## 3 Results
### Construction of the Nitrogen Content Model Based on the Wavelet Coefficients
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sot**} & \multirow{2}{*}{**Flag Leaf**} & \multicolumn{2}{c}{**Upper 1 Leaf**} & \multicolumn{2}{c}{**Upper 3 Leaf**} & \multicolumn{2}{c}{**Upper 4 Leaf**} \\ \cline{3-8}  & & \(R^{2}\) & **RMSE** & **NRMSE** & \(R^{2}\) & **RMSE** & **NRMSE** \\ \hline \multirow{4}{*}{**Flag leaf**} & PLSR & 0.68 **\({}^{**}\)** & 0.07 & 0.06 & 0.58 ** & 0.10 & 0.09 \\  & SVM & 0.51 ** & 0.08 & 0.16 & 0.58 ** & 0.12 & 0.22 \\  & RF & 0.49 ** & 009 & 0.07 & 0.45 ** & 0.12 & 0.10 \\  & MLR & 0.64 ** & 0.08 & 0.06 & 0.54 ** & 0.09 & 0.08 \\ \hline \multirow{4}{*}{Upper 1 leaf} & PLSR & 0.60 ** & 0.14 & 0.08 & 0.57 ** & 0.18 & 0.11 \\  & SVM & 0.59 ** & 0.13 & 0.15 & 0.52 ** & 0.17 & 0.19 \\  & RF & 0.45 ** & 0.15 & 0.09 & 0.43 ** & 0.19 & 0.11 \\  & MLR & 0.55 ** & 0.13 & 0.08 & 0.51 ** & 0.17 & 0.10 \\ \hline \multirow{4}{*}{Upper 2 leaf} & PLSR & 0.33 ** & 0.16 & 0.12 & 0.28 * & 0.12 & 0.09 \\  & SVM & 0.53 ** & 0.09 & 0.08 & 0.49 ** & 0.21 & 0.19 \\ \cline{1-1}  & RF & 0.38 ** & 0.10 & 0.07 & 0.48 ** & 0.23 & 0.17 \\ \cline{1-1}  & MLR & 0.54 ** & 0.14 & 0.10 & 0.53 ** & 0.11 & 0.09 \\ \hline \multirow{4}{*}{Upper 3 leaf} & PLSR & 0.44 ** & 0.10 & 0.10 & 0.52 ** & 0.10 & 0.10 \\ \cline{1-1}  & SVM & 0.42 ** & 0.11 & 0.16 & 0.32 * & 0.13 & 0.22 \\ \cline{1-1}  & RF & 0.24 ** & 0.10 & 0.10 & 0.44 ** & 0.15 & 0.14 \\ \cline{1-1}  & MLR & 0.50 ** & 0.09 & 0.08 & 0.52 ** & 0.11 & 0.11 \\ \hline \multirow{4}{*}{Upper 4 leaf} & PLSR & 0.31 ** & 0.08 & 0.10 & 0.28 * & 0.13 & 0.17 \\ \cline{1-1}  & SVM & 0.33 ** & 0.10 & 0.16 & 0.31 * & 0.11 & 0.16 \\ \cline{1-1}  & RF & 0.16 * & 0.10 & 0.12 & 0.12 & 0.13 & 0.16 \\ \cline{1-1}  & MLR & 0.37 ** & 0.08 & 0.10 & 0.35 * & 0.13 & 0.16 \\ \hline \hline \end{tabular} Note. \({}^{*}\) indicates a significant correlation at the 0.05 level and ** indicates a significant correlation at the 0.01 level.  
\end{table} 
Table 5: Model accuracy of estimating nitrogen content based on vegetation indices. Based on the wavelet coefficients obtained by the CWT, the sensitive wavelet energy coefficients were screened, and the results are shown in Table 6.  
Figure 4: Correlation matrix diagrams of wavelet coefficients and leaf nitrogen content in different leaf positions. (**a**) Flag leaf; (**b**) upper 1 leaf; (**c**) upper 2 leaf; (**d**) upper 3 leaf; (**e**) upper 4 leaf. The overall correlations between leaf nitrogen content and wavelet coefficients at different leaf positions improved with the increasing leaf positions. The correlations between the nitrogen content of the flag leaf and upper 1 leaf were the highest, with the correlation coefficient up to 0.87, but the corresponding wavelet transform scale differed. For the band range of 1500-2500 nm, the flag leaf had the highest global sensitivity index and the corresponding scales were 3, 4, 5, 8, and 9. The coefficient with the highest correlation between the nitrogen content of the flag leaf and the wavelet coefficient was concentrated in the band between 2170 and 2180 nm with a correlation coefficient of 0.87 and the corresponding scale of 4. The global sensitivity index of the flag leaf was the highest in the band range between 1500 and 2500 nm, and the corresponding scales were 2, 3, 4, 5, and 6. The coefficient with the highest correlation between the nitrogen content of the upper 1 leaf and wavelet coefficient was concentrated between the 1700-2200 nm band, while the correlation coefficient was 0.87, and the corresponding scale was 3. The global sensitivity index of the upper 2 leaf was the highest between the 1500-2000 nm band range, and the corresponding scales were 3, 4, and 5. The coefficient with the highest correlation between the nitrogen content of the upper 1690-1700 nm band, with the correlation coefficient of 0.79, while the corresponding scale was 3. The band range between 1300 and 2500 nm showed best global sensitivity index for the upper 3 leaf with corresponding scales 3, 4, and 8.