## 3 Results
### Construction of Nitrogen Content Model Based on the Spectral Reflectance  
The original spectra of different leaf positions of winter wheat were FD, SD, and CR transformed. The correlation between spectral reflectance and nitrogen content of leaves at different leaf positions was analyzed, and the bands with strong correlations were selected as sensitive bands, and the results are shown in Table 2.  
Overall, the correlation between nitrogen content and transformed spectra increased with leaf position, with high correlation in leaves of all positions. The sensitive bands of the flag leaf were concentrated in the visible light-near infrared bands between the 640-790 nm band. The spectral bands with the highest correlation (\(\mid\!r\!\mid\!=\!0.89\)) with flag leaf nitrogen content were FD (649), FD (659), and SD (685). The sensitive bands of the upper 1 leaf were concentrated in the near-infrared bands of the FD and SD between 900 and 1000 nm. The \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sort**} & \multicolumn{2}{c}{**Flag Leaf**} & \multicolumn{2}{c}{**Upper 1 Leaf**} & \multicolumn{2}{c}{**Upper 2 Leaf**} & \multicolumn{2}{c}{**Upper 3 Leaf**} & \multicolumn{2}{c}{**Upper 4 Leaf**} \\ \cline{2-10}  & **(\(\lambda\))** & \(\mid\!r\!\mid\) & **(\(\lambda\))** & \(\mid\!r\!\mid\) & **(\(\lambda\))** & \(\mid\!r\!\mid\) & **(\(\lambda\))** & \(\mid\!r\!\mid\) & **(\(\lambda\))** & \(\mid\!r\!\mid\) \\ \hline
1 & FD (649) & 0.89 & FD (929) & 0.84 & FD (1232) & 0.76 & FD (727) & 0.75 & FD (1421) & 0.67 \\
2 & FD (659) & 0.89 & FD (950) & 0.82 & FD (1234) & 0.76 & FD (621) & 0.74 & FD (1610) & 0.59 \\
3 & SD (685) & 0.89 & FD (926) & 0.83 & FD (1229) & 0.75 & SD (543) & 0.74 & FD (1418) & 0.59 \\
4 & SD (748) & 0.88 & FD (927) & 0.83 & FD (1233) & 0.74 & SD (589) & 0.73 & FD (1598) & 0.57 \\
5 & SD (780) & 0.87 & SD (925) & 0.83 & FD (1231) & 0.74 & SD (542) & 0.73 & FD (1618) & 0.57 \\ \hline \hline \end{tabular} Note: the \(\mid\!r\!\mid\) represents the absolute value of the correlation coefficient and the \(\lambda\) indicates the band position.  
\end{table} 
Table 2: Correlation analysis of different transform spectra and leaf nitrogen content in different leaf positions and the results of screening sensitive bands. Figure 2: The spectral curves for different leaf positions of wheat. **(a)** Leaf original spectral curves; **(b)** first-order differential spectral curves; **(c)** second-order differential spectral curves; **(d)** leaf continuum removal spectral curves. spectral band with the highest correlation (\(|\,r|=0.84\)) with the nitrogen content of the upper 1 leaf was FD (929). The sensitive bands of the upper 2 leaf were concentrated in the FD near-infrared bands between 1200 and 1250 nm. The spectral bands with the highest correlation (\(|\,r|=0.76\)) with the nitrogen content of the upper 2 leaf were FD (1232) and FD (1234). The sensitive bands of the upper 3 leaf were concentrated in the visible light bands with the FD and SD between 540 and 730 nm. The spectral band with the highest correlation (\(|\,r|=0.75\)) with the nitrogen content of the upper 3 leaf was FD (727). The sensitive bands of the upper 4 leaf were concentrated in the mid-infrared bands of the FD, between 1400 and 1620 nm. The spectral band with the highest correlation (\(|\,r|=0.67\)) with the nitrogen content of the upper 4 leaf was FD (1421). Using the spectral sensitive bands selected in Table 2, the prediction models of leaf nitrogen content at different leaf positions of wheat were constructed based on PLSR, SVM, RF, and MLR, respectively. In this study, the nitrogen content estimation models were established using 2/3 of the sample data, and the accuracy of the models was validated using the remaining 1/3 of sample data, and the results are shown in Table 3.  
The results showed that the best model for nitrogen content estimation for flag leaves was PLSR with 0.61 and 0.58 \(R^{2}\) values for modeling and validation.