## 3 Results and Discussion  
### Effect of Adaptation to Cd Stress on Biomass of the Middle European Ecotype of S. nigrum  
There were no distinct morphological differences in the state of roots and shoots between both investigated varieties of the Middle European ecotype of _S. nigrum_ L. ssp. _nigrum_, i.e., between the naturally non-adapted one (N0) and that adapted in the first-generation to high Cd concentration in the soil (A50). Both N0 and A50 varieties retained similar growth and healthy appearance within the entire range of Cd treatments of the soil (from 0 to 50 mg kg\({}^{-1}\)DW, which corresponded to the actual measured mean concentrations of Cd in the soil from 0.22 mg kg\({}^{-1}\) to 50.49 mg kg\({}^{-1}\)) (Table 1). However, more intense green color of leaves and analytical data clearly show trends related to adaptation of _S. nigrum_ to high Cd concentrations in soil. The non-adapted variety N0 displayed a mild downward trend in root biomass with increasing Cd contents in the soil, scarcely by 36% at T50 compared to T0. Shoot biomass showed an even weaker decline (by 26%), at several-fold bigger biomass of shoot than that of root. Shoot to root (S/R) biomass ratio ranged from 5.7 to 7.3 (mean 6.5) throughout the entire soil treatment range, i.e., from T0 to T50 (C\({}_{\text{soil}}\) = 0.22-50.49 mg kg\({}^{-1}\) Cd, DW). In turn, declining trends of biomass of the adapted variety A50 with increase of Cd stress from T0 to T50 appeared to be distinctly weaker. It reached 30% for roots and only 5% for shoots, at S/R biomass ratio varying from 6.0 to 8.4 (mean 7.2). The increase of S/R ratio at T50 was mostly due to a relatively smaller impact of Cd stress on shoot biomass and increasing resilience mechanisms, particularly in A50. The mean root biomass of the adapted variety A50 in T0-T20 range (C\({}_{\text{soil}}\) = 0.22-20.44 mg kg\({}^{-1}\) Cd, DW) was roughly from 3 to 10% lower than that of the non-adapted one N0, while the mean shoot biomass of A50 was more uniformly, from 8 to 9% lower, respectively. However, at higher Cd concentrations in the soil (T30-T50, C\({}_{\text{soil}}\) = 30.09-50.49 mg kg\({}^{-1}\) Cd, DW), the mean biomass both of root and shoot of the adapted (A50) variety surpassed that of the non-adapted (N0) one. The differences in A50 vs. N0 biomass showed clearly increasing trend (from 3% to 29%, and from 10% to 18% in root and shoot, respectively), although there was no statistical significance (Table 1). This evidently resulted from the generally high resistance of the Middle European _S. nigrum_ ecotype to Cd, but a weaker impact of high Cd concentrations in the soil on the adapted plant growth. There are no data on other ecotypes of S. nigrum grown under the same soil/climate conditions. However, identical comparative pot experiments performed on cinnamon soil type (Xantic Ali-Udic Cambisols) [20], with the use of remote Asian ecotypes, two Chinese (Shenyang SY and Hanzhong HZ) and one Japanese ones (Kyoto KT) [5], showed very close root and shoot biomass values of the Middle European and the Asian ecotypes at T0 treatment, but considerably lesser resistance of the Asian ecotypes to growing Cd content in the soil. Already at C\({}_{\text{soil}}\) > 10 mg kg\({}^{-1}\) Cd, fast decrease of biomass (DW) of the Asian ecotypes was observed. The reported reduction of root biomass was up to 2.5-4-fold (by 62-76%), and that of shoot up to over 3-fold (by 69-72%) at C\({}_{\text{soil}}\) \(\approx\) 50 mg kg\({}^{-1}\) Cd compared to control (T0 treatment). In contrast, at the same Cd treatment range, the Middle European ecotype showed a decline scarcely up to 36% and 26% of root biomass, and by 26% and 5% of shoot biomass in the N0 and A50 varieties, respectively. Much lesser S/R ratio (3.4-3.8) of the Asian ecotypes than of the Middle European N0 and A50 varieties (S/R = 7.3 and 8.1, respectively) at C\({}_{\text{soil}}\) \(\approx\) 50 mg kg\({}^{-1}\) Cd indicated a considerably stronger adverse impact of high Cd stress on the Asian ecotype growth.