## 3 Results and Discussion
### Effect of Adaptation to Cd Stress on Biomass of the Middle European Ecotype of S. nigrum
tororum_, reduced concentrations of Cd in roots of a plant growing in Cd-contaminated soil is mainly due to Cd-induced drought stress in the root system. The transcriptional regulation of dehydration-related genes that inhibit water transport in aquaporins thus causing a drought effect, constitutes defensive mechanism against Cd penetration into plant tissues [21,37]. It results also in reduced Cd flow from roots through stem to leaves. In contrast, in A50, reduced Cd content in root compared to N0, was caused solely by increased xylem sap long range flow rate at the lack of barriers to Cd uptake from the soil. Compared to the Asian ecotypes, both Middle European varieties showed similar Cd accumulation capacity within the tolerable to the Asian ecotypes Cd\({}_{\rm soil}\) range, not causing biomass reduction (e.g., C\({}_{\rm R}\) = 57.8-81.4 mg kg\({}^{-1}\), C\({}_{\rm S}\) = 78.7-102 mg kg\({}^{-1}\), and L\({}_{\rm T}\) = 856-1348 \(\mu\)g pot\({}^{-1}\) for SY, HZ and KT [5] vs. C\({}_{\rm R}\) = 50.8-55.5 mg kg\({}^{-1}\), C\({}_{\rm S}\) = 81.1-90.7 mg kg\({}^{-1}\), and L\({}_{\rm T}\) = 1067-1085 \(\mu\)g pot\({}^{-1}\) at C\({}_{\rm soil}\) = 10 mg kg\({}^{-1}\) Cd for N0 and A50 Table 2). However, sensitivity to Cd stress at C\({}_{\rm soil}\) > 10 mg kg\({}^{-1}\) Cd results in declining biomass and total Cd accumulation capacity, at deteriorating physiological parameters but increased  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**T** & **C\({}_{\rm soil}\) mg kg\({}^{-1}\)** & **C\({}_{\rm n}\) mg kg\({}^{-1}\)** & **E\({}_{\rm n}\)** & & **TF** & **L\({}_{\rm n}\)\(\mu\)g pot\({}^{-1}\)** & **LTF** \\  & **Symbol** & **N0** & **A50** & **N0** & **A50** & **N0** & **A50** & **N0** & **A50** & **N0** & **A50** \\ \hline
**20** & 20.44 \(\pm\) 1.96 & & & & & & & 1423.8 \(\pm\) & 1419.2 \(\pm\) & & \\  & & & & & & & & 183.9 & 344.7 & & \\  & & & & & & & abA & & abA & \\
**30** & 30.09 \(\pm\) 1.52 & & & & & & & 1046.1 \(\pm\) & 1172.8 \(\pm\) & & \\  & & & & & & & 500.7 bA & 490.5 bA & & \\
**50** & 50.49 \(\pm\) 2.29 & & & & & & 2106.3 \(\pm\) & 2269.3 \(\pm\) & & \\  & & & & & & & 85.9 aA & 240.6 aA & & \\ \hline  & & & & & & & & & & \\  & & & & & & & & & & \\  & & & & & & & & & & \\  & & & & & & & & & & \\  & & & & & & & & & & \\  & & &Cd concentrations in tissues of the Asian ecotypes. Opposite, the Middle Europan varieties showed high resistance to Cd stress over the entire range of studied concentrations in the soil, up to Csoil = 50 mg kg\({}^{-1}\) Cd. They displayed lesser Cd concentrations in plant tissues, but higher total accumulated loads of Cd and their overall good physiological status (e.g., C\({}_{\text{R}}\) = 301-342 mg kg\({}^{-1}\), C\({}_{\text{S}}\) = 318-387 mg kg\({}^{-1}\), and L\({}_{\text{T}}\) = 1083-1781 \(\upmu\)g pot\({}^{-1}\) for SY, HZ and KT [5] vs. C\({}_{\text{R}}\) = 324-280 mg kg\({}^{-1}\), C\({}_{\text{S}}\) = 175-165 mg kg\({}^{-1}\), and L\({}_{\text{T}}\) = 2106-2269 \(\upmu\)g pot\({}^{-1}\) at Csoil = 50 mg kg\({}^{-1}\) Cd for N0 and A50--Table 2). Within this study, no limit of Cd tolerance of the Middle European ecotype of _S. nigrum_, either non-adapted, or adapted to Cd stress was reached. Although, among very scarce European studies on _S. nigrum_, one study on the exposure to Csoil = 100 mg kg\({}^{-1}\) Cd, i.e., to twice as big concentration as the maximum studied here, of the local South-West European ecotype, revealed still high Cd accumulation capacity and declining tolerance [12]. Concentrations of Cd in root and shoot (DW) of this ecotype appeared to be comparable to that assessed for the Middle European ecotype and reached C\({}_{\text{R}}\) = 377 mg kg\({}^{-1}\), C\({}_{\text{S}}\) = 107 mg kg\({}^{-1}\) at Csoil = 100 mg kg\({}^{-1}\) Cd vs. C\({}_{\text{R}}\) = 324-280 mg kg\({}^{-1}\), and C\({}_{\text{S}}\) = 175-165 mg kg\({}^{-1}\) at Csoil = 50 mg kg\({}^{-1}\) Cd assessed for the Middle-European ecotype at two times lesser Cd concentration in soil.