The relationship between the ratio of far-red to red leaf SIF and leaf chlorophyll content: Theoretical derivation and experimental validation

Leaf chlorophyll content (LCC) is an important indicator of photosynthetic capacity . Sun-induced chlorophyll fluorescence (SIF) is an optical signal emitted from the leaf interior, providing a unique technique for accurately estimating LCC. The far-red to red ratio of chlorophyll fluorescence ( F ratio ) has been used to empirically estimate LCC in some previous studies. While these studies support the use of the F ratio for LCC estimation, its theoretical underpinning remains less well-defined and its effectiveness across a wider range of scenarios remains unclear. In this study, we established the relationship between the F ratio and LCC using the light use efficiency (LUE)-based SIF model and spectral invariant radiative transfer theory. Firstly, the LUE-based SIF model demonstrates that the change in the leaf F ratio is controlled by the ratio of the fluorescence escape fraction (i.e., f esc from the photosystem to the leaf surface) at the corresponding bands. Secondly, a f esc modeling approach is presented using the spectral invariant theory and thus the f esc ratio is linked to LCC. Theoretical analysis shows that the F ratio has a strong correlation with LCC, which explains over 90 % of the variation in F ratio . Both experimental measurements and model simulations from a radiative transfer model Fluspect were used to validate the relationship between LCC and three F ratio (i.e., F ratio ↑ , F ratio ↓ and F ratio tot ), which were derived from the upward and downward SIF of leaves, as well as the total SIF observed from both sides. The Fluspect simulations were used to assess the sensitivity of the F ratio -LCC relationship to the leaf structure. Two types of experimental measurements, including the field measurements of three crops and the laboratory measurements of 20 tundra plants, were employed to examine the species dependence of the F ratio -LCC relationship. The performance of F ratio for LCC estimation was evaluated and compared with spectral indices and the PROSPECT model using the experimental measurements and leave-one-out cross-validation (LOOCV) approach. Both the Fluspect simulations and the experimental measurements indicate that the F ratio is strongly correlated with LCC for a wide range of leaf scenarios. The F ratio -LCC relationship remains relatively stable across different leaf structures and plant species , since the relationship is almost consistent. The LOOCV of experimental measurements shows that the F ratio provides promising and robust LCC estimates, with the F ratio tot performing the best. The F ratio tot outperforms spectral indices, reducing the RMSE for LCC estimation by 19.5 %–93.9 %. Furthermore, compared to the PROSPECT model, the F ratio achieves a reduction in RMSE by 30.4 %–77.8 %. These results demonstrate that the F ratio is effective for estimating LCC of diverse plant species. This study advances our understanding of the relationship between the F ratio and LCC, supporting the use of SIF signals for remote sensing of LCC. • The relationship between the far-red to red ratio of leaf SIF and LCC was derived. • The SIF ratio is strongly correlated with LCC. • The SIF ratio-LCC relationship remains relatively stable across different leaf structures and plant species. • The SIF ratio is promising and robust for LCC estimation. • The SIF ratio outperforms most spectral indices and the PROSPECT model inversion.