## 4 Discussion
### Further Understanding the Relationship Between SIF and the PSII Operating Efficiency  
Our measurements of autumn senescent crops showed that F760 and PAR were similar in their diurnal patterns, which was consistent with the results of previous studies [11; 30]. However, \(F_{q}^{\prime}/F_{m}^{\prime}\) did not show similar diurnal patterns to PAR in a single day (Figure 4). This finding indicated that the relationship between F760 and \(F_{q}^{\prime}/F_{m}^{\prime}\) was not linear in senescent crops. It was further found that Fy760 was not linearly related to \(F_{q}^{\prime}/F_{m}^{\prime}\) (Figure 4), although Fy760 is more linked to photosynthesis than F760 [1]. This nonlinear relationship between \(F_{q}^{\prime}/F_{m}^{\prime}\) and F760 (or Fy760) revealed that photosynthesis at autumn senescence might not be precisely captured by the canopy SIF. Simultaneously measured leaf-based photosynthesis parameters could provide a further understanding of the nonlinear relationship between the SIF and \(F_{q}^{\prime}/F_{m}^{\prime}\). The values of Ci showed contrary diurnal dynamics with the PAR variation but did not show consistent diurnal patterns with gsw (Figure 6c,d). It can be concluded that Ci is not only determined by the aperture of leaf stomata but also by intracellular processes related to the PAR variation. It has been demonstrated that respiration is dependent on the ambient temperature and also on the PAR values, since the PAR values drive the diurnal variation in temperature [27; 41], which was also found in our study based on their diurnal patterns (Figure 8). Therefore, respiration also plays a role in determining the concentration of Ci. The role of respiration also increased, especially in the late afternoon, when the PAR values were relatively lower and the air temperature was lower, as well, which could be explained by the fact that enhanced respiration at lower temperatures can provide more chemical energy for plants to prevent the damage of lower temperatures on plant organs [27; 41]. This synchronous variation in respiration with lowering temperatures could be supported by the diurnal changes in Ci and air temperatures, particularly for the measurements of sweet potato (Figure 8b). The effect of respiration on photosynthesis was also revealed by inconsistent variations between A and \(F_{q}^{\prime}/F_{m}^{\prime}\). The continued decrease in A in soybean and the sharp decrease in A in sweet potato, contrary to the increase in the synchronously measured \(F_{q}^{\prime}/F_{m}^{\prime}\) (Figures 4 and 6), indicated that the respiration intensity was increasing, and more photosynthesis products (i.e., sugar) were consumed to provide the chemical energy required by other intracellular processes. Figure 8: Diurnal patterns of intercellular carbon dioxide concentration (Ci) accompanied by the air temperature and short-wave radiation for (**a**) soybean results and (**b**) sweet potato results. The air temperature and short-wave radiation were collected with an interval time of one hour in a meteorological station that was \(\sim\)300 meters away from our experimental site. Significant differences (\(P<0.05\)) between the mean values in the figure are marked by different letters above symbols, while the letters with a same letter showed that the difference is not significant. For example, as shown in Figure a, the letters “ab” and “bc” corresponding to the second and third measurement of Ci represent that their difference is insignificant, while the letters “ab” and “c” corresponding to the second and fourth one represent that the difference is significant. In addition, we found that the intensity of the PSII operating efficiency in the autumn leaves was not constant and changed with the solar time (Figure 7). This heterogeneity of \(F^{\prime}_{q}/F^{\prime}_{m}\) means that the different sites of the leaf surface did not provide the same levels of chlorophyll fluorescence to canopy SIF, even at the same time.