## 4 Conclusions  
The study in its entirety confirmed the assumption that different ecotypes of cosmopolitan Cd hyperaccumulator _S. nigrum_ L. ssp. nigrum, which originate from remote geographical locations, may significantly differ with respect to Cd hyperaccumulative capacity, tolerance and regulation mechanisms. It appeared that the studied Middle European ecotype displayed much bigger tolerance to abiotic stress compared to the Asian ecotypes [5]. In addition, this tolerance can be significantly enhanced by adaptation through using seeds from the first generation of plants grown in the soil with high content of Cd (50 mg kg\({}^{-1}\)). This manifested itself in the mild root and shoot biomass reduction at T50 and much higher shoot to root biomass ratio (S/R), in both the non-adapted N0, but particularly in the adapted A50 variety compared to the Asian ecotypes. These ecotypes, exposed to the same contents of Cd in the soil (in the range from 0 to 50 mg kg\({}^{-1}\) Cd), show fast reduction of biomass already at C\({}_{\text{soil}}\) = 10 mg kg\({}^{-1}\) Cd. Substantial differences between the Asian and Middle European ecotypes, and N0 and A50 varieties with respect to Cd enrichment and translocation in plant tissues was also observed. While the Asian ecotypes continued to accumulate growing contents of Cd under the critical stress conditions and showed fast declining biomass, the Middle European ecotype retained good health status and strong resistance to Cd stress over the entire range of the studied Cd concentrations in the soil, up to C\({}_{\text{soil}}\) = 50 mg kg\({}^{-1}\) Cd. Simultaneously, it showed lesser Cd concentrations in plant tissues, but similar or higher accumulation of Cd loads, both in shoots and total, in shoots and roots, compared to the Asian ecotypes. Adaptation to Cd stress in the A50 variety resulted in enhancing all physiological parameters/factors crucial for plant development, growth and tolerance to abiotic stress. These parameters comprise chlorophyll a and b, carotenoids, RuBisCO, first line defense (neutralizing) antioxidants (CAT, SOD and its isozymes, GPX), and second line defense (scavenging) antioxidants (APX, AsA, GR, GSH). It was admittedly confirmed by clearly lower Cd stress indicated bylower MDA generation and higher GSH/GSSG ratio for the adapted A50 variety compared to the non-adapted N0 one. Overall, the reported Asian ecotypes of _S. nigrum_ ssp. nigrum_ appear to show somewhat better Cd phytoremediation performance than the Middle European ecotype within Cd tolerance range of Asian ecotypes, mostly at the moderate soil contamination level up to 10 mg kg\({}^{-1}\) Cd. On the other hand, the Middle European ecotype stands out in much higher threshold of tolerance to Cd stress, reaching beyond the highest tested Cd content in soil (50 mg kg\({}^{-1}\) Cd), probably close to 100 mg kg\({}^{-1}\) Cd. Moreover, the adapted A50 variety of the Middle European ecotype shows further considerable enhancement of the plant resistance to Cd stress and the highest Cd accumulation capacity and healthy state of the plants at the biggest tested Cd concentrations in the soil. This makes the Middle European ecotype and its adapted A50 variety particularly useful to sustainable decontamination of heavily polluted \"hot spots\" in the degraded post-industrial areas. It should be added that significant differences between remote ecotypes of Cd hyperaccumulator _Solanum nigrum_ L. ssp. _nigrum_ in Cd accumulation capacity and tolerance to Cd stress suggest possible considerable differentiation of molecular mechanisms responsible for these properties in the multitude of ecotypes and genotypes growing worldwide. Some recent research indicates occurrence of differential responsive mechanisms to low and high Cd exposure in the same plant [43], thus even bigger differentiation at the molecular level can be expected in the remote ecotypes/genotypes. Selection of the most efficient Cd hyperaccumulating ecotypes/genotypes of _S.