## 2 Materials and Methods
### Spectral Reflectance Measurements  
In order to obtain the reflectance spectra of the leaves we started with the characterization of the illumination, acquiring the source emission spectrum:  
Set-up (Figure 1):  
1. All the spectra were acquired in a dark chamber, to avoid the influence of ambient light
2. Four light sources were fixed in the four top corners of the chamber, providing uniform illumination of the sample area: Two LEDs Lexman14/100W warm white and two LEDs Derlight 6/28W Blue/Red, covering the visible and near-infrared spectral bands (400-750 nm) without significant temperature increase on the test chamber.
3. The spectra were obtained after the lamp's warming-up time.  
Before the acquisition of the spectra reflected by the plant leaves, for each plant batch, the spectrum of 2 diffuse reflectance standards was acquired. The spectrum of the dark standard (SRS-02, Labsphere Inc., North Sutton, NH, USA) was used as indicative of the noise in all the acquired spectra, such as ambient light in the chamber or signal noise. The spectrum of the white standard (SRS-099, Labsphere Inc.) was used as indicative of the lamp spectra and allowed for the normalization of the plant spectra.  
For the acquisition of the reflectance spectra, either for the diffuse reflectance standards, or for the plant leaves, the following steps were taken:  
Procedure:  
1. The lamps were turned on and allowed to stabilize (reduced warm up time for LED light sources).
2. The surface of the standard to be analyzed was positioned in the test chamber, facing the light sources.
3. The collecting tip of the optical fibre of the spectrometer was directed normal to the diffusing surface, at a fixed distance.
4. A sample of the light reflected by the standard diffusing surface was collected by the fibre in a quasi-normal geometry.
5. The raw spectrum of the collected light was processed with the software Ocean Optics SpectraSuite; with the following parameters: integration time: 500 msec; average: 1; smoothing: 5; gain level: 1. It was saved as an ASCII data set of wavelength vs. intensity.
6. After the standard spectra acquisition, one plant pot at a time was taken to the chamber, replacing this standard, in a way that the surface of the leaves, approximately flat, kept the same distance to the optical fibre tip.
7. For each plant, spectra were acquired with the following parameters: integration time: 1000 msec; average: 1; smoothing: 5; gain level: 1  
This procedure was applied to the control plants batch (S0) and to the plants with moderate (S1) and severe stress (S2) caused by withholding watering for 7 days (S1, measured at 28 DAS) and 12 days (S2, measured at 33 DAS). Plant spectra data were normalized to the white standard (SRS-099 spectra), as in (6):  
\[\mathrm{R(\lambda)=[I(\lambda)-D(\lambda)]/[W(\lambda)-D(\lambda)], \tag{6}\]  
Figure 1: Set-up for spectra acquisition: (**A**) white LED; (**B**) blue/red LED; (**C**) white diffuse standard; (**D**) fiber tip.  
where R(\(\lambda\)) is the sampled plant reflectance at each wavelength of the spectra, I(\(\lambda\)) is the intensity of the collected light reflected by the sample, for each wavelength, and D(\(\lambda\)) and W(\(\lambda\)) are the intensity of the light reflected by the dark or white standard, respectively, for each wavelength. These parameters were computed for the same acquisition conditions.