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 (ΦF) at the photosystem from canopy measurements, the escape probability (fesc) must be considered.This study compares ΦF 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 fesc through simulations with the radiative transfer model SCOPE. Findings revealed that existing fesc models inadequately predicted the correct value range for red fesc especially at canopy level. In this presentation, we will discuss modifications to address this limitation.Subsequently, modified models for red fesc and an existing model for far-red fesc were employed to analyze the dynamics of leaf and canopy red and far-red fluorescence under increasing drought and heat stress. Incorporating fesc led to a closer agreement between simultaneously measured leaf and canopy SIF signals, with improved r2 values for red fesc (0.3 to 0.49) and far-red fesc (0.36 to 0.52). Comparing the quantum yield dynamics of red and far-red fluorescence (ΦF,687 and ΦF,760) under increasing stress revealed a significant decrease in both leaf and canopy ΦF,687, as well as leaf ΦF,760, as drought and heat intensified. Contrarily, Canopy ΦF,760 did not exhibit the same trend, displaying wider spread and lower median under low stress conditions. We performed a sensitivity analysis of ΦF,687 and ΦF,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 ΦF,760, especially under unstressed conditions. Our findings underscore the necessity for further research to unravel the causes of discrepancies in leaf and canopy-scale ΦF,760. Conversely, the underutilized ΦF,687 demonstrates significant potential for evaluating plant responses to drought and heat stress.