**3. Results**

### *3.1. Influence of Different MS Salt Concentrations on Induction and Growth of In Vitro Jewel Sweet Potato Shoots*

The induction and growth of in vitro shoots from explants depend on the amount of salts dissolved in the medium. In this experiment, the standard MS salt composition was changed to 33% (MS1), 50% (MS2), 100% (MS), and 150% (MS3) of the normal concentration (100% MS).

After 7 days of culture, the shoot tip explants showed no response to the culture medium (Figure 1a), while the stem pieces bearing the axillary buds showed to form the axillary shoots and roots (Figure 1b). Observations in the following weeks did not record the response of shoot tip explants.

**Figure 1.** Response of the explants after 7 days of culture: (**a**) shoot tip; (**b**) stem pieces bearing the axillary bud.

On the other hand, the results also showed that the medium containing different concentrations of MS salts had a significant effect on the growth of axillary shoots and

roots of sweet potato after four weeks of culture. The reduction in salt content compared with basal MS medium showed good results for the sprouting of axillary buds, as well as axillary shoots' subsequent growth. The best growth indices (including axillary shoot length and root length) were obtained from explants on the MS2 medium (Figure 2). High concentrations of mineral salts in the culture medium (MS3) showed a negative effect on axillary shoots' and roots' growth. Based on these obtained results, MS2 medium (containing 50% salt content of the salt concentration in basal MS medium) was used for the following experiments.

**Figure 2.** Influence of different MS salt concentrations on in vitro shoots' growth of jewel sweet potato after four weeks of culture. (\*) indicate a significant interaction between the evaluated parameter at 0.05 probability levels. Means with different letter(s) within the bars differ significantly at a 0.05 probability level using LSD.

#### *3.2. Influence of Different PGRs on Proliferation and Growth of In Vitro Jewel Sweet Potato Shoots*

The MS2 medium (containing 50% salt content of the salt concentration in basal MS medium) supplemented with a combination of 0.5 mg/L IAA with various cytokinins such as 0.5–2.0 mg/L BAP, 0.5–2.0 mg/L Kn, and 0.1–1.0 mg/L TDZ was used for the rapid proliferation of in vitro jewel sweet potato shoots (Table 1, Figure 3).

The obtained results demonstrate that increased cytokinin concentration in the nutrient medium reduced the explants' ability to form axillary and adventitious shoots, whose growth was also reduced due to higher hormonal concentrations. It was found that the best results were obtained in the medium containing 0.5 mg/L BAP and 0.5 mg/L Kn combined with 0.5 mg/L IAA, and the worst in the medium containing TDZ combined with 0.5 mg/L IAA. The media with BAP combined with 0.5 mg/L IAA exhibited average results (Table 1).

Differences were also observed for such parameters as shoot length and the number of leaves per shoot (Table 1). The experiments demonstrated that the MS2 medium supplemented with 0.5 mg/L Kn combined with 0.5 mg/L IAA provided the highest mean shoot length value (6.1 cm) and the number of leaves per shoot (9.2). The second effective medium was one containing 0.5 mg/L BAP combined with 0.5 mg/L IAA; its shoot length was 5.2 cm, and the number of leaves was 8.0. In the other medium, the in vitro shoots grew slowly and formed very short internodes, which was especially evident for the medium containing 1.0 mg/L TDZ combined with 0.5 mg/L IAA.

Cytokinins combined with IAA, 0.5 mg/L

**Figure 3.** Combined effect of auxin and cytokinin on explants survival rate (%) after 45 days of culture. Values of germination percentage were arcsine √X transformed prior to statistical analysis. Means with a different letter(s) within the bars differ significantly at a 0.05 probability level using LSD.

**Table 1.** Combined effect of auxin and cytokinin on proliferation and 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 the Fisher's least significant difference (LSD) test.

In addition, the combined addition of cytokinin and auxin (IAA) showed the in vitro rooting effect. The results showed that the MS media supplemented with 0.5 mg/L BAP and 0.5 Kn combined with 0.5 mg/L IAA gave the highest number of roots and mean root length (0.5 mg/L BAP: 2.02 roots/shoot, 8.95 cm; 0.5 mg/L Kn: 1.98 roots/shoot, 8.52 cm).
