Comparing the quantum use efficiency of red and far-red sun-induced fluorescence at leaf and canopy under heat-drought stress

Sun-Induced chlorophyll Fluorescence (SIF) is the most promising remote sensing signal to monitor photosynthesis in space and time. However, under stress conditions its interpretation is often complicated by factors such as light absorption and plant morphological and physiological adaptations. To ultimately derive the quantum yield of fluorescence (Φ_F) at the photosystem from canopy measurements, the so-called escape probability (f_esc) needs to be accounted for.

In this study, we aim to compare Φ_F measured at leaf- and canopy-scale to evaluate the influence of stress responses on the two signals based on a potato mesocosm heat-drought experiment. First, we compared the performance of recently proposed reflectance-based approaches to estimate leaf and canopy red f_esc using data-supported simulations of the radiative transfer model SCOPE. While the leaf red f_esc showed a strong correlation (r² ≥ 0.76), the canopy red f_esc exhibited no relationship with the SCOPE retrieved red f_esc in our experiment. We therefore propose modifications to the canopy model to address this limitation.

We then used the modified models of red f_esc, along with an existing model for far-red f_esc to analyse the dynamics of leaf and canopy red and far-red fluorescence under increasing drought and heat stress conditions. By incorporating f_esc, we obtained a closer agreement between leaf and canopy measurements. Specifically, for red f_esc, the r² of the two variables increased from 0.3 to 0.50, and for far-red f_esc, from 0.36 to 0.48.

When comparing the dynamics of the quantum yield of red and far-red fluorescence (Φ_F,687 and Φ_F,760) under increasing stress, we observed a statistically significant decrease of both leaf and canopy Φ_F,687 as well as leaf Φ_F,760, as drought and heat conditions intensified. Canopy Φ_F,760, on the contrary, did not exhibit the same trend, since measurements under low stress conditions showed a wider spread and lower median than under high stress conditions. Finally, we analysed the sensitivity of Φ_F,687 and Φ_F,760 to changing solar incidence angle, by comparing the variability of the measurements without and with mesocosm rotation. Our results suggest that the variation in Φ_F,760 strongly increased with changing solar incidence angle. These findings highlight the need for further research to understand the causes of discrepancies between leaf and canopy scale Φ_F,760. On the contrary, the underutilised and understudied Φ_F,687 showed great potential in assessing plant responses to drought and heat stress.