## 4 Discussion
Furthermore, associated with anthocyanin biosynthesis, other chromophores, absorbing at wavelengths lower than 440 nm, are frequently accumulated in the leaf cell vacuoles, as is the case of UV-absorbing flavonoids and other phenolics [48], possibly contributing to the high frequencies observed in the vicinity of 400 nm in Figure 7. Indeed, we also found significant differences on the absorption spectrum of anthocyanins around 400 nm, but in this case a decrease in absorption was found at S2, when compared to S0 (Figure 2d). Additionally, the carotenoid content is known to change in drought stressed plants, in particular the content of violaxanthin, antheraxanthin and zexaanthin, the three xanthophylls involved in the xanthophyll cycle [49]. Our results show some decrease of carotenoids content between S0 and S2 (Figure 2c). In vitro, carotenoids (xanthophylls and caroten) show three distinctive absorption peaks, ranging from 416/440/465 nm for violaxanthin to 426/449/476 nm for \(\beta\)-carotene [50; 51]. Therefore, in vivo, carotenoids and chlorophylls have overlapping absorption bands in the blue range, making it difficult to distinguish between them. However, there is an absorption peak at 520 nm which can be attributed to carotenoids [52]. However, only on XGBoost this wavelength showed a significant discriminant power, suggesting that alterations in carotenoids composition played a minor role in our results. Actually, we did not find such 520 nm absorption peak in our results, albeit the shoulder of the chlorophylls Soret band observed in the total photosynthetic pigments extract at 470 nm might be attributed to the presence of carotenoids (Figure 2d). By using a non-invasive methodology, our study shows that leaf spectral readings are good indicators of soil water content in the plant model _A. thaliana_. We are aware that the translation of knowledge from _Arabidopsis_ to agricultural species may bring new challenges, as always happens when knowledge is translated from model organisms to commercially valued organisms. Nevertheless, we produced discriminative models with a good potential to detect naturally occurring drought stress, and based on the results obtained on our data set, RF was the best performing algorithm. We also identified characteristics that are important for stress phenotyping in plants, which can be applied before crop growth cycles are affected, contributing to a more cost-effective management of plant breeding. **Author Contributions:** Conceptualization, J.M.d.S. ; methodology, A.B., P.M.P.C., S.S., M.C.P., A.R.M. and A.B.d.S. ; software, P.M. and A.B. ; formal analysis, A.B. ; investigation, P.M.P.C. ; writing--original draft preparation, J.M.d.S., S.S., M.C.P. and A.B. ; writing--review and editing, J.M.d.S., P.M.P.C., S.S., A.B. and A.R.M. ; visualization, A.B. ; supervision, J.M.d.S. and S.S.; project administration, J.M.d.S. ; funding acquisition, J.M.d.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by FCT research units BioISI (UID/MULTI/04046/2019) and LASIGE (UIDB/00408/2020 and UIDP/00408/2020), and R&D project INTERPHENO (PTDC/ASPPLA/28726/2017). **Acknowledgments:** We acknowledge the support of [PERSON] on the experimental work. **Conflicts of Interest:** The authors declare no conflict of interest.