*2.1. Climate and Light Conditions*

All experiments were carried out in two climatic chambers developed in the Federal Scientific Agroengineering Center VIM, Russia (Figure 1A,B). The chambers are of the same size–2500 × 1600 × 1700 mm. Each chamber has only one level for plant cultivation. The maximum plant height is 1500 mm. The total usable area for growing was 3.8 m<sup>2</sup> . The total power consumption was 3 kW. The intensity of light was measured by a TKA-VD spectrocolorimeter (TKA Scientific Instruments, Saint-Petersburg, Russia) at 15-cm height in each climatic chamber.

**Figure 1.** Scheme of the experimental setup. Two climatic chambers equipped with LEDs (**A**) or sodium lamps (**B**) and the corresponding spectra of the lighting regimes are shown in plots (**C**–**E**).

Climatic chamber 1 (Figure 1A) is equipped with a system of lighting based on LEDs, and it has two spectral regimes of irradiation. The first regime of lighting is characterized by the visible spectrum shown in Figure 1C. It was applied at the earlier stage of plant growth (the first 5 days) and has the distribution of photosynthetic phonon flux density (PFD) over the frequency ranges as shown in Table 1. The second regime of lighting was applied after 5 days of cultivation and was intended to stimulate the reverse response of phytochromes. This is achieved by reducing the intensity in the red region of the LED spectrum (Figure 1E). The PFD distribution for this regime is shown in Table 1.

Lighting in the second climatic chamber (Figure 1B) was provided by two tubular sodium lamps (yellow color) and one metal halide lamp (white color). The averaged PFD of the chamber is 132.3 µmol m−<sup>2</sup> s −1 . The temperature settings in both climatic chambers at the first stage of cultivation were 19–20 ◦C during the day and 16–18 ◦C during the night; those of at the second stage of cultivation were 20–22 ◦C and 18–20 ◦C, respectively. The relative humidity in the chambers was maintained at 65–70% throughout the experiments. The temperature of the nutrient solution was 18–20 ◦C.

Preparation of the climatic chambers for sowing the seeds was as follows: (1) sanitization of the inner and outer surfaces of the chambers; (2) equipping the chambers with containers of mineral wool used as a substrate for the plants; (3) setting up the spectral composition of LEDs corresponding to the initial stage of cultivation; (4) regulation of the automatic watering system for the plants.

**Table 1.** Averaged biometric parameters of the aerial part of green and red lettuce (*Lactuca sativa* L.) under conditions of a regulated agroecosystem on the 15th day of cultivation. Values represent mean ± SE (*n* = 12). The different letters indicate significant differences among treatments mean non-significant differences among treatments according to Duncan's test (*p* ≤ 0.05).

