Introducing the Optical Gas Exchange System (OGES) concept

Field and long-term measurements of leaf-level photosynthetic gas exchange have been pivotal to understand and model the environmental regulation of leaf photosynthesis (Hari and Makela 2003). Now, scientists seek for means to understand and model the environmental regulation of planetary photosynthesis, by means of remotely sensed data. In addition to gas exchange, photosynthesis generates multiple optical clues that are embedded in the intensity and spectral properties of the light reflected or emitted via chlorophyll fluorescence (ChlF) by vegetation. These signals can be recorded with optical sensors and nowadays measured across a continuum of scales: from leaves to entire ecosystems (Porcar-Castell et al. 2021). However, we are still missing the mechanistic understanding that is required to fully exploit the potential of these data, largely due to the lack of concomitant and long-term observations of photosynthetic gas exchange and optical signals at the scale of a leaf, the smallest scale at which they can be measured in vivo. The Optical Gas Exchange System (OGES) aims at filling this gap by providing field and long-term measurements of photosynthetic gas exchange, pulse amplitude modulated (PAM) chlorophyll fluorescence, and leaf spectral reflectance. By doing so, the OGES provides both a comprehensive view of the photosynthetic regulation (both from the light and carbon reactions point of view) (Oivukkamäki et al. 2024), as well as a unique dataset to disentangle the connection between optical and photosynthetic dynamics and their environmental and physiological regulation. In this communication we present preliminary results from the OGES system tests conducted in the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR) II in Hyytiälä during summer 2023 and 2024. Data integrates observations from a µPAM system (Walz GmbH, Germany) to detect ChlF and track the acclimation of the light reactions, an OctoFLOX spectrometer system (JB Hyperspectral, Germany) and the existing gas exchange measurements in SMEAR-II Station in Hyytiälä, which can resolve shoot-level dynamics of CO 2 , H 2 O or VOC fluxes. In addition, nocturnal LED-induced Fluorescence (LEDIF) methods (Atherton et al. 2019) are being developed for added versatility and information content. Preliminary results indicate that it is possible to automatically record the temporal variations in leaf reflectance and fluorescence spectra under field conditions. Implications, following steps and challenges are discussed. A first concept version demonstrating the potential of integrated long-term µPAM and gas exchange measurements is described in Oivukkamäki et al. (2024).