*3.4. Correlations*

Most of plant growth parameters of *T. officinale* (i.e., the total plant FW, the upper plant part FW, the root FW, the marketable leaf FW, the leaf number per plant, the rosette diameter, the maximum leaf length and the maximum leaf width) were found to be significantly positively correlated with the leaf nitrate concentration, root P, leaf and root K, leaf Ca, leaf Mg, leaf K/Na and Ca/Na ratios, root Zn and root Cu but significantly negatively correlated with leaf and root Na concentrations (see Supplementary Material Table S1). In particular, the correlation coefficients between the rosette diameter, maximum leaf length and maximum leaf width of *T. officinale* with leaf Na concentration were r = −0.81, r = −0.80 and r = −0.78, respectively, whereas the relevant correlation coefficients with leaf K were r = 0.99, r = 0.99 and r = 0.98. By contrast, the determined growth parameters of *R. picroides* were not significantly correlated with the majority of minerals content. Significant correlations were detected mainly between the rosette diameter (and the maximum leaf length) of the species with the leaf N (r = −0.79), P (r = −0.85), K (r = 0.70), Na (r = −0.68), Mn (r = −0.74), Zn (r = −0.77) and B (r = 0.78), root K (r = 0.64) and root Na (r = −0.79), as well as with proline (r = −0.88) and malic acid contents (r = −0.81). Significant correlations between plant growth parameters and ion contents under salinity have been indicated by several researchers [71,110,134], while Bosiacka et al. [47] reported that the strongest correlations were found between soil salinity and the leaf Na, Mn, Ca, Fe, K and Zn content of three *Taraxacum* microspecies.

### **4. Conclusions**

The wild edible greens *R*. *picroides* and *T. officinale* tested in the present study responded differently to salinity treatments indicating different tolerance mechanisms. In particular, plant growth of *R. picroides* was negatively affected only when grown under nutrient solution with EC values equal to 10 mS cm<sup>−</sup><sup>1</sup> (EC-10 treatment), whereas *T. officinale* was more sensitive and plant growth rapidly decreased when EC increased at 6 mS cm<sup>−</sup>1. The leaf and root Na and Cl concentration changes under salinity could partially explain the aforementioned salt-tolerance differentiation between the two species as the more salt-tolerant *R. picroides* accumulated more Cl and Na in the leaves as compared to the sensitive *T. officinale.* Therefore, the higher salt tolerance of *R. picroides* could be due to its ability to develop a better adaptation mechanism of water uptake, to effectively accumulate

osmolytes such as proline and to keep high shoot K probably through a more efficient K/Na selectivity, in combination with an increased Zn uptake ability under salinity stress. Moreover, the studied species differed in the contribution of secondary metabolites such as phenolic compounds to the overall antioxidant mechanism, since it seems that in *R. picroides*, phenolic compounds have an important role in plant defense against abiotic stressors, whereas in *T. officinale,* no such effect was observed. In conclusion, the response of *R. picroides* to moderate and high salinity (EC-6 and EC-10) is of grea<sup>t</sup> importance for its commercial exploitation under saline soils or in regions where irrigation water is of low quality. However, the ability of the species to adapt to saline conditions that are unsuitable for most leafy greens as well as the relevant adaptation mechanisms should be further studied.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/horticulturae7070160/s1, Table S1: Correlation coefficients of plant growth parameters and chemical composition of *Reichardia picroides* and *Taraxacum officinale*.

**Author Contributions:** Conceptualization, A.A.A.; methodology, A.A.A., A.A., P.P., M.B., N.V. and I.C.K.; software, A.A., P.P., M.B. and N.V.; validation, I.C.K. and A.A.; formal analysis, A.A.A. and I.C.K.; investigation, A.A.A., A.A., P.P., M.B. and N.V.; resources, A.A.A.; data curation, A.A., S.A.P. and I.C.K.; writing—original draft preparation, A.A.A., A.A., S.A.P. and I.C.K.; writing—review and editing, A.A.A., S.A.P. and I.C.K.; visualization, A.A.A.; supervision, A.A.A.; project administration, A.A.A., S.A.P. and I.C.K. 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:** The study did not report any data.

**Acknowledgments:** The authors thank George Georgiopoulos for providing seeds and Anastasios Kotsiras for his technical advice for nutrient solution preparation.

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