## 4 Discussion
### Effect of Temperature and Precipitation on Plant Photosynthesis  
Temperature is one of the most important abiotic factors that regulate plant photosynthesis. Within certain limits, a low temperature decreases the activity of enzymes and the effective rate of carboxylation in the photosynthesis processes, thus affecting the efficiency of photosynthesis [34]. Within the range of plant growth, an increase in temperature will lead to an increase in photosynthesis [48]. However, stresses induced by particularly higher temperatures decrease SIF values through the decrease in red and far-red fluorescence [24; 25]. [PERSON] et al. [49] has also argued that in subtropical vegetation, the end of the growing season was positively correlated with the pre-season minimum temperature. However, when the temperature reaches zero degree or lower, plant photosynthesis declines [5; 25]. Our results (see Figures 6 and 7) indicate that different plants show different sensitivities to temperature, and that temperatures show a hysteresis effect on plant photosynthesis in cropland and grassland areas, but not in forests. The forested lands considered in this study are mainly distributed at higher latitudes, where temperatures are lower, and temperature is integral for tree growth. However, forests did not appear to be sensitive to the temperatures recorded in the previous 16 days, but rather were more responsive to current mean and minimum temperature levels. Thus, when the growing season starts in a forest, photosynthesis increases and is mainly affected by the current temperature, and is most sensitive to the daily minimum temperature. Higher temperatures can also lead to a decrease in SIF if the temperature exceeds a certain threshold. [PERSON] et al. (2013) found that in Amazonia, when the temperature is lower than 30 \({}^{\circ}\)C, SIF and EVI increased with the increase in temperature, and when the temperature was higher than 30 \({}^{\circ}\)C, EVI increased, but SIF decreased. Many researchers have explored how water stress affects vegetation SIF and have found similar conclusions. [PERSON] et al. [4] used Amazonia as the study region and used GOSAT SIF to monitor how seasonal water stress affected plant productivity. They concluded that the drought season easily led to a decline in plant photosynthesis and SIF. Under short-term, mild drought events, plant photosynthesis decreases due to stomatal closure, recovering to its initial level after drought [50]. However, long-term and severe drought will lead to severe photoinhibition and reduced photosynthesis rates [51; 52]. The mean annual precipitation of the forest regions considered in this study is approximately 460 mm [12], which is not enough for forest growth. Because of the existence of permafrost in these forest regions, rain that falls to the land surface is fixed by vegetation and cannot infiltrate deep soil [6]. Therefore, we can infer that precipitation is not a factor affecting photosynthesis strength. For cropland and grassland, the previous 16-day temperature measurement is more sensitive to SIF, which indicates that the cumulative temperature is a more sensitive factor for cropland and grassland areas. Abundant water is an essential prerequisite for cultivation. The croplands located in Songnen Plain (one of China's most important grain production bases) receive an annual precipitation of 400-600 mm, which is sufficient for crop growth. However, no significant correlation was found between SIF and precipitation in the croplands. With regard to grassland areas, a significant correlation was found between SIF and current precipitation. A stronger relationship was found between SIF and previous 16-day precipitation. This indicates that precipitation does contribute to SIF in grassland areas, but the contribution is limited, as evidenced by the lower R\({}^{2}\) values. The soil in this region is typically grassland zonal soil, which corresponds to climate and vegetation type.