Locating the shift in the plant fluorescence role under stress conditions

Sun-induced fluorescence (SIF), which serves as a proxy of plant and ecosystem net carbon assimilation (or gross primary production, GPP, at the ecosystem level), has been observed to exhibit sustainability under optimal conditions. The recent evidence demonstrated a break of this relation under stress conditions, and a change in the fluorescence role for excessive energy dissipation. In this study, our aim was to identify the physiological alterations responsible for the transition. To achieve this, we utilize both natural and induced stress conditions to investigate the changing role of fluorescence. Initially, we examine the impact of sustained drought on the correlation between plant leaf-level gas exchange and Pulse Amplitude Modulation (PAM) parameters, in conjunction with plant Sun-Induced Fluorescence (SIF) and other spectral indices. In the subsequent phase, we use chemical inhibitors to assess the reaction of both susceptible and tolerant plants. The findings indicated that Sun-Induced Fluorescence (SIF) primarily interacts with the Non-Photochemical Quenching (NPQ) pathway. During the initial phases of rehydration, the SIF signal decreases correspondingly with the decline in photosynthetic activity. Subsequently, as NPQ levels reach saturation, the intensity of the SIF signal begins to rise. The use of photosystem inhibitors reinforces our observations from the drought experiment. The sensitive accession displays a rapid surge in the fluorescence signal, coinciding with a complete cessation of carbon assimilation. Conversely, in the tolerant accession, a simultaneous decrease in the fluorescence signal occurs alongside a partial decline in the rate of carbon assimilation. The results underscore the dual roles of plant fluorescence within the plant. Distinguishing between these two phases can assist in monitoring both plant and ecosystem responses under stress conditions.