# Estimation of Chlorophyll Content in Wheat Based on Optimal Spectral Index
## 2 Materials and Methods
### Construction of Vegetation Index  
Since the vegetation spectrum is mainly water absorption after 1100 nm, the reflectance of the spectral curve of wheat in the water absorption zone will decrease. Moreover, it is irregular, so spectral reflectance data from 400 to 1000 nm were used to analyze and construct an estimate of the chlorophyll content model [24].  
Based on the spectral reflectance curves of wheat, the general trend of spectral reflectance of wheat in different growth stages is basically the same, and the results are shown in Figure 2. The overall trend is low reflectivity in the visible band (400-700 nm) and high reflectivity in the near-infrared band (800-1000 nm). It can be seen that the spectral curve characteristics are closely related to the chlorophyll content, and the chlorophyll content can be estimated using the reflectance spectrum of wheat.  
The correlation between the chlorophyll content and the spectral reflectance of wheat was obtained by analyzing the correlation between the denoised hyperspectral reflectance of wheat at the flag-picking stage, flowering stage, grain-filling stage, and maturity stage and the corresponding chlorophyll content. Figure 3 shows that there were significant differences in the correlation between spectral reflectance and chlorophyll content in leaves at different growth stages. However, the absolute value of the correlation coefficient is at  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Child-Bearing Period** & **Minimum Value** & **Maximum Value** & **Mean Value** & **Variance** \\ \hline flagging stage & 47.8 & 58.8 & 54.5 & 6.6 \\ flowering period & 45.1 & 57.8 & 51.4 & 10.8 \\ pushulation period & 37.8 & 53.2 & 47.4 & 16.4 \\ maturity & 5.1 & 38.4 & 17.2 & 69.8 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Statistics of chlorophyll content.  
Figure 2: Changes in wheat spectrum during the growth period.  
most 0.32, which indicates that the correlation between chlorophyll content and spectral reflectance is weak. On the whole, the correlation between the original spectral reflectance and chlorophyll content in the four growth stages was low.  
In order to solve the problem that the spectral characteristics are easily influenced by the differences in the physiological information of the crops themselves [25,26], the correlation matrix method was used to construct a spectral index sensitive to wheat information to optimize the estimation of chlorophyll in wheat, and the following spectral index was selected, which is detailed in Table 2. In the spectral wavelength range of 400-1000 nm, the spectral index was calculated for the combination of all wavelengths of the original hyperspectral [27], and then the correlation between all spectral indices and chlorophyll content was analyzed. The wavelength positions of \(i\) and \(j\), where the maximum correlation coefficient was located, were used as the optimal wavelength combination, and the calculated spectral index value was the optimal spectral index with the highest correlation with the chlorophyll index of wheat.