Abstract
Solar-induced chlorophyll fluorescence (SIF) and photochemical reflectance index (PRI) are expected to be useful for remote sensing of photosynthetic activity at various spatial scales. This review discusses how chlorophyll fluorescence and PRI are related to the CO2 assimilation rate at a leaf scale. Light energy absorbed by photosystem II chlorophylls is allocated to photochemistry, fluorescence, and heat dissipation evaluated as non-photochemical quenching (NPQ). PRI is correlated with NPQ because it reflects the composition of xanthophylls, which are involved in heat dissipation. Assuming that NPQ is uniquely related to the photochemical efficiency (quantum yield of photochemistry), photochemical efficiencies can be assessed from either chlorophyll fluorescence or PRI. However, this assumption may not be held under some conditions such as low temperatures and photoinhibitory environments. Even in such cases, photosynthesis may be estimated more accurately if both chlorophyll fluorescence and PRI are determined simultaneously. To convert from photochemical efficiency to CO2 assimilation, environmental responses in stomatal conductance also need to be considered. Models linking chlorophyll fluorescence and PRI with CO2 assimilation rates will contribute to understanding and future prediction of the global carbon cycle.
Highlights
Carbon assimilation by photosynthetic organisms, i.e., gross primary production (GPP), is one of the most important drivers of global carbon cycling and climate
GPP has been obtained from vegetation indices such as normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) using the light use efficiency (LUE) model proposed by Monteith (1972)
Several studies have compared solar-induced Chl fluorescence (SIF) yield (SIF divided by the absorbed light) with Fs (e.g., Cendrero-Mateo et al 2016; Helm et al 2020). These studies showed that SIF yield is significantly correlated with Fs, but the relationship is not necessarily proportional. These results suggest that SIF is useful to estimate ΦF, but should be treated carefully when the value is used quantitatively
Summary
Carbon assimilation by photosynthetic organisms, i.e., gross primary production (GPP), is one of the most important drivers of global carbon cycling and climate. Even when the Chl content is the same, leaf CO2 assimilation rates change depending on the environmental variables at the site. In stress environments such as strong light, extremely low and high temperatures and drought, the rate of energy consumption for CO2 fixation and photorespiration is lower than the potential rate of photochemistry and electron transport.
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