## 4 Discussion
### Temporal Changes in Foliar Nutrient Contents Lead to Two Distinct Nutrient Groupings
Earlier research on tomato plants has also shown that too low or high concentrations of Mn in leaves decrease leaf Cab concentrations [73]. Overall, these studies support the negative correlations found here between Group 2 nutrients and Cab (Figure 7), which again would support the indirect role of Cab, in addition to leaf morphology, in mediating the correlation between Group 2 nutrients and our spectral indices. Foliar Pigment Contents and Leaf Morphology Mediating the Leaf-Level Relationship between Nutrients and Spectral Indices  
Nutrients from Group 1 were found to be negatively correlated with leaf level F685, Fratio, red edge reflectance, and CCI (Figure 7), while MTCI was positively correlated with the same macronutrients. Opposite patterns, albeit much weaker, were observed between these indices and Group 2 nutrients, which also presented weaker correlations with Cab. Likewise, nutrients that did not correlate with Cab, such as Ca, Fe, and Cd, presented the weakest correlations with spectral indices at the leaf level, further emphasizing the role of Cab in mediating these leaf-level correlations. The NDVI was poorly related to foliar nutrient content dynamics, which is consistent with previously reported limitations of the NDVI to track foliar N contents due to signal saturation at moderately low foliar Cab contents [74; 75]. Unlike the NDVI, the MTCI, which has been formulated to track Cab contents [52], was positively correlated with foliar Cab and therefore directly related to the nutrients from Group 1 (Figure 7). In turn, the negative correlation between Cab and red edge reflectance is consistent with the effect of increased Cab amounts shifting the red edge feature towards higher wavelengths [76], thus decreasing the reflectance in the measured red edge region (Table 3), making it less susceptible to Cab saturation compared to the red reflectance bands [77]. Leaf-level reflectance indices using the red edge reflectance region have been previously shown to correlate well with foliar N [78] and Cab [76] amounts in potato plants or with foliar Cab and Mg contents in different wheat genotypes [79], which is in line with our results that expand these observations to other macro and micronutrients. In turn, the negative correlation between leaf-level F685 and Fratio, and nutrients from Group 1 and Cab (and the opposite for Group 2 nutrients) (Figure 7) could be explained in terms of the re-absorption of red fluorescence photons by chlorophyll inside the leaf [36]. Specifically, at low Cab contents, where reabsorption is minimal, a marginal increase in Cab leads to an increase in PAR absorption and therefore an increase in fluorescence across all wavelengths (F685 and F760). In contrast, at higher Cab contents, like the ones observed in our leaves (Figure 5), a marginal increase in Cab has a very small effect on PAR absorption due to self-shading inside the leaf but can further increase the reabsorption of fluorescence in the red fluorescence bands that overlap with the chlorophyll absorption spectrum [28; 36; 37]. This phenomenon would explain the poor correlation between F760 and Cab (and, by extension, nutrients) observed in our study (Figure 7), as well as the negative correlation between F685 or the Fratio and Cab and their subsequent relationship with the nutrient groups. [PERSON] et al. [25] recently reported how steady state PAM fluorescence (Ft), integrated from 697-750 nm as measured with a FluorPen FP110 (PSI, Brno, Czech Republic), was strongly correlated to foliar N contents. This contrasting observation could be due to the lower foliar Cab contents in their almond experiment relative to our potato leaves, where far-red fluorescence would be less saturated, or to the different spectral coverage between studies. Foliar N has also been known to be detectable by red fluorescence and the red to far-red ratio, for example, in wheat [43] and turf grasses [80]. However, as [PERSON] et al.