*3.4. Plant–Soil Interaction*

Principal component analysis (PCA) did not reveal significant correlations between *B. prostrata* fresh and dry biomass and soil properties in 0–20 cm soil layers (Figure 4A). There were significant positive correlations between *B. prostrata* fresh and dry biomass

and the sum of salts and the sum of the contents of anions Ca2+, Mg2+, and SO4 <sup>2</sup><sup>−</sup> in the 20–40 cm and, to a lesser degree, 40–60 cm soil layers (Figure 4B), as well as with K<sup>+</sup> content in 40–60 cm layers (Figure 4C).

PCA revealed the negative dependencies of genetic polymorphism parameters (*P*, *H*e, and *H*o) on K+, Ca2+, and sulfate ions contents and, to a lesser degree, on the sum of salts and the sum of anions in the 0–20 cm soil layers (Figure 4D). In addition, a negative correlation was observed between *P, H*e, and *H*<sup>o</sup> from one side and Mg2+, K+, Ca2+, and SO4 <sup>2</sup><sup>−</sup> contents in 20–40 cm soil layers from the other side (Figure 4E). There were no correlations between genetic polymorphism parameters and soil properties in the 40–60 cm layers (Figure 4F).

**Figure 3.** Genetic polymorphism in seven populations of *Bassia prostrata.* (**A**)—observed heterozygosity (*H*o) of polymorphic loci; (**B**)—expected heterozygosity (*H*e) of polymorphic loci; (**C**) polymorphic loci proportion of population (*P*), average (for all loci) observed heterozygosity (*H*o), and average (for all loci) heterozygosity (*H*e) of seven populations of *Bassia prostrata*.

**Figure 4.** Principle component analysis (PCA) of growth (**A**–**C**), genetic diversity (**D**–**F**) parameters of *Bassia prostrata*, and salinity of 0–20 cm (**A**,**D**), 20–40 cm (**B**,**E**), and 40–60 cm (**C**,**F**) soil layers. K+, Na+, Ca2+, Mg2+, Cl<sup>−</sup>, SO4 <sup>2</sup>−, HCO3 −—ions content in soil; Ss—sum of salts; Sa—sum of anions in soil; FW—fresh plant biomass; DW—dry plant biomass; W—water content in leaves; *P*—proportion of polymorphic loci; *H*o—observed heterozygosity; and *H*e—expected heterozygosity of *B. prostrata*.

#### **4. Discussion**

The habitats of *Bassia prostrata* in this study were characterized by significant diversity in the degree and chemistry of soil salinity; high salinity occurred at different soil depths (Table 1). *B. prostrata* has wide edaphic plasticity and can grow on various soil genesis, e.g., chestnut, light-chestnut alkaline soils, and solonetz, as well as on soil-forming rocks of different compositions, from light sandy to heavy loamy, stony, and gypsum [30,36].

Our results revealed differences in correlations between *B. prostrata* aboveground biomass accumulation and seed genetic polymorphism and the chemistry and degree of salinity of different soil layers. The genetic diversity level was affected by the salinity degree and the chemistry of the uppermost soil layers (0–20 cm, 20–40 cm), and biomass accumulation was mainly affected by the salinity of the 20–40 cm and 40–60 cm soil layers. Such differences may be associated with different seasons of aboveground biomass and seed pool formation. *B. prostrata* biomass accumulation (before flowering) occurs mainly in the summer, the hottest and driest season: 23–26 ◦C, 40–43% humidity, and 65.7 mm precipitation (Figure 1B). In the summer, the drying of the uppermost soil layers can be observed, and plants receive water and dissolved salt ions from lower soil layers, affecting biomass formation. Our study showed a positive dependence of *B. prostrata* productivity on the degree of salinity in 20–40 cm soil layers (Figure 4B). *B. prostrata*, as a halophyte, requires a certain amount of salt in the substrate for optimal growth [37] and has high productivity in soils with 20 dS/m (EC) salinity [31]. The content of the main plant nutrient K+ in seven soil habitats decreased from the upper to lower layers, whereas the Na<sup>+</sup> concentration increased (means of K+/Na+ were 0.45 and 0.01 in the 0–20 cm and 40–60 cm soil layers, respectively; Table 1). Despite the fact that plants growing in saline habitats have acquired mechanisms that allow for selective uptake of K+ when Na<sup>+</sup> dominates in the substrate [37], in *B. prostrata* plants, K+ content in tissues decreased when Na+ exceeded 100–200 mM NaCl [38]. Thus, the selective absorption of K<sup>+</sup> from the 40–60 cm soil layer under conditions of increased competition with Na+ affects *B. prostrata* biomass accumulation in natural habitats (Figure 4C).

Ca2+ and Mg2+ ions are also essential mineral nutrients. Ca2+ is a universal signal in all eukaryotic cells and participates in many other cellular processes, for example, in the maintenance of cell membrane integrity, cation–anion balance, and osmoregulation [39,40]. Mg2+ is an activator of more than 300 enzymes, in particular, photosynthetic and respiratory ones, which are also needed for DNA and RNA synthesis [41,42]. It is well known that Ca2+ plays a protective role in a plant's response to salinity. Much less is known about the role of Mg2+ in the salt tolerance of plants [39]. However, it was shown that low concentrations of mixed salts with CaCl2, and MgSO4 are necessary for the successful seed germination of *B. prostrata* [43]. Our study showed that Ca2+ and Mg2+ contents contributed significantly to soil salinity in *B.prostrata* habitats (Table 1). Positive correlations between biomass accumulation and Ca2+ and Mg2+ contents in the 20–40 cm soil layer (Figure 4B) indicate their necessity for *B. prostrata* growth. The influence of magnesium in this soil layer can be associated with the optimal K+/Mg2+ ratio. The K+/Mg2+ ratio for soils and plant tissues is critical to maintaining optimal plant nutrition and, hence, plant productivity [42]. The K+/Mg2+ ratio (0.09 ± 0.03) in the 20–40 cm soil layer in *B. prostrata* habitats was less than that of the 0–20 cm soil layer (0.56 ± 0.17) but higher than that of the 40–60 cm soil layer (0.02 ± 0.01).

*B. prostrata* seeds are formed in autumn, during a cooler and rainier period (1–16 ◦C, 49–81% humidity, 77.1 mm precipitation; Figure 1B) when the upper soil layers are moist and plants receive water and dissolved salt ions from them. At the same time, the need for water decreases due to lower air temperatures and higher humidity. Therefore, the formation of seed genetic diversity in *B. prostrata*, upon which the future stability of populations in changing environments depends [12,44,45], is affected by the salinity level and ionic composition of the 0–40 cm soil layers. In heterogeneous environments, the processes of gene flow, mutation, and sexual reproduction generate local genetic variation, providing material for local adaptation [45]. The influence of soil factors such as soil type, pH, moisture, and soil layer depth on population genetic diversity has been demonstrated in different plant species [15,17–19]. A nine-year experiment on the influence of soil moisture and nitrogen, phosphorus, and potassium content in soil on allozyme frequency revealed an allele–habitat association in *Festuca ovina* [15]. It was found that in natural populations the *Pgi*-2-2 allele is

significantly associated with soil moisture and is affected by nutrient/water treatments [15]. Negative correlations between *B. prostrata* genetic diversity with inorganic ion content (except for Na+ and Cl−) and the sum of salts in the 0–40 cm soil layers (Figure 4D,E) indicate selection in favor of homozygotes. Since isozymes (allozymes) were also used in our study, a question arises regarding the functional significance of enzymes under selection. Loci *G6pd* and *Me* were the most polymorphic among the *B. prostrata* populations (Figure 3A). They encode the enzymes glucose-6-phosphate dehydrogenase (G6PD) and malik-enzyme (NADP-Me), respectively, which are associated with the regulatory nodes of dark respiration and photosynthesis. G6PD is a key enzyme in the alternative apotomous oxidative pentose phosphate pathway (OPPP), whose role is enhanced under stress [46]. Malik-enzyme is involved in photosynthesis and is especially active in C4 species, and it plays a vital role in the tolerance to salt stress [47]. The adaptive–compensatory reactions of plants under stress are always associated with additional energy costs, which leads to a change in the balance between photosynthesis and respiration [46]. Any shifts in this balance are reflected in the total plant productivity. Selection leads to local adaptation, and the strength of local adaptation depends on the strength of selection. Strong selection leads to strong local adaptation, which is significantly affected by landscape heterogeneity [48]. The negative influence of Ca2+, Mg2+, and SO4 <sup>2</sup><sup>−</sup> contents in the 0–40 cm soil layer on heterozygosity indicates the formation of the local adaptation of *B. prostrata* to magnesium–calcium sulfate soil salinity. The detected level of sodium chloride salinity did not negatively impact seed genetic polymorphism (Figure 4D,E). This is probably due to the necessity of these ions in maintaining water balance in the aboveground organs of *B. prostrata* (Figure 4C).

#### **5. Conclusions**

Our study demonstrates that in natural habitats the productivity and seed genetic polymorphism of halophytes may be affected by the salinity of different soil layers. These differential plant responses to vertically heterogeneous soil salinity could be attributed to seasonal variables during biomass accumulation (summer) and seed formation (autumn). An excess of some ions in the uppermost soil layers can lead to increased local adaptation to a certain type of salinity and the appearance of genotype-environment associations. Genotype–environment association analyses may allow us to develop adaptive measures for natural resource management, pasture improvement, and the phytoremediation and restoration of lands with different salinity chemistries.

**Author Contributions:** Conceptualization, E.S., M.L., and K.T.; methodology, E.S. and M.L.; software, M.P.; validation, E.S., K.T. and M.L.; formal analysis, A.K.; investigation, E.S., A.K. and M.L.; resources, E.S. and K.T.; data curation, A.K.; writing—original draft preparation, E.S.; writing—review and editing, E.S., K.T. and M.L.; visualization, M.P.; supervision, K.T.; project administration, E.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Science and Technology Research Partnership for Sustainable Development (SATREPS), in collaboration with the Japan Science and Technology Agency (JST, JPMJSA2001) and the Japan International Cooperation Agency (JICA), and by the Ministry of Science and Higher Education of the Russian Federation (theme No. 122042700044-6) and FGUR-2022-001- 1021060307664-8-4.1.4.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are available from the authors upon request.

**Acknowledgments:** We are grateful to the administration of the Elton Regional Nature Park for the opportunity to conduct research in the park.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
