Improving the Interpretation of Sun-Induced Chlorophyll Fluorescence under Stress: Insights from Leaf- and Canopy-Scale Measurements

Sun-Induced chlorophyll Fluorescence (SIF) stands out as a promising remote sensing signal for monitoring photosynthesis over time and space. However, its interpretation becomes intricate under stress conditions due to factors such as light absorption and plant adaptations. To derive the quantum yield of fluorescence (Φ_F) at the photosystem from canopy measurements, the escape probability (f_esc) must be considered.

This study compares Φ_F measured at leaf- and canopy-scale to assess the impact of stress responses, using a potato mesocosm heat-drought experiment as a basis. Initially, we evaluated the performance of reflectance-based approaches for estimating red and far-red f_esc through simulations with the radiative transfer model SCOPE. Findings revealed that existing f_esc models inadequately predicted the correct value range for red f_esc especially at canopy level. In this presentation, we will discuss modifications to address this limitation.

Subsequently, modified models for red f_esc and an existing model for far-red f_esc were employed to analyze the dynamics of leaf and canopy red and far-red fluorescence under increasing drought and heat stress. Incorporating f_esc led to a closer agreement between simultaneously measured leaf and canopy SIF signals, with improved r² values for red f_esc (0.3 to 0.49) and far-red f_esc (0.36 to 0.52). Comparing the quantum yield dynamics of red and far-red fluorescence (Φ_F,687 and Φ_F,760) under increasing stress revealed a significant decrease in both leaf and canopy Φ_F,687, as well as leaf Φ_F,760, as drought and heat intensified. Contrarily, Canopy Φ_F,760 did not exhibit the same trend, displaying wider spread and lower median under low stress conditions. We performed a sensitivity analysis of Φ_F,687 and Φ_F,760 to changing leaf-to-sun angle by comparing measurements with and without mesocosm rotation. It revealed a notable increase in the coefficient of variation of Φ_F,760, especially under unstressed conditions. Our findings underscore the necessity for further research to unravel the causes of discrepancies in leaf and canopy-scale Φ_F,760. Conversely, the underutilized Φ_F,687 demonstrates significant potential for evaluating plant responses to drought and heat stress.