## 3 Results and Discussion
### NIRS Calibration and Validation of PLS Models
#### 3.1.2 Sweet Potato  
The sweet potato reference values of the first trial, for both control and water scarcity conditions, were chemically determined (Table S3). On average, drought determined the decrease in the tuber and shoot Pt, N, CaOx, \(\Delta^{13}\)C and \(\delta^{15}\)N content. However, this decrease was significant (\(p\leq 0.05\)) for Pt-shoot, N-shoot, and for \(\Delta^{13}\)C and \(\delta^{15}\)N in both organs. Contrariwise, \(\delta^{13}\)C and TC increased slightly with drought in both organs. The \(\delta^{13}\)C increase under stress conditions was significantly different (\(p\leq 0.05\)) from the control. However, M and St showed an opposite variation between the sweet potato organs. Still, from these major constituents, St is the most abundant constituent produced in tubers, followed by the Pt and M. Instead, the shoots show Pt as the main abundant constituent, followed by M and St. The St-tuber and Pt-shoot are chemotypes because they are the constituents with the highest concentration in sweet potato organs [34]. St-tuber showed an average range of 43.39 and 42.33 g/100 g DW, while Pt-shoot ranged significantly (\(p\leq 0.05\)) between 16.36 and 13.04 g/100 g DW when under stress. The overall Pearson correlations displayed 62 significant correlations in the first trial, among the quality constituents of sweet potato's tuber and shoot organs, of which 29 were highly correlated (\(|\text{r}|\geq 0.50\)) (Table S4). For example, the highest significant correlations were registered for \(\delta^{13}\)C-tuber with \(\Delta^{13}\)C-shoot (\(-0.93\)) and \(\delta^{13}\)C-shoot (\(-0.88\)), and for M-shoot with Pt-shoot (0.89) and St-shoot (\(-0.79\)). Still, Pearson correlations performed according to the acc. 's locality show differences (Table S4) due to different genotypic responses to drought. The chemotype Pt-shoot had greater correlations than St-tuber chemotype, with the St-tuber not showing significant correlation among the acc. of Canary Islands and Guinea-Bissau. Instead, the ones from Madeira Island had significant and negative correlation for St-tuber with M-tuber (\(-0.76\)), TC-shoot (\(-0.59\)), and St-shoot (\(-0.48\)), and positive to M-shoot (0.58). On the other hand, the Pt-shoot had greater significate correlations in those three locations, among them with M-shoot for Canary Islands (0.94), Guinea-Bissau (0.86), and Madeira Island (0.82). The creation of PLS equations from both the calibration models and chemical analysis (used as a reference method) from the sweet potato flour organs was performed through NIRS spectra collected from tubers (Figure 3A) and shoots (Figure 3B) calibration sets. NIRS spectra of the validation sets were also obtained for the tubers (Figure 3C) and shoots (Figure 3D) independent samples from the first trial. Overall, NIR spectra peaks for both sweet potato plant organs lead mainly to starch (1460, 1930, 1966, and 2286 nm) and proteins (2176, 2296, and 2466 nm). These peaks result from overlapping absorption by mostly overtones and combinations of vibrational modes comprising C-H, O-H, and N-H stretching-bending vibrations [10,18]. Good fitting prediction models amongst the quality constituents were recorded in this study, with low SEC and SECV, and with high \(r^{2}_{\text{cv}}\), \(r^{2}_{\text{pred}}\), and RPD values (Table 3), which agrees with previous studies [10,16,18,51]. The TC-tuber and \(\delta^{15}\)N-tuber models were the exceptions, showing the lowest prediction performance through the smallest \(r^{2}_{\text{cv}}\) (0.687, 0.727, respectively), \(r^{2}_{\text{pred}}\) (0.705, 0.769), and RPD (1.79, 1.92) values. The \(\delta^{13}\)C-tuber and \(\delta^{13}\)C-shoot models had the lowest discrimination in the prediction of the independent samples, showing only 17.8% and 57.9% prediction confidence, with \(r^{2}_{\text{pred}}\) of 0.178 and 0.579, respectively.