Integration of electron flow partitioning improves estimation of photosynthetic rate under various environmental conditions based on chlorophyll fluorescence

Electron transport rate (ETR), estimated from chlorophyll fluorescence, is a widely-used indicator of photosynthetic activity. However, net photosynthetic CO2 assimilation rate (A) does not linearly correlate with ETR when the fraction of electron partitioning into photosynthesis and photorespiration changes under fluctuating environmental conditions (CO2, light, temperature and soil moisture). Here, we propose a practical approach to estimate A based on ETR with integration of stomatal conductance (g(s)) and leaf temperature, taking intercellular CO2 concentration (C-i)- and temperature-dependent electron partitioning into account. A estimated with the present approach was in good agreement with A measured under i) various CO2 concentrations and light intensities in seedlings of Japanese beech and Japanese white birch, and ii) under 400 and 800 mu mol mol(-1) CO2 and temperatures of 15-40 degrees C, and iii) in progress of drought stress after a water withholding in seedlings of Japanese white birch. Furthermore, canopy-level CO2 uptake could be estimated based on the spectroscopic and flux observations over a Japanese beech stand, under a wide range of environmental conditions. The present approach is advantageous because it does not require biochemical information, such as the maximum rates of Rubisco carboxylation and electron transport, providing a more forward approach to estimate terrestrial carbon flux through remote sensing approaches.

Automated summary

The Electron transport rate (ETR) is used as an indicator of photosynthetic activity but does not linearly correlate with the net photosynthetic CO2 assimilation rate (A) under changing environmental conditions. A practical approach to estimate A based on ETR, stomatal conductance (g(s)), and leaf temperature, taking into account intercellular CO2 concentration (C-i) and temperature-dependent electron partitioning, showed good agreement with measured A in various conditions. This approach can also be used to estimate canopy-level CO2 uptake in different environmental conditions without requiring biochemical information.