**5. Conclusions**

All tested parameters were affected by each osmotic material used in our experiments. In general, shoot length was the most inhibited, while changes in rooting parameters both stimulated and reduced growth, depending on the genotype. Usually, the survival rates decreased significantly in treatments with strong osmotic pressure. Responses of genotypes were affected by the type and concentration of osmotic agent. Comparing genotypes, we can conclude that C103 tolerated more osmotic stress than C107, although both of them are drought-tolerant. Out of the 27 total breeding lines examined, nine genotypes (C8, C63, C14, C2, C5, C58, C12, C11 and C30) were shown to be worthy of further investigation.

Besides their high survival rate SI values, their rooting parameters were stimulated or hardly inhibited comparing to other genotypes, and their shoot length SI values were also high in several cases. We found significant interactions between genotypes and osmotic agents and their concentrations. In fact, breeding lines with high survival rates showed high SI values in the PEG 6000 or D-mannitol treatment or in both, but not in treatments with PEG 600. In general, the PEG 6000 10.0% and D-mannitol 0.2 M treatments proved to be the most suitable for differentiating genotypes according to their osmotic stress tolerance. The PEG 6000 and D-mannitol-containing media were tested at different concentrations, but they resulted in very similar inhibitory effects in terms of shoot length, root length, and survival rate (Table 1). In fact, the rate of absorbed and metabolized D-mannitol is not known. The very strong inhibitions observed in the PEG 600 treatments may be attributable mainly to its toxicity, because their resulting osmolality values were higher than those of PEG 6000 treatments but lower than those of media with D-mannitol (Table S5). However, a significant root growth stimulating effect was also observed in the referent lines, which is probably due to the adaptation response to osmotic stress. There are ongoing in vivo greenhouse and field experiments with selected breeding lines to confirm their drought tolerance.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/horticulturae8070591/s1, Figure S1: Survival rate SI (SISR) values of potato genotypes under osmotic stress induced by PEG 6000 added to the medium at levels of 5.0% (c), 7.5% (b) and 10.0% (c). Figure S2: Shoot length SI values (SISL) of potato genotypes under osmotic stress induced by PEG 6000 added to the medium at levels of 5.0% (a), 7.5% (b) and 10.0% (c); Figure S3: Root length SI (SIRL) values of potato genotypes under osmotic stress induced by PEG 6000 added to the medium at levels of 5.0% (a), 7.5% (b) and 10% (c); Figure S4: Root number SI (SIRN) values of potato genotypes under osmotic stress induced by PEG 6000 added to the medium at levels of 5.0% (a), 7.5% (b) and 10% (c).; Figure S5: Survival rate SI (SISR) values of potato genotypes under osmotic stress induced by PEG 600 added to the medium at levels of 2.5% (a), 5.0% (b) and 7.5% (c); Figure S6: Shoot length SI (SISL) values of potato genotypes under stress treatment induced

by PEG 600 added to the medium at levels of 2.5% (a), 5.0% (b) and 7.5% (c).; Figure S7: Root lengths SI (SIRL) values of potato genotypes under osmotic stress induced by PEG 600 added to the medium at levels of 2.5% (a), 5.0% (b), 7.5% (c); Figure S8: Root number SI (SIRN) values of potato genotypes under osmotic stress induced by PEG 600 added to the medium at levels of 2.5% (a), 5.0% (b), 7.5% (c); Figure S9: Survivor rate SI (SISR) values of potato genotypes under stress treatment induced by D-mannitol added to the medium at levels of 0.1 M (a), 0.2 M (b) and 0.3 M (c); Figure S10: Shoot length SI (SISL) values of potato genotypes under stress treatment induced by D-mannitol added to the medium at levels of 0.1 M (a), 0.2 M (b) and 0.3 M (c); Figure S11: Root length SI (SIRL) values of potato genotypes under stress treatment induced by D-mannitol added to the medium at levels of 0.1 M (a), 0.2 M (b) and 0.3 M (c); Figure S12: Root number SI (SIRN) values of potato genotypes under stress treatment induced by D-mannitol added to the medium at levels of 0.1 M (a), 0.2 M (b) and 0.3 M (c).; Table S1: Ranking of breeding lines cultured on medium supplemented with PEG 6000 including each SI value; Table S2: Ranking of breeding lines cultured on medium supplemented with PEG 600 including each SI value; Table S3: Ranking of breeding lines cultured on medium supplemented with D-mannitol including each SI value; Table S4: Ranking of breeding lines cultured on medium supplemented with PEG 6000, PEG 600 or D-mannitol including each SI value; Table S5: Osmolality values of MS media supplemented with different osmotic agents used in experiments.

**Author Contributions:** Writing—original draft preparation, A.H., N.M.-D. and K.M.-T.; writing review and editing, A.H., N.M.-D., K.M.-T., J.D. and L.Z.; visualization, N.M.-D. and A.H.; supervision, K.M.-T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the Hungarian Government; project number: AGR\_PIAC\_13 \_1\_2013\_0006 ("Research on disease resistance of potato for decreasing the effect of climate change").

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** The research was supported by the Thematic Excellence Programme (TKP2021- EGA-20) of the Ministry for Innovation and Technology in Hungary, within the framework of the Biotechnology thematic programme of the University of Debrecen.

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