## 5 Conclusions  
Flash droughts can cause a rapid SM depletion, which can limit the ability of crops to adjust their physiological activities in a timely manner, ultimately posing a serious threat to food security. Therefore, there is an urgent need to investigate the characteristics of flash droughts in agricultural areas and the consequences of these droughts on crop photosynthetic capacity. Southern China's agricultural regions are vital for both the livelihoods of a large population and economic output. Further research on the high risk of flash droughts in this region is needed to provide crucial early warning information to protect agricultural production and the associated economy. This study explored the characteristics of flash droughts and their impacts on SIF in the agricultural regions of southern China from 2001 to 2020 using high-resolution datasets for SM and SIF. The main conclusions are as follows:  
(1) The southern areas of agricultural regions exhibited relatively high SIF values. This may be attributed to the relatively abundant P-PET and SM. The general upward trends of SIF in most regions of the study area indicate that crop growth improved in these regionsduring the last two decades, contributing favorably to agricultural yield increases. The increase in SIF is mainly due to increases in temperature, P-PET, and SM, with sufficient water and heat resources for the growth of crops. (2) The hotspots of flash droughts were located primarily in the eastern parts of the UYR and LYR, with a high percentage of the most rapid category events (Case 4 events) in these regions. The average duration of flash droughts in the southern agricultural region was 6-12 pentads, long enough to exert adverse effects on crops. Although there was a decreasing trend in flash droughts from 2001 to 2020, generally increasing trends were observed in the extended study period from 1981 to 2020. This suggests that the southern agricultural regions remain vulnerable to flash droughts under long-term climate warming. Additionally, P-PET provides a relatively effective early warning signal for SM-derived flash droughts (agricultural type), which should facilitate the identification of meteorological flash droughts. (3) Crops in the southern agricultural region exhibited a notably negative response to flash droughts. The response frequency of SIF was basically more than 80% and the response time was basically within two pentads. This result implied that flash droughts present a great threat to crops in the study area, as there is not sufficient time to take mitigation measures. In late July 2019, severe flash drought occurred in the southern agricultural zones, especially in the MYR and LYR regions. This drought was triggered by a rapid decline in P-PET and a rapid increase in T\({}_{\text{max}}\) (VPD), ultimately leading to a rapid depletion of SM. Consequently, this drought caused a rapid decline in the photosynthetic capacity of crops in the affected region, resulting in a significant reduction in food production. Overall, SIF exhibits promising advantages in terms of vegetation response to flash droughts. The cumulative and lagged effects on ecosystems as flash droughts evolve into prolonged droughts need to be explored in depth in the next study. The results of this study may provide great potential for leveraging SIF as a tool to monitor ecosystem resistance against flash droughts, thereby enhancing the ability to assess the potential impacts of droughts on ecosystems and develop appropriate countermeasures for different vegetation types under different climatic characteristics. In agricultural regions with hotspots of flash droughts, there should be a prioritizing of crop varieties that exhibit robust resistance to flash droughts to optimize the regional cropping structure, thereby enabling resilience against the adverse effects of drought.