*2.2. Cultivation Conditions*

The research was conducted in a climate chamber for growing green crops. Plants were grown in plastic trays on racks equipped with periodic flood hydroponic systems (Figure 1a). Seeds were sown in cubes with a mineral wool substrate. To avoid the negative effect of far-red radiation on seed germination, plant lighting was turned off before germination in the climatic room. After the appearance of the cotyledons, the lighting was turned on again. After seedlings were found and the first true leaves appeared, three plants were left in each cube. For growing plants, seven light-insulated racks of three tiers each were used. The cultivation area for each light treatment was 0.68 m2. The planting area was 50 plants per 1 m2. The microclimate in the room was maintained by an automatic system. The day/night air temperature was 25/22 ◦C with a relative humidity of 75%. The ventilation system maintained the concentration of carbon dioxide and corresponded to atmospheric values of 400–450 ppm. Additional CO2 was not used. Nutrient solutions were prepared using Flora Series® hydroponic fertilizer complex (Terra Aquatica, Fleurance, France). The electrical conductivity of the nutrient solution was maintained within 1500–1600 μS/cm.

#### *2.3. Irradiation Conditions*

Irradiation of plants was carried out by irradiators based on white light LEDs with a colour temperature of 4000 K Samsung LM281b+ 2835 (Seoul, South Korea). The specific power consumption per one shelf of the rack 0.68 m2 was 90 W m<sup>−</sup>2. The daytime integral of light (DLI) in all variants of the experiments was ~15.6 mol m−<sup>2</sup> day−1. Measurements of the photon flux density and the spectral composition of irradiation were carried out using an MK350D Compact Spectrometer (UPRtek Corp. Miaoli County, Taiwan). The spectral composition of the radiation is presented in Figure 1b.

On the first rack, round-the-clock irradiation of plants was implemented 24 h a day. This mode was chosen because some studies on growing plants with a long photoperiod show some potential advantages of a 24-h irradiation regimen [21,22]. The PAR irradiance was ~180 μmol s−<sup>1</sup> m<sup>−</sup>2. Daily electricity consumption was 5.7 kWh.

On the second rack, impulse irradiation of plants with a frequency of 1 kHz and D = 67% was implemented since early studies showed the maximum efficiency of light use in this mode of lettuce illumination [23]. The PAR irradiance (average value) was ~180 μmol s−<sup>1</sup> m<sup>−</sup>2. Daily electricity consumption was 6.1 kWh.

**Figure 1.** Climatic chamber with plants (**a**), the spectral composition of phytoirradiators radiation (**b**), operating parameters of scanning irradiation (**c**).

On the third rack, scanning irradiation of plants was implemented with five irradiators, which were switched on in pairs according to the "running lights" principle. The work cycle was divided into five steps (Figure 1c). The duration of each step was adjusted so that DLI for each plant on the shelf was ~15.6 mol m−<sup>2</sup> day<sup>−</sup>1. The PAR irradiation at the plant level varied from ~45 μmol s−<sup>1</sup> m−<sup>2</sup> to ~285 μmol s−<sup>1</sup> m<sup>−</sup>2. Daily electricity consumption was 5.8 kWh.

The lettuce of 'Azart' and 'Lollo Rossa' varieties were irradiated 16 h a day on the fourth rack (control variant). The PAR irradiance was ~275 μmol s−<sup>1</sup> m<sup>−</sup>2. Daily electricity consumption was 6.0 kWh.

#### *2.4. Biometric Measurements of Lettuce Plants*

On the 25th and 35th days of cultivation, the fresh and dry mass of the aerial parts of plants was weighed using a Sartorius LA230S balance (Laboratory Scale, Göttingen, Germany). To obtain dry matter, the selected plants were dried to constant weight in an oven at a temperature of 105 ◦C. The leaf area was determined on an LI-COR LI-3100 AREA METER photoplanimeter (LI-COR, Inc., Lincoln, NE, USA). The nitrate concentration was measured by an ionometric method using the ion-meter "Itan" (Tom'analit, Tomsk, Russia).

#### *2.5. Spectrophotometric Measurements of the Pigment Composition of Leaves*

Quantitative pigments were analysed by extracting them from plant tissues with solvents. The optical density of the pigment extract was determined on a SPECS SSP-705 (Moscow, Russia) spectrophotometer at wavelengths 662, 644, and 440 nm using cuvettes with an absorbing layer thickness of 10 mm. The concentration of chlorophylls a, b and carotenoids was calculated using the Holm–Wettstein formula for 100% acetone [24]. The measurements were carried out on the 25th and 35th days of cultivation.

#### *2.6. Reflection Measurements in Lettuce Leaves*

Leaf reflectance spectra were measured using a portable PolyPen RP 410 UVIS system (Photon Systems Instruments, Drásov, Czech Republic). Leaf reflectance measurements were taken 35 days after the start of cultivation. The spectra of ten plants were measured under each illumination and at each cultivation period. Three spectral measurements were carried out on different leaves of each plant.

Using the PolyPen RP 410 UVIS software, the main reflectance indices were automatically calculated. In this case, we analysed, first of all, the normalized difference vegetation index (NDVI) [25], which is a widely used indicator of the photosynthetic biomass of plants, the Zarco–Tejada–Miller index (ZMI) [26] and Gitelson–Merzlyak Indices 1 and 2 (GM1 and GM2) [27], which are highly sensitive to the content of chlorophylls, carotenoid reflectance indices 1 and 2 (CRI1 and CRI2) [28], which are sensitive to the content of carotenoids, and anthocyanin reflectance index (ARI) [29], which is sensitive to the content of anthocyanins.
