## 3 Results
### Construction of the Nitrogen Content Model Based on Vegetation Indices  
The correlation analysis between vegetation indices constructed in Table 1 and leaf nitrogen content at different leaf positions was carried out, and the correlation coefficient matrices were drawn. The results are shown in Figure 3. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{
\begin{tabular}{c} **Leaf** \\ **Position** \\ \end{tabular} } & \multirow{2}{*}{**Model**} & \multicolumn{4}{c}{**Modeling Accuracy**} & \multicolumn{4}{c}{**Verification Accuracy**} \\ \cline{3-7}  & & \(\mathbf{R^{2}}\) & **RMSE** & **NRMSE** & \(\mathbf{R^{2}}\) & **RMSE** & **NRMSE** \\ \hline \multirow{3}{*}{**Flag leaf**} & PLSR & 0.61 ** & 0.08 & 0.06 & 0.58 ** & 0.19 & 0.14 \\  & SVM & 0.47 ** & 0.09 & 0.08 & 0.42 & 0.23 & 0.21 \\  & RF & 0.26 ** & 0.12 & 0.09 & 0.23 & 0.22 & 0.16 \\  & MLR & 0.55 ** & 0.08 & 0.06 & 0.55 ** & 0.19 & 0.13 \\ \hline \multirow{3}{*}{Upper 1 leaf} & PLSR & 0.47 ** & 0.16 & 0.09 & 0.55 ** & 0.16 & 0.09 \\  & SVM & 0.37 ** & 0.16 & 0.18 & 0.36 & 0.20 & 0.22 \\  & RF & 0.28 ** & 0.18 & 0.11 & 0.30 * & 0.20 & 0.12 \\  & MLR & 0.55 ** & 0.14 & 0.08 & 0.57 ** & 0.16 & 0.10 \\ \hline \multirow{3}{*}{Upper 2 leaf} & PLSR & 0.43 ** & 0.09 & 0.08 & 0.52 ** & 0.11 & 0.09 \\  & SVM & 0.32 ** & 0.10 & 0.20 & 0.35 & 0.11 & 0.21 \\  & RF & 0.32 ** & 0.10 & 0.08 & 0.38 * & 0.11 & 0.09 \\  & MLR & 0.39 ** & 0.09 & 0.08 & 0.47 ** & 0.11 & 0.10 \\ \hline \multirow{3}{*}{Upper 3 leaf} & PLSR & 0.13 ** & 0.13 & 0.12 & 0.11 & 0.13 & 0.12 \\  & SVM & 0.42 ** & 0.13 & 0.22 & 0.45 ** & 0.15 & 0.26 \\  & RF & 0.12 & 0.10 & 0.10 & 0.21 & 0.16 & 0.14 \\  & MLR & 0.15 * & 0.13 & 0.12 & 0.17 & 0.12 & 0.11 \\ \hline \multirow{3}{*}{Upper 4 leaf} & PLSR & 0.17 * & 0.11 & 0.14 & 0.20 & 0.14 & 0.18 \\  & SVM & 0.40 ** & 0.12 & 0.19 & 0.42 ** & 0.13 & 0.20 \\ \cline{1-1}  & RF & 0.11 & 0.11 & 0.14 & 0.15 & 0.15 & 0.18 \\ \cline{1-1}  & MLR & 0.17 * & 0.09 & 0.11 & 0.14 & 0.14 & 0.18 \\ \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 3: Model accuracy of estimating leaf nitrogen content based on sensitive bands. Based on the results of the correlation analysis between the nitrogen content of leaves at different leaf positions and vegetation indices, the vegetation indices with high correlation were screened and the results are shown in Table 4.  
The results showed that the correlations between the nitrogen content of leaves at different leaf positions and vegetation indices improved with increasing leaf positions. The vegetation indexes with the highest correlation with the nitrogen content of flag leaf were EXR and GLA (\(\left\lvert r\right\rvert=0.78\)). The vegetation indices with the highest correlation with nitrogen content at different leaf positions in the upper 1, upper 2, upper 3, and upper 4 were GBVI, WI, NDWI, and TCARI (\(\left\lvert r\right\rvert=0.75\), 0.70, 0.66, and 0.48), respectively. Figure 3: Correlation matrix between vegetation indices and leaf nitrogen content for different leaf positions. (**a**) Flag leaf; (**b**) upper 1 leaf; (**c**) upper 2 leaf; (**d**) upper 3 leaf; (**e**) upper 4 leaf. Based on the results of correlation analysis between leaf nitrogen content and vegetation indices, the top five vegetation indices with high correlation at different leaf positions were screened for model construction, and the results are shown in Table 5.  
Overall, the accuracy of leaf nitrogen content estimation gradually improved with the increase in leaf position.