Linking chlorophyll a fluorescence to photosynthesis for remote sensing applications: mechanisms and challenges

Abstract

Chlorophyll a fluorescence (ChlF) has been used for decades to study the organization, functioning, and physiology of photosynthesis at the leaf and subcellular levels. ChlF is now measurable from remote sensing platforms. This provides a new optical means to track photosynthesis and gross primary productivity of terrestrial ecosystems. Importantly, the spatiotemporal and methodological context of the new applications is dramatically different compared with most of the available ChlF literature, which raises a number of important considerations. Although we have a good mechanistic understanding of the processes that control the ChlF signal over the short term, the seasonal link between ChlF and photosynthesis remains obscure. Additionally, while the current understanding of in vivo ChlF is based on pulse amplitude-modulated (PAM) measurements, remote sensing applications are based on the measurement of the passive solar-induced chlorophyll fluorescence (SIF), which entails important differences and new challenges that remain to be solved. In this review we introduce and revisit the physical, physiological, and methodological factors that control the leaf-level ChlF signal in the context of the new remote sensing applications. Specifically, we present the basis of photosynthetic acclimation and its optical signals, we introduce the physical and physiological basis of ChlF from the molecular to the leaf level and beyond, and we introduce and compare PAM and SIF methodology. Finally, we evaluate and identify the challenges that still remain to be answered in order to consolidate our mechanistic understanding of the remotely sensed SIF signal.

Connection of the Photochemical Reflectance Index (PRI) with the Photosystem II Quantum Yield and Nonphotochemical Quenching Can Be Dependent on Variations of Photosynthetic Parameters among Investigated Plants: A Meta-Analysis

“…In contrast, very long-term stress can change the content of chlorophyll and the pool size of the xanthophyll cycle pigments. It is known that similar changes can also influence PRI [48,122].…”

Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees

“…Further studies are warranted to evaluate the variability in the leaf-level fluorescence signals in relation to photosynthetic phenology, and to compare F S to SIF in response to seasonally changing conditions. Here we used F S as an indicator of likely SIF signals, but key differences in instrumentation and measurement protocols exist between these two methods [10].…”

“…A small fraction of absorbed light can also be dissipated through re-emission as chlorophyll fluorescence at a longer wavelength than what is absorbed [8]. Fluorescence is driven by the amount of absorbed photosynthetically active radiation (APAR), but is also modified by NPQ [10].…”

“…Chlorophyll fluorescence has often been measured using the pulse-amplitude modulation (PAM) technique, which is restricted to the leaf-level due to the requirement of a saturating pulse [10]. With high-spectral resolution spectrometers that can resolve atmospheric absorption bands (Fraunhofer lines), fluorescence can now be passively detected from a distance as a small signal present in the "gaps" of the solar spectrum [27]).…”

“…As a newly defined index, the CCI has not previously been considered in the complementarity hypothesis [15]. We predict that, on seasonal timescales, the CCI will be similar to SIF, sensitive to both canopy structure-influencing absorbed radiation (APAR) and photosynthetic downregulation [10]. Both metrics should be indicators of GPP in both deciduous and evergreen vegetation (Figure 1).…”