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Sun-induced chlorophyll fluorescence (SIF) is a promising new tool for remotely estimating photosynthesis. However, the degree to which incoming solar radiation and the structure of the canopy rather than leaf physiology contribute to SIF variations is still not well characterized. Therefore, we investigated relationships between SIF and variables that at least partly capture the canopy structure component of SIF. For this, we relied on high-quality SIF observations from ground-based instruments, high-resolution airborne SIF imagery and the most recent satellite SIF products to cover large ranges in spatial and temporal resolution and diverse ecosystems. We found that the canopy structure-related near-infrared reflectance of vegetation multiplied by incoming sunlight (NIRVP) is a robust proxy for far-red SIF across a wide range of spatial and temporal scales. Our findings indicate that contributions from leaf physiology to SIF variability are small compared to the structure and radiation components. Also, NIRVP captured spatio-temporal patterns of canopy photosynthesis better than SIF, which seems to be mostly due to the greater retrieval noise of SIF. Compared to other relevant structural SIF proxies, NIRVP showed more robust relationships to SIF, especially at the global scale. Our results highlight the promise of using widely available NIRVP data for vegetation monitoring and also indicate the potential of using SIF and NIRVP in combination to extract physiological information from SIF.

Terrestrial evapotranspiration (ET) is an important flux that links global cycles of carbon, water and energy and is largely driven by transpiration (T) through leaf stomata in vegetated areas during the growing season. ET, however, remains one of the most uncertain hydrological variables at the global scale. In this study, we proposed a semi-mechanistic model for estimating terrestrial T by deriving an analytical solution between solar-induced chlorophyll fluorescence (SIF) and stomatal conductance (gc) as well as vapor pressure deficit (VPD), combining theories on the photosynthetic pathway and optimal stomatal behavior. The relationships of SIF-ETR and ETR-gc·VPD0.5 was calibrated by the Soil-Canopy Observation of Photosynthesis and Energy (SCOPE) model. This model was validated by hourly canopy SIF and concurrent eddy covariance flux observations at both forest and cropland ecosystems. Results showed that the SIF combined with VPD can better predict gc than using SIF alone with a more consistent seasonal trends found in both SIF and gc·VPD0.5. The correlation between gc·VPD0.5 and SIF was stronger than those between gc and SIF and between gc and VIs. Canopy T was accurately predicted from SIF at both hourly (R2 > 0.65) and daily (R2 > 0.76) scales and was also successfully estimated using SIF observations from the TROPOspheric Monitoring Instrument (TROPOMI) at cropland ecosystems. In comparison with empirical relationships of directly linking gc with SIF or VIs, the proposed model produced latent heat flux (λE) estimation in best agreement with measured values at all three sites. Our model could be a step forward in understanding the coupling of carbon and water cycles and may be used in ecosystem models for improving ET estimation over large areas.

Under natural sunlight illumination, the chlorophyll fluorescence emitted by the vegetation represents less than 3% of the reflected light in the near infrared part of the spectrum. This small amount is difficult to quantify except at certain wavelengths, where the solar spectrum is attenuated (Fraunhofer lines). An instrument measuring the in-filling of the atmospheric oxygen absorption band at 760 nm by chlorophyll fluorescence has been designed and constructed at the “Laboratoire pour l'Utilisation du Rayonnement Electromagnetique” in Orsay, France. The system was calibrated against a pulsed fluorimeter (FIPAM), especially developed for monitoring chlorophyll fluorescence at distance. The penetration of diuron, a herbicide acting on photosynthesis, was monitored by the passive instrument for several days on a corn canopy. A good agreement was found between gas exchange and variable chlorophyll fluorescence at the canopy level and variable fluorescence at the leaf level. The potential application of the passive chlorophyll fluorescence measurements for long range vegetation remote sensing is discussed.

We modified a PAM fluorimeter for remote detection from 0.5 m to 1 m distance by using a laser diode for excitation. It permitted us to compare directly and simultaneously measurements of relative fluorescence yield to the measurements of lifetime with the τ-LIDAR performed under natural conditions. The existence of a linear relationship, and therefore the equivalence between the lifetime and yield for chlorophyll fluorescence estimation in vivo was confirmed here for several plant types and under both optimal conditions and conditions of water stress. We have also shown that fluorescence lifetime measurements, with the τ-LIDAR, can be used to perform complex fluorescence quenching analysis in fluorosensing, like the one developed in the laboratory for near-contact measurements with PAM-fluorimetry. Water stress effects on fluorescence changes are especially pronounc ed in maize plants due to the C4 type of metabolism and the absence of photorespiration. This would permit the use of the steady state level of fluorescence (Fs, τs) for stress detection, provided that the irradiance is known or is estimated.

Fluorescence or backscattered signals measured over a plant canopy after a picosecond laser shot (nadir viewing) are composed of elementary contributions coming from different illuminated leaves, branches, twigs, and the soil background. They are affected by time delays, depending on their levels within the canopy. The global signals measured are then rather complex and depend on canopy architecture and fluorescence characteristics. Also, an amplitude decorrelation exists between fluorescence and backscattered signals, due to the difference existing between reflectance and fluorescence properties of plant canopy components. For this reason it was necessary to develop a specific method for retrieving the mean fluorescence lifetime of a complex plant canopy under field condition. In a first step, the position and the reflectance characteristics of each reflecting canopy element is determined from the deconvolution of the backscattered signal. In a second step, the parameters obtained are introduced into the fluorescence function in order to determine the mean fluorescence lifetime. The validity of the deconvolution method has been tested for numerous situations, using simulated laser shots on a 3-D canopy mockup. The results obtained show a very good agreement between estimated and input parameters.