**5. Conclusions**

Centaury shoots grown under NaCl-induced stress decreased the content of photosynthetic pigments, total phenolic compounds and DPPH. The activities of SOD, CAT and POX were also reduced under salt stress conditions. All these results indicate that centaury is a salinity-sensitive plant species. However, the MDA concentration was decreased while H2O2 concentration did not drastically change under stress conditions caused by NaCl, which indicate that centaury can be also be considered a salinity-tolerant species. Under salt stress conditions, proline content also did not significantly change which is not an attribute of salinity-tolerant species. In addition, it is possible that centaury has a preference for other osmolytes, rather than proline. In salt stress conditions, Na+ and Cl<sup>−</sup> ions can act as "cheap osmolytes". In addition, the effective removal of Na+ from the cytosol does not result in excessive ROS generation, eliminating the high activity of antioxidant mechanisms. Therefore, it is necessary to investigate the mechanisms that regulate the transport of ions in centaury in order to reveal if this important medicinal plant is a halophytic species. The results presented in this work also shown that SNP, a widely used NO donor, improved centaury tolerance to salinity (Figure 8). SNP showed a positive effect on total *Chl* and carotenoid content and affected lipid peroxidation, proline and total phenolic content, DPPH concentrations as well as antioxidant enzyme activities in centaury shoots grown under salt stress caused by NaCl. In addition to NO, SNP releases cyanide and iron ions as toxic by-products, and thus limits its potential application in agriculture. Therefore, nanoparticles that release NO, as well as S-nitrosothiols and S-nitrosoglutathione, the natural reservoirs of NO in biological systems, have been suggested as alternatives to SNP application.

**Figure 8.** Schematic illustration showing how SNP affects centaury shoots during salt stress in vitro.

**Author Contributions:** M.T.-M. and N.S. contributed to all in vitro experiments and manuscript preparation. M.T.-M., N.S., M.Ð., S.M. and M.M. contributed to all experimental work requiring biochemical analyses. Z.G. contributed to data analyses and interpretation of the results. A.S. supervised the whole study and also contributed in preparing the final manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the data are contained within the manuscript.

**Acknowledgments:** This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, contract numbers 451-03-68/2022-14/200007 and 451-03-68/2022-14/200178.

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

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
