**5. Conclusions**

In our studies, variations in incident light spectral quality simulated with LEDs influenced growth and development in lettuce plants in several ways via direct effects on photosynthesis and control of plant photomorphogenetic responses. PSA structure and growth activity were significantly affected in the distinct light treatments, and these changes influenced source–sink relations in plants through the assimilate demand, etc. (indirect light effects on photosynthesis).

Our studies have shown that monochromatic blue light retarded lettuce plant growth, and monochromatic red light accelerated it. In plants exposed to blue light, the assimilating leaf area growth was retarded (both source and sink simultaneously!), and even an increased photosynthesis rate could not compensate for this delay. In the combined spectrum, farred-light action was also important as far as it had triggered the shade-avoidance response and enhanced plant assimilate demand.

For the first time, it was found that the light of different PAR spectral regions affected the carbon isotope composition of leaf biomass. The strongest and most opposite in direction effects of monochromatic blue and red light were observed. Continuous blue-light treatment resulted in the <sup>12</sup>C enrichment of lettuce plant leaf biomass by about 3‰, whereas continuous red-light treatment resulted in <sup>13</sup>C enrichment of the same value. The effects of light of the other PAR spectral regions studied were considerably less significant. Daily variations in the leaf tissue carbon isotope composition were not significant.

Further research is needed to assess light-induced isotopic effects in plants and the mechanisms underlying them. These studies also could provide significant starting points for the development of the dynamic (changing in time) lighting regimes combining the advantages of the distinct spectra studied above at certain periods of plant growth. Thus, plant acclimation and photosynthetic improvements in response to added far-red and green-light wavelengths to the main red–blue spectrum have already been studied along with the changing red-to-blue-light ratio [59].

It is known that photorespiration can serve as an energy sink preventing the overreduction in the photosynthetic electron transport chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic CO<sup>2</sup> assimilation and provides metabolites for other metabolic processes, e.g., glycine for the synthesis of glutathione, which is also involved in stress protection [60–62]. Therefore, another area of interest could be studies on plant stress responses and stress tolerance mechanisms including lightinduced stress, e.g., extremely high PPFD or abnormal spectral environment adaptation.

**Author Contributions:** Conceptualization, I.G.T. and A.A.I.; methodology, I.G.T., V.A.L. and A.N.S.; software, N.N.S.; validation, I.G.T.; formal analysis, I.G.T.; investigation, I.G.T., D.A.T., M.P.L., A.S.S., A.N.S. and V.A.L.; resources, M.P.L.; data curation, I.G.T.; writing—original draft preparation, I.G.T. and A.A.I.; writing—review and editing, I.G.T. and A.A.I.; visualization, D.A.T.; supervision, I.G.T.; project administration, I.G.T.; funding acquisition, I.G.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was conducted with the support of the Ministry of Science and Higher Education of the Russian Federation in accordance with agreement № 075-15-2020-905, 16 November 2020, and a grant in the form of subsidies from the federal budget of the Russian Federation. The grant was provided for state support for the creation and development of a world-class scientific center: "Agrotechnologies for the Future".

**Data Availability Statement:** Data sharing is not applicable to this article.

**Acknowledgments:** We would like to thank Ivan S. Chuksin for his technical assistance in setting up plant sampling during carbon isotope determination.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
