## 4 Discussion
### Synergies of Optical, Thermal, and Canopy Height Observations for PLS-R Modeling
Further research would be needed to confirm this finding across different illumination sources and conditions. Regions around 515 nm and 700 nm have been reported to be very suitable to estimate A and indirectly g\({}_{\rm s}\) due to their ability to detect chlorophyll changes [42; 75]. Similar to our result for soybean, [PERSON] et al. (2015) [55] found maximum VIP scores between 670 and 680 nm when using PLS-R to predict GPP and NEE in pasture and rise sites under Mediterranean climate. In addition, a recent study concluded, based on PLS-R VIP analysis, that most important bands to predict wheat g\({}_{\rm s}\), T\({}_{\rm r}\), and A from hyperspectral data were mainly placed in the VIS and red-edge regions [57]. Wavelengths around 780 nm presented one of the highest VIP values in soybean g\({}_{\rm s}\), T\({}_{\rm r}\), and A, in agreement with mARI being sensitive to T\({}_{\rm r}\). Note that narrow wavebands (831 and 850 nm), often combined with water absorption bands in SWIR to obtain water and moisture indices [42], were not used in VI calculations. In Figure 9, soybean T\({}_{\rm r}\) and g\({}_{\rm s}\) VIP showed also a spike centered at the green band 540 nm, which might be partly due to artifacts caused by artificial illumination sources (Figure 4). Nonetheless, [PERSON] et al. (2015) [15] also found that this band was one of the bands contributing the most to estimate leaf water potential to detect water stress under solar irradiance conditions. In relation to artifacts caused by the lamps (Figure 4), to some extent, they should be removed when calculating reflectance by the normalization with incoming irradiance and the normalization prior to PLSR processing. Even though it is possible that some of these artifacts could be related to temporal changes in the lamp's irradiance caused by voltage oscillations, other authors have found similar effects in reflectance due to lamps irradiance spikes. [PERSON] & [PERSON] (2006) [103] investigated reflectance of healthy and water stressed pepper plants under seven artificial illumination sources and they found that high pressure sodium and metal halide lamps, with irradiance spikes similar to those in our high pressure mercury and halogen lamps, introduced spikes in the reflectance spectra. Thus, peaks in the spectral irradiance distribution of the lamps, compared to the solar irradiance, may modify mostly plants reflection while the absorbed light might not be affected in the same proportion, and, therefore, plants release the extra light. Nevertheless, these small reflectance artifacts do not seem to show an inconvenient to effectively detect plant changes in reflectance due to water stress under these type of lamps [103]. Unlike soybean, the contribution of wavelengths to maize g\({}_{\rm s}\), T\({}_{\rm r}\), and A PLS-R modeling was more spread over the whole spectra with lower VIP scores (Figure 9). Nonetheless, there were a few common regions with spikes at two red-edge wavelengths (around 700 and 760-780 nm). Note that bands around 770 nm were near to h\({}_{\rm c}\), explaining similar trend. This was not the case for 700 nm presenting negative loading in the first LV (Figure 10). These bands are closely related to the rNDVI (705 and 750 nm), which relate to structural changes to detect drought [76]. Bands at 705 nm and 700-740 nm relate to water stress while 760 nm accounts for water absorption [42]. In line with this and using PLS-R, strong relations were found between red-edge bands and V\({}_{\rm cmax}\) (correlated to A) in nine agricultural crops [58]. In Figure 9, green bands (around 520 and 560 nm) for predicting maize g\({}_{\rm s}\) and T\({}_{\rm r}\) showed important contribution, but not same information as h\({}_{\rm c}\) (Figure 10). These bands were not used in VI calculations, despite being close to PRI and its variants (Table 2). Thus, they could be potential wavelengths to predict maize g\({}_{\rm s}\) and T\({}_{\rm r}\) under water stress. NIR band (843 nm) was also important (Figure 9).