Conference Agenda

The FLEX satellite mission was designed and proposed almost 20 years ago. Since then a portfolio of instruments to measure solar-induced fluorescence (SIF) was developed in various scientific studies. This portfolio includes instruments that measure SIF on the level of single leaves, top-of-canopy imaging systems to map the distribution of SIF in natural canopies, UAV-based cameras to map entire fields, tower-based systems to record time series of SIF and reflectance as well as airborne sensors for large scale mapping of SIF. These experimental approaches were applied on a wide range of ecosystems and under various environmental conditions with the goal to furher develop the scientific understanding on the mechanistic link between SIF and the actual state of vegetation photosynthesis and stress response.

With this presentation we will give a critical review on the current knowledge, the uncertainties and knowledge gaps that is based on various experimental campaigns and the activities. We assess the various experimental results and try to provide some answers on how accurately FLEX will be able to quantify actual rates of photosynthesis and vegetation stress, the two main objectives of the mission.

Actual photosynthetic rates and functional stress responses are manifested on the level of single leaves. While we have a good scientific knowledge on the acclimation and adaptation of photosynthesis and the mechanisms of stress response on this small scale, it is a scientific challenge to translate the 300x300 m FLEX pixels to the meachanistic regulation on single leaves. We will review our current knowledge on the dynamic variations that occur on the leaf level, revisit our approaches to correct and normalize for canopy structural effects, compare the variations of SIF across different plant species to finally develop an understanding on the certainties and uncertainties we have during this scaling process.

We will quantitatively describe the spatio-temporal dynamics of fluctuating light and SIF emission within natural canopies, provide an answer, which leaves contribute to the top-of-canopy SIF signal, review the variations that occur under the normal diurnal cycle and those variations, which are introduced by drought, extreme temperatures, pests and diseases, and finally touch the question how SIF emitted from different species within one pixel mixes. We will give some answers on how deeply we possibly can look into the regulatory properties of the photosyntetic machinery from the FLEX satellite plattform.

FLEX data will combine SIF with visible and near-infrared reflectance and thermal information, which in the future can be complemented with ground-based time series and mechanistic understanding of plant functioning. Thus, FLEX data shall be used in combination with other data sources to constrain models of carbon and water fluxes and to predict vegetation health and stress. We will discuss the potential for higher-level data products, which will help to better understand the functioning of our vegetation in times of global change and extreme events.

Sun-induced chlorophyll fluorescence (SIF) holds great promise as a non-invasive, remotely-sensed proxy for tracking photosynthetic activity across spatial scales. The effective translation of canopy SIF into meaningful physiological information, however, requires a deeper mechanistic understanding of the relationship between the quantum yields of fluorescence (ΦF) and photosystem II photochemistry (ΦPSII). This relationship is complicated by two major factors: (1) the confounding effects of canopy structure, which through scattering and reabsorption dictate the escape probability (fesc) of fluorescence photons, and (2) the dynamic, non-linear, and condition-dependent interaction between photochemical and non-photochemical quenching pathways that govern both ΦPSII and ΦF. These challenges are particularly relevant for the red SIF emission peak (F687), which is more directly linked to PSII activity but suffers from strong chlorophyll reabsorption. Consequently, it remains unclear which methods best retrieve the intrinsic photosystem-level signal and how the ΦPSII-ΦF relationship unfolds diurnally and seasonally across species with different functional strategies. This uncertainty complicates the physiological interpretability of snapshot satellite SIF observations, which are sampled at a single, fixed time of day.

In this study, we evaluated contemporary methods for correcting red and far-red SIF for canopy escape probability and investigated the diurnal and seasonal dynamics of the ΦPSII-ΦF relationship under a gradient of environmental stress. We collected 92 days of continuous, concurrent measurements of canopy SIF (at 687 and 760 nm) and pulse-amplitude-modulation (PAM) fluorometry on two temperate tree species with contrasting drought response strategies: the anisohydric European beech (Fagus sylvatica) and the isohydric small-leaved lime (Tilia cordata). Measurements were conducted in the ARBOfun research arboretum in Leipzig, Germany. We compared several reflectance-based approaches to estimate fesc and derived photosystem-level fluorescence yields. Daily conditions were classified into five Hydrometeorological Condition Groups (HCGs) based on vapor pressure deficit, radiation, and soil moisture to analyze stress responses. The predictive power of ΦF for ΦPSII was assessed using both fixed time windows (simulating satellite overpasses) and adaptive, condition-specific integration periods.

We used a correction for the escape probability of SIF based on the Near-Infrared Reflectance of vegetation (NIRv). This NIRv-based correction significantly strengthened the correlation between corrected photosystem-level red fluorescence (F687,PS) and both absorbed photosynthetically active radiation and electron transport rate for both species. We found that the quantum yield of red fluorescence (ΦF687) was a substantially stronger and more reliable predictor of ΦPSII than the far-red signal (ΦF760). The diurnal relationship between ΦPSII and ΦF687 was non-linear and non-monotonic, exhibiting distinct, species-specific trajectories explained by their isohydric (T. cordata) or anisohydric (F. sylvatica) strategies. Our results showed that due to these complex dynamics, seasonal predictions of ΦPSII based on snapshot measurements aligned with satellite overpass times (e.g., 10:30 or 13:30 local solar time) were poor. In contrast, using adaptive, HCG-specific integration windows that captured periods of positive relationship between ΦPSII and ΦF687 significantly improved estimation accuracy and robustness.

These findings underscore the critical importance of accounting for species-specific physiology and diurnal dynamics to accurately interpret SIF data. They highlight the stronger physiological information content of red SIF over far-red SIF and demonstrate that using fixed-time satellite snapshots risks misrepresenting photosynthetic function, especially under stress conditions. For future satellite missions like FLEX, our results call for a prioritization of red SIF retrieval and the development of interpretation frameworks that integrate knowledge of diurnal dynamics and plant functional types.

The FLEX-ITA (FLEX Inland Water and Terrestrial Airborne Measurements and Scientific Exploitation) project, funded by the Italian Space Agency, aims at establishing a network of experts in airborne and ground-based campaigns to support the validation of ESA’s FLEX mission products. While the FLEX-ITA project includes two components, concerning broadly contrasting environments: a "land component" for early detection of crop water stress and an "aquatic component" for characterizing lake phytoplankton, this contribution will present the activities and outcomes of the former.

The “land” experimental component of FLEX-ITA involved two extensive airborne and ground campaigns conducted in 2024 and 2025 in Tuscany, Italy. In 2025, the field campaign was conducted in synergy with the FRM4FLUO project, which allowed the validation of the FLEX-ITA airborne SIF maps (AisaIBIS and Hyplant, Specim, Spectral Imaging Ltd) by comparison with near-surface point measurements from permanently installed FloX systems (JB Hyperspectral Devices GmbH) and spatially distributed observations collected using the UAV-mounted AirFLOX system. Thanks to cooperation with the PRIN 2022 SCOOP project (CUP B53D23018190006), it was possible to study the response of four different species (sudangrass, sunflower, tomato and maize) to evolving water stress, and in some cases, to re-watering. To answer the question whether SIF can reliably detect early crop water stress and/or recovery of the physiological functions upon re-watering, the experimental investigations within the two above-mentioned measurement campaigns, involved a comprehensive set of measurements of plant physiological (leaf photosynthesis measured with CIRAS-4 and LI-6800 Portable Photosynthesis Systems; NPQ derived from MINI-PAM-II fluorometer; leaf water potential measured with Scholander-type pressure chamber), biochemical (leaf chlorophyll content measured with SPAD/Dualex leaf clip sensors) and biophysical (fAPAR - Sunscan, Delta-T) parameters, together with meteorological and soil water conditions.

The FLEX-ITA project results highlight the potential of airborne imaging systems for SIF satellite measurements validation. Moreover, by measuring in-situ the dynamic interplay of the three fundamental dissipation pathways of absorbed light energy in various crop species, FLEX-ITA addresses the open gap in SIF research on the mechanistic link between SIF and photosynthesis, especially under stress conditions, and provides insights regarding the universality of the SIF-GPP relationship.

Chlorophyll fluorescence is a primary signal for monitoring plant photosynthesis through remote sensing. However, the common assumption that the relationship between fluorescence and photosynthetic efficiency is linear often fails under environmental stress conditions. To address this challenge, this work synthesizes four studies—ranging from seasonal field trials to controlled diurnal monitoring—to characterize how nitrogen, water, and thermal stress drive non-linear energy partitioning.

Field experiments on durum wheat revealed that plants grown under high-light conditions exhibited low photosynthetic efficiency (between 0.10 and 0.40). Under these conditions, high activation of non-photochemical quenching (NPQ) dominated the energy dissipation pathway. This decoupled the link between photochemistry and fluorescence, causing standard linear models to fail. To further investigate these drivers, we conducted multi-species experiments on maize, wheat, camelina, tomato, and barley. These revealed a three-phase response—usage, protection, and damage—regulated by the plant's NPQ capacity. Notably, drought accelerates these phase transitions, whereas nitrogen deficiency delays them. This understanding was further refined through continuous diurnal monitoring of tomato plants under water and heatwave stress. The results confirm that water deficit shifts the interaction between fluorescence, NPQ, and photosynthetic efficiency into non-linear patterns. These effects become even more pronounced when drought is combined with thermal stress.

These findings highlight the importance of considering nonlinear dynamics when interpreting fluorescence-based remote sensing data. Moving beyond linear assumptions and exploiting nonlinear dynamics provides a more robust framework for interpreting fluorescence remote sensing data. These findings are essential for developing next-generation stress-detection algorithms for the FLEX mission.

Effective monitoring of drought impacts based on satellite observations can be achieved by combining atmospheric information with vegetation indices (VIs) derived from optical remote sensing. However, VIs generally indicate drought impacts only after vegetation damage has progressed to stages that are often irreversible. Indicators of plant physiological functioning, in contrast, offer the possibility of detecting drought stress at a much earlier stage. Solar-induced chlorophyll fluorescence (SIF), which is emitted directly from the photosynthetic apparatus, provides such physiological information (Drusch et al., 2017). Under abiotic stress conditions, enhanced dissipation of excess energy as heat through non-photochemical quenching (NPQ) leads to a reduction in fluorescence yield, which can be observed remotely as changes in SIF (Berger et al., 2022; Damm et al., 2018).

Satellite observations of top-of-canopy (TOC) SIF have been available globally since 2018 from the TROPOMI sensor aboard Sentinel-5P (Guanter et al., 2021; Köhler et al., 2018). However, TOC SIF is strongly influenced by incoming radiation and canopy structure. These influences must be accounted for in order to derive fluorescence yield at the leaf level, expressed as the quantum efficiency of fluorescence (ΦF), which more directly reflects the actual physiological state of vegetation. In this study, ΦF is calculated following Equation (1), where the vegetation index NIRv (NDVI × NIR) serves as a combined proxy for the fraction of absorbed photosynthetically active radiation (fAPAR) and the fluorescence escape probability (fesc) (Badgley et al., 2017; Dechant et al., 2020; Liu et al., 2023). SIF at 743 nm as well as the reflectance used to compute NIRv are derived from TROPOMI data, while photosynthetically active radiation (PAR) is obtained from MODIS observations.

ΦF = pi*SIF743canopy/(NIRv*PAR) (1)

This study introduces a new multi-year (2018–2023) ΦF dataset for Germany at 0.05° spatial resolution and daily temporal coverage. To evaluate ΦF as an early indicator of drought stress in agricultural and forest ecosystems, it is compared with anomalies in subsurface water storage (SSWS) derived from coupled ParFlow/CLM simulations, which serve as a proxy for plant water availability (Belleflamme et al., 2023). Periods of prolonged negative SSWS anomalies were identified and cross-referenced with watch and warning phases of the Combined Drought Indicator of the European Commission. Cross-correlation coefficients were calculated for multiple time lags using spatially aggregated daily data smoothed with a two-day rolling average.

The highest cross-correlation between ΦF and SSWS anomalies was observed at a two-day lag, with correlations decreasing thereafter, indicating a short delay of 2 days in the ΦF response to declining subsurface water availability in both agricultural and forest ecosystems. In contrast, non-normalized canopy SIF and conventional vegetation indices (NIRv and NDVI) showed weak and inconsistent correlations during drought periods within the response window of 7 days. These results demonstrate that ΦF is sensitive to early reductions in plant water availability and underline the importance of normalizing and downscaling canopy-level SIF for drought monitoring. This approach for deriving ΦF has already been applied to generate a pan-European dataset, which will enable a more comprehensive assessment of spatio-temporal drought dynamics and ecosystem-specific responses in the near future.

Sun-induced fluorescence (SIF) has emerged as a promising tool for assessing photosynthetic activity and plant stress as it directly reflects changes in the regulation of absorbed light energy within the photosynthetic apparatus. Several studies have demonstrated SIF’s ability to detect abiotic stresses, including nutrient deficiencies, drought and temperature, across leaf to canopy scales and using different platforms. In contrast, potential of SIF to detect biotic stress in real field under different environmental conditions like elevated CO₂ remains comparatively underexplored. Elevated CO₂ alters photosynthetic activity and carbon assimilation, potentially modifying plant pathogen interactions and the objective of this study is to assess whether SIF can provide insight into the underlying physiological mechanisms and final yield formation from the interaction of elevated CO₂ and biotic stress.

In this study, we conducted trials in 2024 growing season at experimental fields in Campus Klein-Altendorf, Germany using two sugarbeet genotypes with different susceptibility to cercospora infection (high and low) under elevated CO₂ concentration (600 ppm) and ambient CO₂ concentration using a free air carbon dioxide enrichment (FACE) facility and high throughput platform FieldSnake. Half of the trial was inoculated by the fungus Cercospora beticola which causes Cercospora leaf spot (CLS), and the other half was treated with fungicide to prevent CLS. Two field-based chlorophyll fluorescence methods were employed, an FloX system passively measuring reflectance-based vegetation indices and sun-induced fluorescence (SIF Red and SIF Far-Red) and active Light-Induced Fluorescence Transient (LIFT) device to obtain the operating efficiency of Photosystem II (F_q’/F_m’) both at canopy level. Yield, and quality parameters were also measured to estimate and to understand the interaction of factors for entire crop season.

Seasonal dynamics of SIF shown clear responses to elevated CO₂ and CLS progession. SIF distinguishes CLS symptoms earlier than NDVI and at early infection stages in both varieties whereas reductions in PSII efficiency were mainly observed at later disease stages. Under 600 ppm CO₂, inoculated plots exhibited stronger disease effects compared to ambient CO₂ as indicated by SIF. When accounting for reabsorption and scattering effects, SIF response was driven stronger by structural changes than by physiological responses under CLS. Far-Red SIF exhibited a strong relationship with electron transport rate highlighting its role as a proxy for canopy- scale photosynthesis. Despite disease pressure, elevated CO₂ significantly increased beet and sugar yield, partially compensating for CLS effects in both the varieties. Overall, these results demonstrate complex interactive effects of elevated CO₂ and biotic stress on canopy photosynthesis, structure, and yield formation. These findings provide a strong basis for linking ground-based fluorescence observations with satellite remote sensing and support the interpretation and validation of sun-induced fluorescence products from ESA's FLEX mission.