## 4 Discussion
### Temporal Changes in Foliar Nutrient Contents Lead to Two Distinct Nutrient Groupings  
Most of the observed variation in foliar nutrient contents during our study was caused by the temporal variation between our early and late measurements. We found that top canopy foliar contents of macronutrients N, P, K, and Mg tended to decrease over time in response to the combined effects of canopy development, fertilization, and water stress treatments (Figure 4). When considering all the data together, these macronutrients (Group 1) were positively correlated to foliar Cab (Figures 7 and A15). In turn, the contents of micronutrients Cu, Mn, and Zn, as well as macronutrient S (Group 2), tended to increase over time (Figure 4) and were inversely related to Cab (Figures 7 and A15). The patterns of variation in Group 1 and Group 2 nutrients, as well as their covariation with leaf level Cab and canopy level FVC, emphasized the observed relationship between spectral indices and foliar nutrient contents at the leaf and canopy scales, which we discuss in the following sections. It is worthwhile noting the special increase in foliar nutrients observed in the water-stressed I\(-\) treatment, where in addition to Group 2 nutrients, Fe, Ca and Cd also accumulated in the top canopy leaves. These patterns could reflect the impact of the water stress in the I\(-\) plots, which, combined with the high temperatures during our experimental period (Figure 1B), would have interfered with overall canopy development, promoting the accumulation of nutrients in the new leaves at the top of the canopy. In fact, low canopy leaves in I\(-\) started to senesce a few days after our measurements, which would be compatible with a relocation of nutrients to top canopy (younger) foliage under drought stress [62]. The observed increase in SLA in I\(-\) (Figure 5) as well as the decreasing trend in FVC observed in the I\(-\) treatment compared to other treatments (Figure 6) suggest that canopy development in I\(-\) treatment was water-limited. The contrasting temporal dynamics observed between the macronutrient-dominated (Group 1) and Group 2 nutrients could be partly explained by the increased biomass of the plants. This increase leads to a so-called dilution phenomenon [63] caused by an increasing demand for macronutrients in e.g., cell-wall formation, which decreases macronutrient concentrations in the leaves [64] and has also been noted in earlier research on potato plants for N, P, and K [65; 63]. In micronutrients, there was a noticeable pattern of accumulation on the top leaves during the measurement period, with the exception of Fe. This could in part be caused by the relatively high background amount of micronutrients in our experimental fields [66]. The photosynthetic role of Group 1 nutrients K, Mg, N, and P is well documented across species [67; 68; 69], being essential constituents of proteins and chlorophyll (N, Mg), energy transfer and ATP synthesis (P), as well as stomatal movement and chloroplast structure (K). Notably, we found a highly consistent and positive relationship between N, Mg, and Cab throughout the study (Figure A15) indicating that the allocation of these nutrients into chlorophyll synthesis remained stable over time. Positive, albeit less consistent, correlations were also found between Cab, P, and K. It is therefore reasonable to state that Cab dynamics played a central role in mediating the leaf level correlations between spectral indices and Group 1 nutrients, either directly, as in N and Mg, or indirectly. In terms of Group 2 nutrients, previous research by [PERSON] et al. [70] showed that increasing leaf Cu in bean plants (_Vicia faba_ L.) leads to a lowered Cab concentration in leaves. Additionally, it has been demonstrated that higher S levels in wheat have a negative effect on leaf Cab contents [71] and that Zn deficiency decreases leaf Cab amounts in pecan (_Carya illinoensis_) [72] by limiting C fixation and protein synthesis.