A practical approach for extracting the photosystem II (PSII) contribution to near-infrared solar-induced chlorophyll fluorescence

Recent studies have indicated a good potential for using solar-induced chlorophyll fluorescence (SIF) to estimate photosynthetic CO₂ assimilation. SIF can be emitted by both Photosystem I (PSI) and Photosystem II (PSII), but it is the SIF signals from PSII which are related to photosynthetic carbon fixation. However, since top-of-canopy SIF observations (SIF_TOC) always contain contributions from both photosystems, to mechanistically estimate gross primary productivity (GPP) from SIF, it is essential to extract PSII SIF from SIF_TOC. Based on the differences in the relative contribution of PSII across different wavelengths, we propose a practical approach for extracting PSII contribution to SIF_TOC at the near-infrared (NIR) band (f_{PSII_760}) using measurements of SIF_TOC in the red and NIR spectral regions. A leaf-scale concurrent instrument was developed to assess the response of f_{PSII_760} under varying environments. For winter-wheat leaves, as light intensity increased from 0 to 400 μmol m^-2 s^-1, f_{PSII_760} rose from 0.6 to 0.8; with further increase in light intensity to 1800 μmol m^-2 s^-1, f_{PSII_760} consistently decreased to 0.65. There was a slight decreasing trend in f_{PSII_760} with rising temperatures, with values dropping from 0.65 at 15 °C to 0.61 at 40 °C. We found that variations in f_{PSII_760} are due to changes in the fluorescence yield of PSII, with the two having a positively proportional relationship. We also estimated canopy-scale f_{PSII_760} for a winter-wheat study site: f_{PSII_760} varied from 0.61 to 0.83, with a mean value of 0.78 during the peak growing season. A comparison with eddy covariance-derived GPP reveals that GPP estimated with dynamic f_{PSII_760} was more accurate than that calculated using fixed f_{PSII_760}, with R² increasing from 0.6 to 0.84. This study contributes to a deeper understanding of the link between SIF and photosynthetic CO₂ assimilation, paving the way for more effective use of SIF to estimate GPP.