## 1 Introduction
Consequently, a strong linkage is observed between GPP and SIF under drought conditions. However, the complex regulatory interactions between photochemistry, SIF, and NPQ further complicate the GPP-SIF relationship during droughts. The GPP-SIF relationship and its variation patterns under drought conditions have not been fully explored, limiting the application of SIF in estimating GPP and monitoring photosynthesis under environmental stress. The fluorescence yield of photosystem II (PSII) is more sensitive to variations in light intensity, photochemical quenching (PQ), and NPQ than that of photosystem I (PSI). Since the red SIF contains more PSII information, it is considered a more effective photosynthetic indicator and exhibits a stronger correlation with GPP than the far-red SIF [11; 28]. However, whether the red SIF is more suitable for GPP estimation under drought conditions than the far-red SIF remains unknown. Based on [PERSON]'s light use efficiency model, GPP and SIF can be divided into physiological and non-physiological components [27; 29]. The SIF quantum yield (\(\Phi\)F) and light use efficiency (LUE) represent the physiological components of SIF and GPP, respectively. The diversity of the LUE-\(\Phi\)F relationship determines variations in the GPP-SIF relationship [22; 30; 31], but how this physiological link changes under drought conditions remains unclear. Incident radiation or photosynthetically active radiation (PAR) is another crucial non-physiological component driving variations in SIF and GPP, determining their diurnal patterns and serving as a mediator in the SIF-GPP relationship [13; 32]. Studies have shown that the promotive effect of PAR on SIF and GPP weakens under drought stress [27; 31; 33], but its impact on the GPP-SIF relationship requires further investigation. The canopy structure is another critical non-physiological component that drives SIF and GPP. The observed SIF by sensors from the canopy only represents a portion of the total SIF emitted by all leaves that escape from the canopy. In contrast, GPP represents the cumulative GPP of all leaves in the canopy. The processes of scattering and reabsorption within the canopy add complexity to GPP estimation derived from SIF. Numerous studies suggest that the canopy structure plays a significant role in affecting the quantitative relationship between SIF and GPP, especially canopy structural changes caused by drought stress, such as changes in the leaf angle, leaf area index, and chlorophyll content [15; 34; 35]. However, the impact of the canopy structure on SIF, GPP, and their relationship under drought stress remains unclear. Drought alters the water status of vegetation by reducing the availability of water in the soil and atmosphere and affects plant photosynthesis. The plant water stress index (PWSI), derived from evapotranspiration, quantitatively represents drought stress through vegetation's physiological responses and is regarded as a dependable indicator of drought conditions [36; 37]. Generally, when a drought occurs, other meteorological factors, such as radiation, atmospheric moisture, and soil moisture, will all change, thus synergistically affecting vegetation. Therefore, exploring the sensitivity of the SIF-GPP relationship to drought synergistic effect factors will help to enhance the understanding of the relationship between SIF and GPP under drought stress. Oak forests, as a representative forest type in warm temperate monsoon regions, contribute significantly to the carbon cycle of the global terrestrial ecosystem. With a broad geographical distribution, Chinese cork oak stands provide an ideal situation for the examination of forest carbon sinks. In this study, we focused on a Chinese cork oak plantation, conducting continuous observations of the canopy red and far-red SIF, flux, and micrometeorological data from the growing season to analyze the impacts of drought stress on the GPP-SIF relationship.