## 4 Discussion  
### Physiological, Biochemical, Morphological, and Remote Sensing Responses under Water Stress  
Changes in leaf physiology are the result of plant-level regulation, and therefore changes in g\({}_{\mathrm{s}}\), T\({}_{\mathrm{r}}\), and A associated with soil drought have to be interpreted by looking at transpiration responses of the overall canopy. After 4-7 DoM, leaves were practically covering the full soil surface, especially in soybean plants. Thus, soil evaporation could be neglected, assuming canopy transpiration similar to ET. Our data showed that for a 60% soil water reduction, both crops experienced a significant reduction about the same order of magnitude in average canopy transpiration (around 23% for maize and 35% for soybean) (Figure 3). However, maize and soybean plants presented different responses in T\({}_{\mathrm{r}}\) and g\({}_{\mathrm{s}}\) at the leaf level. For soybean, a proportional decline of leaf g\({}_{\mathrm{s}}\) (\(\approx\)34%) and leaf T\({}_{\mathrm{r}}\) (\(\approx\)24%) was observed after the reduction of 35% ET (Figure 3). This suggests that the strategy of soybean plants to cope with drought is through tight stomatal control, also shown by the significant rise of leaf vapor pressure deficit (VPD\({}_{\mathrm{leaf}}\)\(\approx\) 13%) (Figure S1) and T\({}_{\mathrm{L,Rad}}\) (\(\approx\)3%) (Figure 3). From this study, we found that T\({}_{\mathrm{L,Rad}}\) is a key remote sensing index to track drought responses in soybean plants, as it captures the warming effect from less cooling from lower T\({}_{\mathrm{r}}\) and g\({}_{\mathrm{s}}\) (Figure 3). Furthermore, \(\Delta\)T was the only index significantly correlated with soybean g\({}_{\mathrm{s}}\) and T\({}_{\mathrm{r}}\) (Figure 6). Similar response in g\({}_{\mathrm{s}}\) of soybean plants has been also shown by others [17; 19; 86]. High stomatal control could be associated to soybean plants not acclimating well to soil water stress due to lack of adjustment in leaf hydraulic conductance [87]. Thus, to avoid hydraulic failure, it tends to close stomata. As in soybean plants, most of the reduction in ET can be linked to reductions in leaf T\({}_{\mathrm{r}}\), water stress impacts on growth (i.e., LAI decreases) did not seem to play a big role (Table 3). The main support for this argument relies on LAI measurements at the end of the season with not significant differences between wet and dry soil water levels (Table 3 and Figure S5). In addition, remote sensing indices traditionally associated with LAI such as NDVI [42; 53; 88], OSAVI [37; 39; 85], and REIP [75; 88], were not significantly affected by changes in soil water (Table 4, Figure 5 and Figure S1). It is possible that, as VIs were calculated only from vegetation pixels (Figure 2), their ability to detect decrease in LAI might be reduced. However, it can be seen that h\({}_{\mathrm{c}}\), a proxy for cell enlargement and associated to growth [7; 17], did not suffer significant changes due to drought (Table 3). Because VIs were obtained from average pixels of top vegetation cover from a high resolution camera (10.5 mm/px), we assumed that most of the detected reflectance by the sensor was coming from top of canopy leaves and it is not considered as a composite plant-canopy reflectance. However, some of the pixels might incorporate reflectance of several overlapping leaves (Figure 2) and leaf scattering effects might be observed on the NIR part of the spectrum [32], which can be also related with changes in leaf angle due to drought effects [10; 32; 89]. By removing soil background pixels (Figure 2), we eliminated most of the reflectivity from soil, supported by similar behavior between NDVI and OSAVI, which minimizes soil effects [70], with significant differences for soybean but not for maize (Table 3). Thus, we could infer that soil brightness effects were low. Nevertheless, for further improvements on reflectance measurements, 'bare soil' reflectance for each soil water level can provide useful information to reduce soil brightness effects [90].