## 4 Discussion
### Limitations of the C4 Crop Experiment  
This research was conducted on a routinely irrigated C4 crop. Due to differences in photosynthesis between C3 and C4 crops and differences in environmental conditions between irrigated crops and natural ecosystems, LUE and SIF\({}_{\text{yield}}\) may exhibit different response patterns in other situations. The results of this study may, therefore, have limited application to other photosynthesis types and different natural stress conditions.  
Several recent studies have found that the slope of the SIF-GPP relationship varies according to the biome or plant functional type (PFT) [15; 19; 61; 70; 71; 72]. For example, [PERSON] et al. [29] reported that the relationship between SIF and GPP was highly dependent on the PFT and that the photosynthesis of C3 and C4 plants exhibited different patterns in response to environmental conditions: generally, C4 plants were found to have higher photosynthetic light-use efficiencies and greater adaptation to a high light intensity than C3 plants [73; 74]. Maize, a typical C4 species with a high photosynthetic capacity, usually exhibits a slight light-saturation effect at the canopy level [61; 75]. This is consistent with our observations that the maize canopy GPP exhibited slight saturation for both the diurnal and daily data sets (Figures 5 and 6). However, the environmental influences on C3 and C4 species are always different. Compared with C3 crops, C4 crops adapt to warm environments and have a higher light capacity for high light conditions and high temperature levels [29]. Previous studies have reported that the quantum yield of C3 plants for CO\({}_{2}\) uptake, generally driven by photorespiration, decreases with an increasing temperature [69], whereas the temperature has no significant effect on the quantum yield of C4 species for CO\({}_{2}\) uptake. The high evaporation demand around noon leads to a decrease in stomatal conductance and lasts longer. This effect is more pronounced in C3 leaves, leading to a further reduction in carbon absorption [6; 76]. Therefore, it can be inferred that the influence of illumination on the SIF-GPP relationship for C3 plants may be greater than that for C4 plants. However, as we were limited by the data sets available in this study, our conclusion only applies to C4 plants.  
In addition, as our study site consisted of irrigated farmland, there was no severe abiotic stress at the site, and our conclusions may have limited application to natural vegetation, especially vegetation under meteorological stress. Some studies have reported that the SIF-GPP relationship can be disturbed by environmental stress [12; 16; 21]. For example, [PERSON] et al. [77] concluded that drought conditions might result in a decrease in steady-state red and far-red SIF. [PERSON] et al. [78] reported that pea plants showed small changes in SIF parameters under cold stress. [PERSON] et al. [79] also observed that both the canopy structure and SIF physiology changed to cope with water stress in a maize field. Therefore, in future work, more attention should be paid to other natural vegetation types as well as to exploring the specific response mechanisms of the GPP-SIF relationship to natural stress.