## 4 Discussion
### Insights from the Impacts of Water Stress and Phenological Stages  
In order to further understand the influence of water stress, it should be considered that SIF at 760 nm was contributed by the combination of two photosystems (photosystems I and II) [1], which could be greatly affected by water stress. Specifically, water stress is known as a limiting environmental parameter that results in the down-regulation of photosynthesis through stomatal closure [48; 49; 50] or metabolic impairment [51; 52]. Down-regulation of photosynthesis generally lowers the absorption of solar radiation, which thereby causes the decline of SIF emissions. This result was also confirmed by our wheat observations, which showed decreasing SIF (F760, AF760, and Fy760) values at seasonal and diurnal scales accompanied with intensifying water stress (Figures 3, 4 and 5a-c). Furthermore, the down-regulation of photosynthesis finally leads to the inhibition of plant growth [53]. This was also demonstrated in our study by the decreasing LAI values with the increasing of water stress over the four plots of wheat (Figure S3). This decreased LAI values under water stress would further lower the absorption of solar radiation due to the lower solar capture, and further lowered the SIF emission (Figure S4).  
Additionally, water stress is often accompanied by high values of Ta (optimal Ta for plants normal growth is about 10-35 \({}^{\circ}\)C) and VPD (in general greater than 2 kPa, meaning low humidity), which could also affect the stomatal conductance of leaves and further decrease photosynthetic capacity [54; 55]. In our study, high values of Ta and VPD were not observed at seasonal and diurnal scales; however, their impacts on SIF radiance cannot be ignored, particularly for Ta in wheat plots, some of which were suffering from water stress. This conclusion can be further confirmed by the relative high correlation between Ta and AF760 at the wheat seasonal scale (Figure 6c). This was because water stress could cause the increase of intercellular CO\({}_{2}\) level and the decrease of stomatal conductance, which would greatly sharpen the response of photosynthesis to the variation of temperature [52].  
Phenological stages were also found to be an important factor in driving the variation of SIF, both on seasonal and diurnal scales. This was because phenological stages can determine the timing of plant development (e.g., LAI) and behavioral phases of organisms, such as bud burst, flowering, leaf coloration, and senescence [56; 57; 58]. That is to say, the impact of phenological stages on SIF may be integrated from the impact of LAI or leaf coloration (i.e., leaf greenness). Their synergistic effects may explain the drop of SIF after May for wheat and after September for maize, although concurrent SWRs were similar to before. (Figures 3 and 5). Their mixed influence on SIF variation might bring much trouble in exploring the detailed relationships between phenological stages and SIF. Moreover, phenological phases of plants are mainly driven by air temperature, photoperiod length, and are less impacted by water availability, nutrients, and soil properties [56; 57; 58; 59]. This makes it more complicated for understanding the impact of phenology on SIF emissions.