*2.7. Measurement of Chlorophyll Fluorescence Parameters*

To measure the activity of the light stage of photosynthesis, a portable fluorimeter FluorPen FP 110/S (Photon Systems Instruments, Drásov, Czech Republic) was used to detect active chlorophyll fluorescence and its further analysis using the PAM method or the OJIP test. FluorPen FP 110/S includes a detector (PIN photodiode with a narrow band filter, working optical range from 667 to 750 nm) and a blue LED emitter (maximum about 455 nm), an ambient light sensor. To assess Fv/Fm, the leaf was preliminarily dark-adapted for at least 20 min [30].

#### *2.8. Raman Spectroscopy of Lettuce Leaves*

Analysis of the Raman spectra (RS) was carried out on the 25th day of cultivation, on undisturbed samples of living tissues of lettuce plants. The measurements were carried out on a Senterra II confocal Raman microscope (Bruker, San Jose, CA, USA) using a red (785 nm) laser. Additional processing of the samples was not carried out, the accumulation time and the number of additions did not change, and corrective substrates (SERS) were not used in the measurements. The resolution was 4 cm<sup>−</sup>1. The measurements were carried out in quadruple repetition.

#### *2.9. Statistical Data Processing*

All experiments were carried out threefold. Statistical processing of measurement results and plotting were performed in Python 3.9 and MS Excel. To estimate the statistical significance by the considered parameters the Independent two-sample *T*-test at *p*< 0.05 significant levels.

#### **3. Results and Discussion**

#### *3.1. Morphological Parameters of Lettuce Plants*

According to biometric indicators, the studied lettuce varieties had different responses to irradiation regimes (Table 1). The indicator of the accumulation of a fresh mass of plants for the semi-head 'Azart' variety had the highest value with continuous illumination for 24 h, and for the 'Lollo Rossa' leaf variety—when using impulse irradiation; at the same time, the differences between these lighting options remained unreliable. The obtained results are consistent with similar studies carried out on red leaf lettuce (*Lactuca sativa* L. cv. 'Sunmang'), the biomass of which did not differ significantly from the control with continuous illumination at a duty cycle of 1 kHz (D = 75%) [2]. In general, the fresh mass of both lettuce cultivars was somewhat less under scanning irradiation, as was the case in studies conducted on rose and bluebells plants [18,19].


**Table 1.** Morphological parameters of lettuce plant of 'Azart' and 'Lollo Rossa' varieties on the 35th day of cultivation. Values represent mean SEM (n = 10). Letters indicate significant differences among treatment and control samples (*p* < 0.05).

According to the accumulation of dry mass, both varieties showed a similar reaction depending on the lighting regime. In the control variant (16/8) and the variant with the use of scanning irradiation, the ratio of dry to fresh mass did not have statistically significant differences, however, in the variant with continuous illumination for 24 h and using the impulse irradiation mode, the accumulation of dry matter of the 'Azart' variety was significantly increased by 23% and 28%, respectively, and for 'Lollo Rossa' the increase in dry weight was not statistically significant. The studied lighting regimes had a different effect on the development of the leaf apparatus of plants of the 'Lollo Rossa' variety and did not have a significant effect on changes in the area of the leaf surface of the 'Azart' variety. The greatest increase in the leaf surface area of the 'Lollo Rossa' variety (13.5%) was observed in the variant with impulse irradiation, while scanning irradiation significantly reduced this indicator compared to the control variant of illumination (by 21%). The head lettuce variety 'Azart' in the variant with the scanning mode of lighting was significantly extended, while the height of the stem increased by 1.8 times and the rosettes of the leaves were non-compact, the plants acquired an unmarketable appearance. Studies of scanning irradiation modes on flower crops showed a different effect: when growing a rose, the height of the stem remained unchanged [18], and when growing a bell, it even significantly decreased [19]. This difference is due to the fact that for these flower crops, scanning irradiation was used as an addition to the main one, and in our case, it was the main one. A significant elongation of the stem of the 'Lollo Rossa' lettuce variety was observed in the variants with impulse and constant 24-h irradiation by 1.8 and 2 times, respectively.
