## 3 Results
### Diurnal Patterns of SIF  
Due to the different canopy pigment contents, both F760 and AF760 in soybean and sweet potato showed different values (Figure 3). As expected, soybean showed larger daily values of F760 and AF760 (1.83 mw\(\cdot m^{-2}\cdot nm^{-1}\cdot sr^{-1}\) and 0.0018 \(sr^{-1}\) for soybean, 0.39 mw\(\cdot m^{-2}\cdot nm^{-1}\cdot sr^{-1}\) and 0.00060 \(sr^{-1}\) for sweet potato) because of its larger canopy pigment levels. In addition, the intensity of daily averaged PAR for the soybean measurements was stronger than that for the sweet potato measurements (989.98 and 602.72 mw\(\cdot m^{-2}\cdot nm^{-1}\), respectively). This difference in PAR may lead to a partial deviation of SIF between soybean and sweet potato. However, the difference in SIF between these two crops still existed even when the parallel PAR measurements were similar, such as the soybean measurements at 14:44 and the sweet potato measurements at 12:50 (1441.58 and 1387.93 mw\(\cdot m^{-2}\cdot nm^{-1}\), respectively; Figure 3). Thus, SIF differences among species were mainly determined by the difference in the canopy  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Pigments** & **Soybean** & **Sweet Potato** \\ \hline Chl a & \(7.18\pm 0.29\) & \(6.10\pm 0.75\) \\ \hline Chl b & \(1.92\pm 0.11\) & \(1.81\pm 0.16\) \\ \hline Chl a + Chl b & \(9.09\pm 0.40\) & \(7.91\pm 0.90\) \\ \hline Chl a/Chl b & \(3.76\pm 0.09\) & \(3.32\pm 0.21\) \\ \hline Car & \(1.13\pm 0.04\) & \(0.89\pm 0.07\) \\ \hline Chl/Car & \(8.09\pm 0.24\) & \(8.80\pm 0.55\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Pigment concentrations (ug/mL of plant extract) with values of mean ± stand error of mean.  
Figure 2: Comparisons of canopy pigment contents between soybean and sweet potato. The difference in the values between soybean and sweet potato were significant at the level of \(P<0.05\), which was determined using the one-way ANOVA (Tukey test). Error bars show the standard error of the mean.  
levels of pigment. We further noted that the ratios of the daily mean F760 and AF760 values between the soybean and sweet potato was approximately 4.69 and 2.94, respectively, which were much higher than the ratios of daily canopy pigment contents among these two crops (1.84 and 1.80, for LAI*Chl a and LAI* total Chl, respectively). This finding suggested that other factors (maybe intracellular physiological processes) were causing the observed SIF differences between the plant functional types.  
Although PAR was not the main driver of the differences in F760 and AF760 among species, the diurnal dynamics of PAR were clearly driving the diurnal patterns of both F760 and AF760 (Figure 3). A consistent diurnal pattern was found between F760 and PAR in all species. However, only AF760 in sweet potato followed the diurnal patterns of PAR relatively well. The measurements of AF760 in soybean inversely related to the PAR value in the morning but positively related to the PAR in the late afternoon. These different behaviors of AF760 in responding to the PAR variations between the soybean and sweet potato indicated that the regulation of the SIF efficiency in response to the PAR changes was different for various species. This different regulation of the SIF efficiency in responding to the PAR variations might provide an explanation for the different magnitude of the SIF values between soybean and sweet potato.