## 4 Discussion
According to the literature, rice plants experiment with a high vegetative growth rate at MT, and a low visible reflectance would indicate a high rate (i.e., chlorophyll content is increasing) [93], but the correlations with yield were weak. Interestingly, we could assume that since tillering to harvest, more variables were affecting the yield besides the chlorophyll content. The importance of visible bands in the other growth stages was less noticeable, suggesting that the crop greenness had a low implication on the yield at PI and GF. The different senses of correlations for the spectral bands B06, B07, B08, and B8A can also be discussed based on the leaf optical properties. The NIR reflectance has no direct correlation with the pigment content; all the radiation is reflected or transmitted by the cell structures of the leaves, increasing with the amount of canopy [94]. The highest amount of canopy indicates high values of vegetative biomass, and this biomass can quickly provide more fertile tillers, the number of grains, and weight if the carbohydrate partitioning is correctly maintained [95]. The relation between NIR reflectance and the canopy amount reported by the literature would explain a positive correlation with the yield variability [45]. The highest correlation was obtained at PI when the vegetative growth ended, and the amount of canopy reached saturation. This correlation was lower at GF; this inconsistency could be associated with higher variability in the sub-growth stage sub-growth panicle presence. That means, at PI, the plants are very uniform in the vegetative canopy; however, at GF, the panicles are visible, and the canopy loses its uniformity, coexisting vegetative with reproductive structures. Moreover, from flowering to harvest, the subgroupt stages within the field can be more variable than in the vegetative period. At this point, the NIR radiation reflected by a canopy with panicles would be different from that without panicles, with differences in the panicle ripening. The panicles cover the leaves, and there is no clear evidence in the literature of how this can modify the reflectance. The last significant region in the results was the SWIR; those spectral bands are correlated with the water content, decreasing the reflectance with a water content increase [96]. A high-water content would indicate a non-stressed plant; thus, when those bands or the LSWI are used, the goal is to find a negative correlation with the yield in the spectral bands and a positive correlation in the index [97]. However, the correlation in the spectral bands was positive at MT, with remarkable collinearity with the NIR region and closer to zero in the other growth stages. LSWI exhibited the worst correlation among all the indices. Given those results, SWIR would not be a suitable region to study rice yield variability. The water sheet maintained on the fields could alter all the optical properties; thus, a low number of plants at MT would allow the satellite to measure the field water better, showing low reflectance values. For example, the values would be higher when the canopy is superior and covers the soil surface. The inundated conditions could explain the low correlation between NDVI and LSWI [98]. B08 and B11 were positively correlated, losing the opposite sense of correlation that NDVI exhibits between B08 and B04. To summarize the collinearity results, visible bands and B05 were highly correlated at the three stages; similarly, the NIR B08 band and B06, B07, and B8A were also highly correlated. Those results showed the dependence between spectral regions (visible, NIR, and SWIR), with the Red edge highly correlated with visible bands or NIR. The improvement given by the Red edge would be negligible, according to our study. This result is controversial with the new studies focused on exploiting the Red edge region, but, as far as our concern, those studies are more focused on other chemical and biological properties rather than the yield [99; 100].