## 2 Materials and Methods
### Lighting Conditions and Lighting Recipes  
Lettuce plants were isolated from external light sources. The only light source was a PHYTOFY(r) RL tunable LED panel (OSRAM GmbH, Munich, Germany), designed for indoor horticulture applications. The pots were placed under the LED module at a constant distance of 55 cm and were switched places every 3 days to compensate for possible inhomogeneous distribution of light intensity by the LED module. Some lighting conditions were common to all the growth tests. A constant photoperiod equal to a 16 h day\({}^{-1}\) was applied, with light supplied continuously from 00.00 to 16.00 without interruptions. The Photosynthetic Photon Flux Density (PFFD) was equal to 150 umol m\({}^{-2}\) s\({}^{-1}\), while the daily light integral (DLI) was equal to 8.64 umol m\({}^{-2}\) d\({}^{-1}\). Two different light recipes were tested for the growth of lettuce, named LED-1 and LED-2. Their spectral composition, expressed as PPFD for the different wavelengths, is shown in Table 2, and is similar to the information given in previous work [41]. Figures 1 and 2 give a graphic representation of the light recipes, obtained from the data of the software supplied by the producer with the LED panel. Figure 1 shows the timespans of different spectra used in LED-1 and LED-2. Figure 2 instead shows the different spectra. LED-1 is made only of one spectrum with blue and hyper-red in a 50/50 ratio (Table 3, Figure 2a). This light recipe was chosen because \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **APV50** & **APNUT** \\ \hline Bulk density (g/cm\({}^{3}\)) & 1.260 & 1.146 \\ True density (g/cm\({}^{3}\)) & 2.6523 & 2.6497 \\ Total porosity (\%) & 51.88 & 56.14 \\ pH & 6.60 & 7.38 \\ Electrical conductivity (dS/m) & 0.268 & 0.220 \\ Water absorption after 24 h & 22.00 & 30.73 \\ (\%) & & \\ \hline \end{tabular}
\end{table} 
Table 2: Properties of the two cores of the slow-release fertilizers [39]. it is more conventional and is more able to guarantee the stable growth of lettuce plants. In fact, the two hyper-red and blue wavelengths are often used for the indoor growth of plants [20; 21; 22; 23]. The LED-2 recipe was designed with a triple aim, namely to promote the growth of lettuce through the alternation of hyper-red and blue, but also with the aim of trying the effect of other wavelengths, and finally to promote the germination of lettuce seeds using only hyper-red and far-red wavelengths. LED-2 is made up of three different spectra (Figure 2b-d). Spectrum LED-2a (Figure 2b) was used during the first 7 days of the growth test to promote lettuce germination (Figure 1b). Hyper-red and far-red wavelengths promote germination of other lettuce cultivars [42; 43] and in LED-2a the hyper-red/far-red ratio was equal to 2 to avoid negative responses by seeds, such as stem elongation or leaf hyponasty, which are typical of the shade avoidance syndrome (SAS) [44]. In the last 21 days, the photoperiod was divided into two parts (Figure 1b). LED-2b spectrum (Figure 2c) was used during the first 8 h of the photoperiod, and spectrum LED-2c (Figure 2d) for the following 8 h. LED-2b and LED-2c spectra differ only in terms of the alternance of blue and hyper red wavelengths (Table 3). This alternance was set to promote the vegetative growth of lettuce plants after the germination phase, as suggested by [22; 23]. The choice of using far-red and green in the LED-2 recipe was determined by their additional effect, which is considered positive for the growth of lettuce when these wavelengths are added in limited percentages to blue and red wavelengths.