# Hyperspectral Imaging to Evaluate the Effect of Irrigation Water Salinity in Lettuce
## 3 Results and Discussion
### Models and Data Processing
#### 3.3.3 Comparison between Spectral Indexes  
Table 2 presents eleven narrow band indexes that are related to pigment, water content and salinity effects, all of them extracted from a literature review on this topic. These eleven indexes, along with LSI index and PCA scores, are applied on the spectra of the leaves from the calibration set (\(n\) = 1000 spectra per leaf, 7000 spectra per salinity class). Table 3 presents the results of the one-way ANOVA's and Tukey-Kramer tests performed on these indexes, calculated considering as grouping variable the salinity treatment. LSI (\(F\) = 3440.2), Green and Indigo Ratio (GIR) \begin{table}
\begin{tabular}{c c} \hline
**Index** & **Formula** \\ \hline NDVI, related to chlorophyll content [42] & NDVI = (R\({}_{500}\) - R\({}_{670}\))/(R\({}_{800}\) + R\({}_{670}\)) \\ NDVI\({}_{705}\), related to chlorophyll content [43] & NDVI\({}_{705}\) = (R\({}_{750}\) - R\({}_{705}\))/(R\({}_{750}\) + R\({}_{705}\)) \\ Red Edge Position Index, related to chlorophyll content [44] & RPI = (R\({}_{750}\)/R\({}_{700}\)) \\ Related to chlorophyll content in anthocyanin free leaves [45] & Chl\({}_{\text{green}}\) = (R\({}_{760}\)/R\({}_{550}\)) - 1 \\ Chl\({}_{\text{red edge}}\) = (R\({}_{800}\)/R\({}_{705}\)) - 1 \\ Plant Senescence Reflectance Index [46] & PSRI = (R\({}_{800}\) - R\({}_{500}\))/(R\({}_{750}\)) \\ Water Index, related to water content [40] & WI = (R\({}_{900}\)/R\({}_{970}\)) \\ Green region NDVI, related to salinity [13] & NDVI\({}_{\text{green}}\) = (R\({}_{550}\) - R\({}_{670}\))/(R\({}_{550}\) + R\({}_{670}\)) \\ Far red region NDVI, related to salinity [13] & NDVI\({}_{\text{far red}}\) = (R\({}_{710}\) - R\({}_{670}\))/(R\({}_{710}\) + R\({}_{670}\)) \\ Simple Ratio Vegetation Index, related to salinity [37] & SRVI = (R\({}_{830}\)/R\({}_{660}\)) \\ Green and Indigo Ratio, related to salinity [47] & GIR = (R\({}_{436}\)/R\({}_{554}\)) \\ \hline \end{tabular}
\end{table} 
Table 2: Indexes used for comparison with Level Salinity Index (LSI) index. Figure 7: Virtual images obtained by applying the Level Salinity Index (LSI) on each pixel of the hyperspectral images of lettuce leaves, from calibration set. Pixels colored from blue to red are showing increasing levels of salinity affectation. (\(F\) = 1340.2), Green region Normalized Difference Vegetation Index (NDVI\({}_{\text{green}}\)) (\(F\) = 1160.7), Chl\({}_{\text{red edge}}\) (\(F\) = 1024.1), NDVI\({}_{\text{705}}\) (\(F\) = 874.2), Chl\({}_{\text{green}}\) (\(F\) = 737.2), NDVI\({}_{\text{far red}}\) (\(F\) = 702.8) and RPI (\(F\) = 686.4) showed consistent trends and all of them were able to distinguish the four groups of salinity treatments. Among these indexes, the three with the highest segregation ability had been specifically defined for sensing salinity effects (LSI, GIR and NDVI\({}_{\text{green}}\)). However, Simple Ratio Vegetation Index (SRVI) (\(F\) = 119.7), also for salinity detection, together with NDVI (\(F\) = 137.3), Plant Senescence Reflectance Index (PSRI) (\(F\) = 119.8) and Water Index (WI) (\(F\) = 53.4), did not give consistent trend or clear distinction between salinity classes. The scores of PC1 showed similar segregating ability as LSI (\(F\) = 2845.88).