Theoretical Foundation for Applying Sun-Induced Chlorophyll Fluorescence in Global Photosynthesis Research

An analytical framework is established to guide the observation and modeling of sun-induced chlorophyll fluorescence and its applications in ecosystem science.

The Science

Recent progress in observing sun-induced chlorophyll fluorescence (SIF) provides an unprecedented opportunity to advance photosynthesis research in natural environments. However, an analytical framework to guide SIF studies and integration with the well-developed active fluorescence approaches is lacking. A set of coupled fundamental equations was therefore derived to describe the dynamics of SIF and its relationship with C3 and C4 photosynthesis. These equations show that although SIF is dynamically as complex as photosynthesis, the measured SIF simplifies photosynthetic modeling from the perspective of light reactions by integrating over the dynamic complexities of photosynthesis. Specifically, the measured SIF contains direct information about the actual electron transport from photosystem II to photosystem I, giving a quantifiable link between light and dark reactions. With much-reduced requirements on inputs and parameters, the light reactions–centric, SIF-based biophysical model complements the traditional, dark reactions–centric biochemical model of photosynthesis. The SIF-photosynthesis relationship, however, is nonlinear because photosynthesis saturates at high light while SIF has a stronger tendency to keep increasing as fluorescence quantum yield has a relatively muted sensitivity to light levels.

The Impact

The theory developed in this study clarifies several conflicting issues in the SIF-photosynthesis relationship, provides a solid foundation for SIF research, and points to future research directions.


Chlorophyll a fluorescence (ChlF) is the emission of red and far-red photons from the excited states of chlorophyll molecules in competition with photochemical and non-photochemical energy uses. It is tightly coupled to photosynthesis at the level of fundamental biochemical and biophysical processes. The feasibility of remotely sensing SIF, which is also referred to as passive ChlF, has stimulated a flurry of research to correlate SIF with gross primary production (GPP) and related variables. This enthusiasm has raised the hope of making concrete progress toward understanding and predicting the dynamics of GPP from canopy to global scales, a recalcitrant challenge that has plagued generations of researchers in ecosystem, plant, and agricultural sciences. However, the precise relationship between SIF and GPP is currently unknown. The theory developed in this study fills this gap. Its application will advance a predictive understanding of several previously underexplored physiological and biophysical processes under natural conditions. Advances can be facilitated by coordinated efforts in plant physiology, remote sensing, and eddy covariance flux observations.

Principal Investigator(s)

Lianhong Gu  
Oak Ridge National Laboratory


The Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.


Gu, L., J. Han, J. D. Wood, C. Y. Y. Chang, and Y. Sun. “Sun-induced Chl fluorescence and its importance for biophysical modeling of photosynthesis based on light reactions.” New Phytologist 223(3), 1179–91 (2019). [DOI:10.1111/nph.15796]