer unprecedented spatio-temporal and spectral resolution of SIF data, raising high expectations towards assessing GPP on a global scale. However, the relationship between SIF and GPP is intricate, varying with environmental conditions due to the influence of a third process — Nonphotochemical Quenching (NPQ). NPQ critically affects this relationship by utilizing the same energy pool. Hence, NPQ is not unfortunately not directly measurable through remote sensing but previous studies have employed the Photochemical Reflectance Index (PRI) as a proxy for NPQ. Yet, a systematic assessment of the PRI-NPQ relationship is still lacking, as previous works were limited to case studies, confined to the studied ecosystem and the environmental conditions during the study period. In this contribution, our primary aim is to contribute to enhancing SIF as a remotely sensed proxy for GPP. We present the initial findings of our investigation into the robustness of the PRI-NPQ correlation under diverse environmental conditions. To achieve this goal, we leverage a unique dataset that includes joint measurements of hyperspectral reflectance (Data: Fluorescence BoX, JB Hyperspectral Devices GmbH) and active chlorophyll fluorescence (Data: MONI-TORING-PAM Multi-Channel Chlorophyll Fluorometer, Walz) from seven European ecosystems spanning the years 2018 to 2022.