# Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index
###### Abstract
This physiology-based vegetation index reflects the decline of reflectance at 531 nm due to the xanthophyll cycle triggered by the photo-protective mechanism of plants when receiving excessive radiation. It has been proved to be closely related to the LUE of evergreens at leaf and canopy scales [28; 29; 30; 31]. The increasing number of applications of PRI have greatly improved the accuracy of gross primary production (GPP) estimation using remote sensing approaches [32]. The response of PRI to environmental stresses can reflect important characteristics of the status of plants. Numerous studies focus on the relationships between PRI and environmental stresses, including water stress [33] and salinity stress [34]. Over the diurnal scale, PRI variations indicate the pigment interconversion via the xanthophyll cycle in response to excessive light [26]. Over longer time scales, the variations of pigment pool size in response to various environmental stresses could be the primary causes of PRI variations [35]. Physiological mechanisms driving PRI variations are not straightforward, and it is important to disentangle the effects of diurnal variation in the xanthophyll cycle (facultative component) and seasonal variation in pigment pool size (constitutive component). To further quantify the contribution of these two components on PRI variations, many PRI-relevant formulations have been proposed. [PERSON] and [PERSON] [36] proposed that the confounding contributions of these two effects (constitutive and facultative) could be isolated using dark-adapted leaves as a baseline for PRI measurements, where the dark-state PRI was defined as \"PRI0\". They found that most of the PRI variations were due to the changes in constitutive pigment pool size associated with species and canopy position. [PERSON] et al. [37] proposed a delta PRI (\(\Delta\)PRI) by subtracting the midday PRI from the early morning PRI (PRI0) to represent the diurnal magnitude of xanthophyll pigment interconversion in response to changing environmental conditions. To date, the physiological controls on PRI variations in mangroves have not been well understood. Empirical studies are needed to explore the complex mechanisms underlying PRI variation across different time scales in mangrove forests. The introduction of PRI has greatly improved the assessments of photosynthetic activity in evergreen ecosystems, while most of these ecosystems experience boreal climate conditions [38; 39]. The ability of PRI to track the ecosystem carbon dynamics of mangrove forests has not been fully explored. Compared to boreal evergreen ecosystems, mangrove forests in subtropical and tropical zones experience much smaller seasonal variation in temperature, and the driving factors of PRI variations may be different. In addition, the photosynthesis rates of mangroves tend to be saturated at relatively lower light levels due to their lower stomatal conductance and intercellular CO\({}_{2}\) concentrations [5]. Given these special characteristics, it is unclear whether PRI can track the long-term temporal variations of mangrove carbon fluxes. Although some attempts have been made to focus on this issue [40; 41], most lack long-term continuous measurements that are necessary for clarifying the relationship between PRI and mangrove carbon fluxes at both seasonal and inter-annual time scales. [PERSON] et al. [42] explored the link between PRI and mangrove carbon dynamics at a seasonal scale but they were not able to reveal any inter-annual variation pattern. Here, based on four-years of continuous measurement of mangrove PRI and EC-based ecosystem carbon fluxes, the purposes of this study are (1) to analyze the response of mangrove carbon fluxes to climate fluctuations and drought stress, (2) to examine the ability of PRI to track the response of the mangrove carbon cycle to climatic anomalies, and (3) to explore the complex mechanisms underlying PRI variations across different time scales.