###### Abstract
While susceptible to soil background effects at early growth stages [18], MTCI has been used for canopy chlorophyll [17] and N [18; 19; 20] estimation on various plant species, while other related red edge indices have been shown to correlate with canopy N status in grasslands and potato fields [21]. These approaches are complementary to studies that make use of absorption features in the short-wave infrared (SWIR) region of the spectrum that relate to N containing proteins [22; 23]. Recent technological advances in both uncrewed aerial vehicles (UAVs) and field spectroscopy offer new opportunities for optical remote sensing in precision agriculture at increased spectral, spatial, and temporal resolution [24; 25]. A range of spectral indices are \begin{table}
\begin{tabular}{c c} \hline \hline
**Nutrient** & **Function in Plant** \\ \hline N & Required for all proteins, e.g., RuBisCO, chlorophyll synthesis, electron transport \\ \hline P & Essential for cellular energy transfer and metabolism: ATP and NADP \\ \hline K & Photosynthesis (through ATP synthesis) and stomatal control \\ \hline Mg & Chlorophyll synthesis, phosphate metabolism, protein (e.g., ATP) activation \\ \hline Ca & Cell wall synthesis, acts as a messenger in nutrient and stress signaling in plant \\ \hline S & Amino acid (cysteine and methionine) and coenzyme synthesis \\ \hline Cu & Protein synthesis (e.g., plastocyanin), nitrogen fixation \\ \hline Fe & Chlorophyll synthesis and chloroplast maintenance \\ \hline Mn & Metabolic processes (glycosylation) and nitrogen assimilation \\ \hline Zn & Regulates plant response to biotic and abiotic stress, protein synthesis \\ \hline Cd & Non-essential, hinders nutrient and water uptake \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Macro- and micronutrients targeted in this study and their primary functions in plants. available that do not only relate to plant greenness, but also capture plant photoprotective dynamics and are thus potentially sensitive to both long-term and short-term impacts of nutrient deficits. In contrast to indices based solely on plant greenness which are not sensitive to rapid physiological adjustments in plants, the emission of chlorophyll fluorescence (ChIF), measurable at the canopy level in terms of solar induced fluorescence, SIF [26; 27; 28], is directly and instantaneously related to both photosynthetic light absorption and photoprotection [29]. Likewise, the photoprotection-sensitive and carotenoid-dependent indices photochemical reflectance index (PRI) [30] and chlorophyll/carotenoid index (CCI) [31] (which in this study are referred to as photoprotection indices), could also provide new opportunities to capture the optical expression of plant stress (e.g., nutrient deficiency) across scales [32]. ChIF is electromagnetic radiation emitted by chlorophyll molecules when exposed to photosynthetically active radiation (PAR). The intensity of the ChIF signal is dependent on the fraction of absorbed PAR (fAPAR) absorbed by the leaf and related to its chlorophyll content, as well as the fluorescence quantum yield, which is connected to the regulation of the photosynthetic machinery [27; 29; 33]. ChIF is emitted from the leaf between 650 nm and 850 nm, and it has two peaks, at around 685 nm (the red peak) and 740 nm (far-red peak). In turn, the shape of the ChIF spectra has been shown to be strongly dependent on the leaf chlorophyll (Cab) content, which mediates the predominant reabsorption of red ChlF relative to far-red ChlF photons both at the leaf and canopy scales [34; 35; 36; 37], affecting the ratio of the red to far-red peaks. At the canopy scale, the SIF signal is influenced by both scattering and re-absorption from the canopy [37]. Since ChIF is one of the three alternative pathways for the utilization of absorbed excitation energy in the leaf, ChIF carries information on the dynamics of the other two pathways: photochemistry leading to photosynthesis and non-photochemical quenching (NPQ) related to photoprotection.