## 3 Results and Discussion
The type of aggregate seems not to influence the response variable C, consistent with Table 8. The different behavior of C in LED-1 and LED-2 could be explained by considering the positive synergy among the Aggregates Number and the LED variables, which act differently according to the light recipe used. In fact, it seems that this synergy is greater for LED-2 than for LED-1. The response variable SR (Figure 5b) shows a behavior similar to that of C, since its value remains almost constant at near 7.9% with the LED-2 light recipe but decreases in a non-linear way with LED-1, from 9.5% to 6.8%. This behavior could be explained by the use of green and far-red wavelengths in the LED-2 light recipe. Green light penetrates the deeper strata of the plant canopy and allows the whole structure of the plant to perform the photosynthesis, promoting the fixation of CO\({}_{2}\) on the abaxial side of leaves better than blue and red wavelengths [29; 31], while red and blue are intercepted by the adaxial side of leaves and the upper canopy, which is saturated [53]. The role of the green wavelength is also recognized in the increase of dried mass in lettuce plants, when green is coupled with other wavelengths and added in a limited amount [54]. Also, the far-red wavelength has a role in helping the development of the dry mass of plants  
Figure 6: Examples of plant morphology occurring with (**a**) 0 aggregates LED-1, (**b**) 6 aggregates LED-1, (**c**) 0 aggregates LED-2, and (**d**) 6 aggregates LED-2. The black reference bar corresponds to 10 mm. when coupled with other wavelengths [55], even if at a low percentage [25]. This would be consistent with the relatively greater values of C and SR found in plants when LED-2 is used, and the number of aggregates is higher. Therefore, the model suggests that using the maximum amount of fertilizer, equal to six aggregates, is more beneficial with the LED-2 light recipe. This is because the use of other wavelengths like green or far-red enhances the CO\({}_{2}\) uptake and biomass development in lettuce when the plants grow in a substrate enriched with nutrients. These nutrients promote the morphologic properties of the lettuce (Figures 4 and 6b,d), allowing the plants to develop a deeper canopy, which hinders the penetration of red and blue light in the lower leaves or in the leaves' abaxial side. On the contrary, when no fertilizer is used, the morphologic development of lettuce is lower, as shown again in Figure 4 and also in Figure 6a,c, and the use of only red and blue is enough to saturate the whole canopy of lettuce plants, since there is no deeper strata. A possible way to enhance the action of green and far-red light could be to add far-red at the end of the 16 h photoperiod, as suggested in [55], to avoid the reduction of the leaves' absorbance of the green spectrum. However, C and SR have higher values when the Aggregates Number is lower and the LED-1 light recipe is used (Figure 5a,b), suggesting that lettuce plants may develop better with only blue and red wavelengths when less nutrients are available. A possible explanation for this phenomenon is that, when plants show lower FW and LA, the canopy is less developed and deep, and the role of green and far-red wavelengths is less important. As pointed out in [56], VF systems have a high potential for the food industry but still show several uncertainties. The main problems pointed out are the choice of lighting, nutrient delivery, irrigation, and climate control systems. The system studied in this work tried to help face some of these challenges. Regarding lighting, this study highlighted the following benefits: the suitability of the two light recipes for the growth of lettuce and the possibility of reducing the plant canopy-LED distance, reducing the volume needed for every single plant. The high scalability of the system can facilitate the creation of bigger facilities.