## 2 Materials and Methods  
### Experiment Design  
Experiments were conducted during May-September of 2018 in Helsinki, Finland, on the University of Helsinki Viikki Campus (60.2269\({}^{\circ}\)N, 25.0186\({}^{\circ}\)E). Potatoes of the variety 'Lady Felicia' were planted at the end of May in 5 m \(\times\) 5 m (see Figure 1) plots with a distance between plants of 30 cm (within row) and 70 cm (between row), with an overall density of 4000 kg/ha. The potato is an important food plant and was selected for this study for its fast growth, broad leaves, and suitability to the northern climate. For the nutrient treatments, plots were supplied with different combinations of two types of commercial fertilizer (Yara, Finland; see details in Appendix A, Table A1): a nitrogen-rich (N) fertilizer lacking other essential nutrients and a more general fertilizer containing all (A) essential nutrients. By doing so, we could adjust the amount of supplied nitrogen independently of the rest, allowing us to better investigate the optical expression of other, less studied nutrients. This resulted in four nutrient treatment levels: a control treatment (N2A2), which represented the typical fertilization levels supplied by potato farmers; a treatment with half the recommended amount of both nitrogen (N) and all (A) nutrients (N1A1); and two more treatments with the recommended amount of nitrogen but only half (N2A1) or only minimal amount of additional nutrients added (N2A0), resulting in the nutrient doses per treatment reported in Table 2. Measurement plots were randomized so that each experimental row contained one plot from each nutrient treatment (Figure 1). Fertilization was applied by hand on June 5 prior to the plants sprouting (June 11). On July 4 and 12, plants were sprayed with Revus-fungicide spray (Sygenta, United Kingdom) containing 250 g of mandipropamid per litre to combat 'potato blight', a serious disease affecting potato plants caused by _Phytophthora infestans_. In parallel with our nutrient treatments, we also conducted a water stress experiment by adjusting the irrigation (I) levels (I\(-\) and I\(+\) in Figure 1). Plants in the water stress experiment were supplied with the control level of nutrients (N2A2). The water stress experiment had a paired design, with each pair having an I\(+\) and an I\(-\) plot. Since the natural rainfall of the 2018 summer in Helsinki was only ca. 10 mm (Figure 1), all the nutrient treatment plots were irrigated two times per week, starting with the planting of the potato seedlings. The irrigation in the nutrient plots was done with sprinklers covering the whole nutrient treatment area. For the separate water stress experiment, the irrigated plots (I\(+\) in Figure 1) were irrigated by furrow irrigation using a hose. Overall, there were 16 plots in the nutrient treatments (with n = 4) and 10 plots in the water stress treatment (with n = 5). Different levels of nutrient and water availability were generated to study spatial differences in plant nutrient uptake as well as the interaction between water stress, leaf nutrient contents, and canopy development (Figure 2). The interaction between leaf nutrient contents and canopy development was further investigated by conducting measurements at two different stages of canopy development: early measurements on the 10-11.7. and late measurements on the 24-25.7. Leaf spectral measurements were done on the 10 and 25 of July around noon (11:00-13:00), while canopy spectral measurements were done on the 10, 11, and 25 between 9:30 and 13:00. Samples for the nutrient measurements were