## 4 Discussion
### Impact of Canopy Structure on the Capacity of Spectral Indices to Track Foliar Nutrient Contents
In terms of ChIP, canopy FY685 (C) and Fratio presented inverted patterns of correlation with foliar nutrients relative to leaf scale measurements, whereas canopy FY760 (C) remained largely uncorrelated to foliar nutrient contents. The strong relationship between FY760 (C) and FVC across the dataset (Figures 7, A12 and A13) points to the importance of canopy structure and cover fraction in determining chlorophyll fluorescence variation at the canopy scale. As with leaf scale, canopy ChlF is also driven by APAR (modulating both FY685 (C) and FY760 (C)) and reabsorption (influencing FY685 (C)). However, in contrast to the leaf level, where these mechanisms are mediated by foliar Cab amounts, at the canopy scale they are additionally mediated by the amount, arrangement, and dynamics of leaves and their angles [28; 37; 88], here pooled together into a fractional vegetation cover (FVC) parameter, which influences canopy APAR and by extension, ChlF emission. The fact that FY760 (C) was positively correlated with FVC across the study would indicate that the effect of LAI on canopy APAR was not yet saturated. In other words, increasing the number of leaves would result in higher canopy absorption and FY760 (C). Increasing FVC did also increase reabsorption of red fluorescence photons as seen by the negative correlation between FVC and Fratio (Figures 7, A12 and A13). Yet, the fact that red fluorescence remained positively correlated to FVC, (albeit much less than FY760 (C)) would suggest that the APAR effect dominated the variation in FY685 (C) over the reabsorption effect at the canopy. This is in contrast to the leaf level where the APAR effect of Cab had saturated, and reabsorption mediated the connection between ChlF and Cab. These contrasting controls could explain why the patterns of correlation between FY685 (C) and Fratio had reversed at the canopy scale, where FVC (which tended to increase over time), instead of Cab (which tended to decrease over time), was driving the variation in ChlF at this scale. Interestingly, the fact that FY760 (C) did not relate to foliar nutrient contents despite being strongly correlated to FVC further emphasizes the decoupling between FVC dynamics and foliar nutrient contents observed in this study. Likewise, being sensible to both APAR and reabsorption effects, F685 and Fratio would appear to have enhanced capacity to track foliar nutrient dynamics. In a recent study, [PERSON] et al. [25] reported positive and significant correlations between foliar N and far-red SIF in a multiyear experiment in an almond orchard. Again, one possible explanation for the contrasting results between this study and that of [PERSON] et al., is that in addition to differing Cab levels, also the nitrogen concentrations in their leaves were about half compared to those in the present study. Besides, major differences in canopy architecture between an almond and potato crop could have also contributed to these contrasting results due to their impact on SIF [28; 88]. As for the greenness indices above, the capacity of FY760 (C) to detect foliar nutrient contents would have most likely increased should we have estimated canopy level nutrients, factoring in the temporal and spatial variation in LAI. In fact, [PERSON] et al. [44] have found a positive relationship between far-red fluorescence and canopy N contents in wheat when upscaling the leaf N measurements to the canopy using destructive leaf area measurements. In line with our hypothesis, canopy scale spectral measurements of PRI had strong positive and negative correlations with nutrient Groups 1 and 2 respectively (Figures 7 and A1-A11 in Appendix B), while canopy level CCI was negatively correlated with Fe, which is in line with other spectral indices that are positively correlated with FVC amounts. In addition, PRI was weakly positively correlated with Cab, pointing to our results confirming earlier research by [PERSON] et al. [89].