# Hyperspectral Imaging to Evaluate the Effect of Irrigation Water Salinity in Lettuce
## 3 Results and Discussion
### Effect of Salinity on Spectral Features  
#### 3.2.1 Non Preprocessed and SNV Reflectance Spectra  
The mean spectrum of each saline treatment was computed (calibration set, \(n\) = 7 leaves per treatment). The raw spectra and the preprocessed SNV spectra were considered for comparison. Figure 3A shows the mean spectra of each saline treatment. It can be seen that the Ct global reflectance is lower than the others, primarily in the infrared range. This fact can be related to certain physical properties inducing lower scattering in the leaves of the Ct leaf samples. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Saline Treatment** & **Water Content (\%)** & **Osmotic Potential (MPa)** \\  & **Mean \(\pm\) STD** & **Mean \(\pm\) STD** \\ \hline Ct & 94.53 \(\pm\) 0.09a & \(-\)0.65 \(\pm\) 0.04a \\ S1 & 94.54 \(\pm\) 0.12a & \(-\)1.08 \(\pm\) 0.09b \\ S2 & 93.15 \(\pm\) 0.18b & \(-\)1.49 \(\pm\) 0.11c \\ S3 & 92.70 \(\pm\) 0.07c & \(-\)2.06 \(\pm\) 0.09d \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Water content and osmotic potential of the lettuce leaves treated without NaCl (Ct) and treated with 50, 100 and 150 mM NaCl (S1, S2 and S3, respectively). Letters indicate the different mean groups according to [PERSON]’s test for a \(p\) level of 0.05. Figure 3: (**a**) Mean relative reflectance spectra of the leaves of each saline treatment. Calibration set, \(n\) = 28 leaves (4 saline treatments \(\times\) 7 leaves). (**b**) Same spectra corrected with Standard Normal Variate (SNV) normalization to eliminate multiplicative effect. (**c**) Zoom in the visible range of SNV corrected spectra. See Table 1 for abbreviations. Crops often have a higher NIR reflectance level under water stress conditions [16; 17; 18] due to air replacing water between the cells and producing optical discontinuities and scattering. [PERSON] [30] noted that crops under salt stress conditions typically show the same symptoms as under water stress conditions. Salinity affects the osmotic potential of the plants and the water availability of the soil; as such, the water absorption capacity is affected [5]. As shown in Table 1, the osmotic potential and water content in the lettuce leaves decreases when the salinity increases, mainly in S2 and S3 treatments. This fact could induce water stress in the plants that could justify the higher NIR reflectance observed in the leaves of those treatments. Other effects observed in the crops under saline stress include the loss of growth and turgor, as well as the destruction of cellular membranes [31]. Moreover, an accumulation of salt in the leaves can alter the refractive index of the water. [PERSON] et al. [32], the increase in NIR reflectance could be caused by an increase in the thickness and density of the leaves. This fact has been observed in leaves under moderate salinity levels [6]. All of these described effects can affect the scattering process, and this scattering can induce a multiplicative effect on the spectra in the NIR region. This multiplicative effect has been removed applying SNV. Figure 3B shows the mean spectra of the leaves in each saline treatment corrected with SNV normalization in order to remove the multiplicative effect. Differences among each spectrum can be observed in two areas (Figure 3C): (1) the peak near 550 nm and (2) the valleys near 480 and 680 nm. The visible region of the spectra depends on the absorbance of the different pigments of the leaves, mainly chlorophyll but also carotenoids and flavonoids. The reflectance peak near 550 nm is due to the significant absorption of chlorophyll in the blue and red regions and the reflection in the green region. The height of the peak of each spectrum can be compared by computing the difference between the reflectance values at 550 and 510 nm. A progressive and systematic decrease in the height of the peak was observed from the Ct to the S3 treatments as the salinity increased.