Diurnal responses of large-scale solar-induced chlorophyll fluorescence to varying heat and water stresses

Heat and water stress induce structural and physiological changes in plants that can become decoupled within a diurnal cycle due to faster physiological responses. Understanding these physiological responses can improve the large-scale modeling of photosynthesis and evapotranspiration. Satellite solar-induced chlorophyll fluorescence observations (SIF_obs) provide both structural and physiological information and are recognized as a reliable indicator for monitoring plans heat and water stresses at large scales. However, the diurnal responses of large-scale SIF_obs and its physiological component, fluorescence efficiency (Φ_f),  to heat and water stresses remain unclear. In this study, we used data from Orbiting Carbon Observatory-3 (OCO-3) and combined a machine learning technique with the near-infrared radiance of vegetation (NIRvR) approach to model four years of hourly SIF_obs and Φ_f data for summer seasons across mainland China. Statistical analyses of the modeled outputs were conducted to investigate the diurnal variations of SIF_obs and Φ_f under varying water and heat stress conditions. Additionally, by comparing modeled SIF_obs variations at different times of the day, we also attempted to investigate the uncertainties in assessing changes in the daily average SIF_obs (ΔSIF_daily) when using daily correction factors to convert polar-orbiting satellite SIF_obs into the daily averages (SIF_daily). Our results revealed that SIF_obs and Φ_f at different times of day exhibited different variations under water and heat stress conditions, both in magnitude and sign, especially in forests. Morning and afternoon SIF_obs generally exhibited larger positive or smaller negative responses than the midday period. In contrast, the morning Φ_f also exhibited larger positive or smaller negative responses than the midday period, but the opposite pattern was found for the afternoon Φ_f. Such diurnal differences in SIF_obs and Φ_f responses became more pronounced on days with higher water and heat stresses. Additionally, morning polar-orbiting satellite SIF observations tended to overestimate ΔSIF_daily, whereas midday observations tended to underestimate it. Such biases also intensified with rising daily water and heat stress levels. Our findings broaden the understanding of the diurnal responses of SIF and especially Φ_f to varying heat and water stresses. The results also highlight the importance of observation time in monitoring plant water and heat stresses from polar-orbiting satellite SIF observations.