Connecting active to passive fluorescence with photosynthesis: a method for evaluating remote sensing measurements of Chl fluorescence

Abstract

Recent advances in the retrieval of Chl fluorescence from space using passive methods (solar-induced Chl fluorescence, SIF) promise improved mapping of plant photosynthesis globally. However, unresolved issues related to the spatial, spectral, and temporal dynamics of vegetation fluorescence complicate our ability to interpret SIF measurements. We developed an instrument to measure leaf-level gas exchange simultaneously with pulse-amplitude modulation (PAM) and spectrally resolved fluorescence over the same field of view - allowing us to investigate the relationships between active and passive fluorescence with photosynthesis. Strongly correlated, slope-dependent relationships were observed between measured spectra across all wavelengths (F , 670-850 nm) and PAM fluorescence parameters under a range of actinic light intensities (steady-state fluorescence yields, F ) and saturation pulses (maximal fluorescence yields, F ). Our results suggest that this method can accurately reproduce the full Chl emission spectra - capturing the spectral dynamics associated with changes in the yields of fluorescence, photochemical (ΦPSII), and nonphotochemical quenching (NPQ). We discuss how this method may establish a link between photosynthetic capacity and the mechanistic drivers of wavelength-specific fluorescence emission during changes in environmental conditions (light, temperature, humidity). Our emphasis is on future research directions linking spectral fluorescence to photosynthesis, ΦPSII, and NPQ.

OCO-2 advances photosynthesis observation from space via solar-induced chlorophyll fluorescence

“…Clearly, more in-depth processbased studies are needed to understand the nature of the SIF-GPP relationship. A particular emphasis should be placed on the covariation between F CO2 and F F at different spatiotemporal scales, which will be key to using SIF as a shortcut to estimating GPP at large scales (40).…”

Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence: Implications for Remote Sensing of Photosynthesis

“…Here, we define leaf‐level ChlF as F t,λ , and canopy‐level ChlF as SIF λ , both measuring the absolute radiant energy flux of ChlF from remote sensing platforms (e.g., W·m −2 ·nm −1 ·sr −1 ; equation ): where APAR is absorbed photosynthetically active radiation (μmol·m −2 ·s −1 ), β λ is the spectrally dependent leaf or canopy escape probability, f PSII is the fraction of photons absorbed by PSII photosystems, Φ is fluorescence yield of PSII and PSI (quanta emitted/quanta absorbed), and S is the spectral shape of PSII and PSI (J·μmol −1 ·nm −1 ·sr −1 ). In contrast, steady‐state fluorescence from PAM (F t ), typically recorded in millivotls, can be expressed as where APAR ML is the absorbed radiant energy from a fixed output ML and the wavelength dependency of β and S is simply measured as the integral of emitted fluorescence somewhere in the 650‐ to 850‐nm range (depending on the manufacturer; Magney, Frankenberg, et al, ), while c is a factor that accounts for the preset sensitivity of the photodiode (mV/[W·m −2 ·nm −1 ·sr −1 ]). The spectral shape ( S ) of ChlF has two peaks centered in the red (~685nm) and far‐red (~740 nm) spectrum, with nonequal contributions from PSI and PSII.…”

Leaf optical properties and dynamics of photosynthetic activity

“…The method, which consists on painting the sample black and measuring the outgoing photon flux density from the integrating sphere before and after painting, was very useful for needles, as it by-passes the need to calculate the gap fraction that otherwise affects the accuracy of transmittance and reflectance spectra (Daughtry et al 1989;Middleton et al 1996;Mesarch et al 1999). Thus, the methodology presented in Study II is expected to facilitate the calculation of leaf PAR absorption, and help avoid the use of constant PAR absorption values in studies dealing with the dynamics of photosynthetic activity (Pieruschka et al 2010;Magney et al 2017).…”

“…10), can be estimated by simply multiplying a general value for photochemical yield by the light intensity (Maxwell and Johnson 2000). However, this study suggests that efforts to estimate individual leaf light absorption are worthwhile, especially in studies comparing parameters related to the photochemical activity of various species (Pieruschka et al 2010;Magney et al 2017), or between the leaves of a particular species that differ in age, canopy position or season. This is true because PAR absorption can greatly differ from the commonly assumed 0.84, and thus obscure the actual dynamics of photosynthesis.…”

“…Directly correlating fluorescence to photosynthetic activity is also not an option. Hence work is in progress to relate passive and active fluorescence approaches (Magney et al 2017), and is necessary to determine the physiological and non-physiological components affecting SIF at various spatial and temporal scales .…”