*3.3. Influence of Different Artificial Light Conditions on Induction and Growth of In Vitro Jewel Sweet Potato Shoots*

In this experiment, we studied the effect of the spectral composition of the light (red and blue spectrum) on the in vitro shoots' growth of jewel sweet potato. The main research results are shown in Table 2. Studies have shown that the addition of red and blue spectra in different proportions to normal illumination did not lead to an increase in the growth of cultivated explants. As a rule, the specific growth rate of the main shoot from axillary buds was about 2–2.5 times less than in the control variant.


**Table 2.** Influence of the ratio of red and blue spectra on growth of in vitro jewel sweet potato shoots after 45 days of culture.

<sup>1</sup> Mean ± standard error (SE), means followed by the same letter are not significantly different at *<sup>p</sup>* ≤ 0.05 according to Fisher's least significant difference (LSD) test.

According to the obtained results, studied treatments had an ambiguous influence on the growth indices of jewel sweet potato shoots. Within the nutrient medium containing MS salts, the studied indicators were less or equal to the control variant. The exception was observed at the R = 30%: B = 70% treatment. In this treatment, the mean number of roots per shoot was 5.25, which is about 1.75 times higher than in the two other treatments.

As regards cultivation of in vitro shoots on medium free of mineral salts with only water, sucrose, and agar, clearer dependences were observed. Under these conditions, the mean shoot length was maximal at the R = 70%: B = 30% treatment. At the R = 30%: B = 70% treatment, the mean number of roots was 5.67, which was nearly 2 times more than in other treatments within the used medium.

When growing in vitro shoots of jewel sweet potato on a nutrient medium without mineral compounds, an inverse relationship was observed between the mean number of roots and the proportion of red and blue spectra. There was an increase in root formation as a result of the increase in the blue spectrum proportion. Shoot growth was observed with the predominance of the red spectrum. It can be seen that, by changing the composition of light, it is possible to regulate the morphogenetic potential of jewel sweet potato.

#### **4. Discussion**

Like most European countries, the sweet potato cultivation area in the Russian Federation is limited, concentrated mainly in southern provinces. In recent years, one of the popular trends in the food industry is the manufacturing of functional and dietary food products. Only in Russia, about 1400 tons of such products are consumed annually, and most of these are imported. The practical requirement is to expand the area of material plants for this industry and sweet potato is one of them. In the present study, we selected the 'Jewel' sweet potato cultivar, which has high nutritional value and is popularly grown in many countries around the world, to study the factors affecting their in vitro propagation ability, thereby creating a premise to expand large-scale production in the direction of gradually replacing imported raw materials for the food industry.

Investigations in the field of plant cell engineering start from a well-grown sterile culture. Many publications have demonstrated that the proper selection of a sterilizing agent, its concentration, and its effect on an explant are vital parts of a study that in many ways determine the success of an experiment [48]. To obtain a sterile sweet potato culture, many authors applied 0.1% HgCl2 solution to soak the explants for 14–15 min [49–51]. However, such a long exposure may cause necrotic lesions in the young and actively growing plant tissues, leading to their premature death. Our experiment, performed in plants of different taxonomic groups, showed that the best sterile explants were obtained from the plant tissues socked in 0.1% HgCl2 solution for 5 min. Similarly, Dewir et al. [4] also obtained good surface sterilization of red-peeled sweet potato explants using 0.1% HgCl2 solution for 5 min.

The success of clonal micropropagation depends on the balanced composition of the nutrient medium, both in terms of mineral and PGR composition. Several reports on other plant species have demonstrated that different MS salt concentrations affect the growth development or regeneration values of plants, such as *Mentha spicata* L. [52], *Bacopa monnieri* L. [53], *Lophophora williamsii* Coult. [54], and *Staurogyne repens* (Nees) Kuntze [46]. In sweet potato, the MS base medium was reported to be the most suitable medium for shoot initiation, shoot proliferation, and rooting in most reports [21–39]. However, in some sweet potato cultivars, 1/2MS medium has been found to be more suitable for shoot proliferation and rooting, e.g., 'purple flesh sweet potato' [37] and 'red-peeled sweet potato' [4]. The results of our study on the sweet potato jewel cultivar also showed that the medium containing 50% MS salt was the most suitable for shoot initiation, shoot proliferation, and rooting. On the other hand, most of the reports also showed that the addition of BAP or Kn combined with IAA resulted in good shoot regeneration and rooting effects [21–23,26,37]. Similar to these reports, our results suggest that a nutrient medium supplemented with 0.5 mg/L BAP or 0.5 mg/L Kn combined with 0.5 mg/L IAA was best for shoot initiation, shoot proliferation, and rooting in sweet potato jewel cultivar.

It is known that the spectral composition of light is an important physical factor influencing morphogenetic processes. It was shown that different light spectra affect the proliferation and differentiation of plant cells in different ways. For example, violet and blue spectra increase the process of photosynthesis, which leads to the rapid formation of more powerful plants [55]. Plant photomorphogenesis depends on the intensity of the red and blue spectrum of light, as well as their ratio. It has been experimentally shown that the spectrum of red light is quite wide. Its different parts are responsible for the regulation of various physiological processes. This may affect the production process as a whole [55]. In addition, the synthesis of auxins depends on red light. Auxins are responsible for root differentiation in an intact plant. The blue spectrum is responsible for the differentiation of buds and the formation of the aboveground biomass. The green spectrum leads to an increase in the effectiveness of the action of various spectra on the morphophysiological processes of the studied objects [56,57]. The results of our study have shown that an increase in the proportion of the blue spectrum stimulates an increase in root formation. The predominance of the red spectrum stimulates the activation of shoot growth. Due to the fact that very few similar studies have been reported previously, present and future studies in this direction are of interest.

#### **5. Conclusions**

This study is one of the first to report the effects of culture conditions on the micropropagation of *Ipomoea batatas* 'Jewel' cultivar. Effects of three important factors (including MS salt concentration, PGRs, and artificial light) on the initiation, proliferation, and growth of in vitro jewel sweet potato shoots obtained from the axillary bud and shoot tip explants were studied. Research results have shown that the medium containing 50% MS salt concentration supplemented with 0.5 mg/L BAP or 0.5 mg/L Kn combined with 0.5 mg/L IAA was the most suitable for induction, proliferation, and growth of in vitro jewel sweet potato shoots. This result will pave the way for further studies on the jewel sweet potato cultivar towards determining the biochemical composition, especially inulin content, biological activity, and adaptability to field conditions in the Russian Federation of plantlets. Thereby creating a premise to expand large-scale production in the direction of replacing imported raw materials for the food industry to meet practical needs.

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

**Funding:** The article was completed within the framework of the thematic plan task for the implementation of research work by the federal state budgetary educational institution of higher education 'Russian State Agricultural University-Moscow Agricultural Academy named after K.A. Timiryazev' by order of the Ministry of Agriculture of Russia at the expense of the federal budget in 2022. Additionally, Sections 2.1 and 2.4 were supported by the Ministry of Science and Higher Education of the Russian Federation in accordance with agreement № 075-15-2020-905, date 16 November 2020, on providing 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'.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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