## 3 Results
### Stepwise Multiple Regression Analysis  
One index per category (the one that obtained the best relationship with yield) was selected for the stepwise multiple regression. The categories considered were: (i) temperature-derived, (ii) chlorophyll fluorescence emission-related, (iii) structural indices, (iv) chlorophyll indices, and (v) carotenoid indices. The indices chosen for this analysis were CWSI, FLD, PRI, MSAVI, and TCARI/OSAVI butthe stepwise multiple regression excluded MSAVI and TCARI/OSAVI from the analysis (Table 3), indicating that these two indices did not contribute to predict yield. This analysis was repeated for all of the indices comprised in the categories iii and iv (structural and chlorophyll indices), and the results obtained were the same (_data not shown_). These categories were therefore eliminated from the analysis and only CWSI, FLD, and PRI were included. The regression model for these three indices explained 77% of the total variability in yield and was significant (\(p<0.001\)). The RMSE yielded 259 kg ha\({}^{-1}\) (Figure 9), which corresponded to a relative RMSE of 10.2%. When NDVI and TCARI/OSAVI were included in the regression analysis, the RMSE was 276 kg ha\({}^{-1}\) (RMSE = 10.8%).  
Figure 7: Relationship between net assimilation (A; \(\mathrm{\SIUnitSymbolMicro m}^{-2}\) s\({}^{-1}\)) and: CWSI (**a**) and (**b**), FLD (**c**) and (**d**) and NDVI (**e**) and (**f**) for two selected varieties of durum wheat (solid symbols) and two varieties of bread wheat (open symbols) on DOY 76 (a), (c) and (e) and DOY 126 (b), (d) and (f). For each case, a linear regression was fitted.  
\begin{table}
\begin{tabular}{l c} \hline
**Index** & **R\({}^{2}\)** \\ \hline (i) Thermal-derived indices \\ \hline CWSI & 0.53** \\ WDI & 0.45** \\ \hline (ii) Fluorescence indices \\ \hline FLD & 0.47** \\ CUR & 0.41** \\ R690/R630 & 0.13* \\ DPi (R) & 0.03 \\ R685/R655 & 0.14* \\ \hline (iii) Structural indices \\ \hline NDVI & 0.10 \\ RDVI & 0.25** \\ OSAVI & 0.20** \\ MCARI1 & 0.17* \\ MSAVI & 0.31** \\ \hline (iv) Chlorophyll indices \\ \hline TCARI & 0.35** \\ \hline \end{tabular}
\end{table}
Table 2: Coefficient of correlation (R\({}^{2}\)) and level of significance of the thermal- and hyperspectral-derived indices related with yield.  
Figure 8: Relationship between yield (kg ha\({}^{-1}\)) and: CWSI (**a**), fluorescence (FLD) (**b**), PRI (**c**) and NDVI (**d**) for durum (solid symbols) and bread wheat (open symbols). Each point represents the average of the three replicates of each variety.  
\begin{table}
\begin{tabular}{l l l l} \hline
**Index** & \(\mathbf{R^{2}}\) \\ \hline TCARI/OSAVI & & 0.41** \\ TVI & & 0.22** \\ SIPI & & 0.17* \\ G & & 0.00 \\ ZTM & & 0.02 \\ VOG & & 0.01 \\ \hline \multicolumn{3}{l}{(v) Carotenoid indices} \\ \hline CAR & & 0.44** \\ LIC3 & & 0.36** \\ RARS & & 0.29** \\ PRI & & 0.49** \\ PRI515 & & 0.23** \\ PRIn & & 0.05 \\ \hline \multicolumn{3}{l}{* \(p<0.05\), **\(p<0.001\)} \\ \end{tabular}
\end{table}
Table 2: _Cont._  
\begin{table}
\begin{tabular}{c c c c} \hline
**Explanatory Variable** & **Beta** & \(\mathbf{t}\) & \(\mathbf{p}\) \\ \hline CWSI & \(-0.468\) & \(-4.499\) & \(0.000\) \\ FLD & 0.383 & 3.141 & \(0.003\) \\ PRI & 0.539 & 6.609 & \(0.000\) \\ MSAVI & 0.078 & 0.465 & \(0.644\) \\ TCARI/OSAVI & \(-0.335\) & \(-1.693\) & \(0.098\) \\ \hline \end{tabular}
\end{table}
Table 3: Results of the multiple regression analysis performed on the selected indicators.  
Figure 9: Comparison of measured yield and the estimation derived from the multiple regression, with CWSI, FLD, and PRI as independent variables. The adjusted regression and the calculation of the RMSE include both durum (solid symbols) and bread wheat (open symbols).