## 3 Results and Discussion
### Effect of Adaptation to Cd Stress on Biomass of the Middle European Ecotype of S. nigrum
This indicates further gradual decrease by about 39% of Cd accumulation in shoot provided that these European ecotypes indeed react similarly to Cd stress. These data show the need of conducting further comparative experiments on different remote eco/genotypes under the same soil/climate conditions and extended range of Cd treatments to unbiasedly select the most appropriate hyperaccumulator and assess Cd tolerance thresholds for each eco/genotype. Apart from the Cd contents in soil, a variety of external factors like soil type and composition, moisture content and temperatures may affect Cd hyperaccumulation capacity and overall phytoremediation efficiency of eco/genotypes of _S. nigrum_. Comparing the results obtained in this study with other data available so far, in addition to different tolerance to Cd stress of various remote Asian/Far East and European ecotypes of _S. nigrum_, two diverse defense mechanisms were indicated. One, observed in Asian/Far East ecotypes is more conservative. It displays almost unrestricted translocation of Cd with xylem sap from root to shoot until deep breakdown of physiological functions of a plant under critical level of Cd stress, and further partial adaptation of a plant to functioning under excessive stress. This manifests itself in constantly high accumulation of Cd in shoot at poor plant condition and deep biomass reduction [5; 16]. Another mechanism was displayed in this study by both N0 and A50 varieties of the Middle European ecotype of _S. nigrum_, but in particular by the adapted variety A50. It is aimed at protecting the physiological functions of a plant through active molecular mechanisms in roots, which control Cd loading capacity into the xylem sap for long-distance transport from root to shoot and storage in leaf vacuoles [5; 38]. This resulted in no Cd stress-induced breakdown in the Middle European ecotype, but also in lower concentrations of Cd accumulated both in root and shoot tissues at critical contents of this metal in the soil (Table 2). Cd accumulation in root was somewhat lower, and in shoot somewhat higher in the adapted A50 than in the non-adapted N0 variety practically in all Cd-fortified soils, however at a non-significant level (_p_ < 0.05). Consequently, compared to N0, A50 displayed lower enrichment factors EF in root, but higher in shoot, and higher translocation factor TF. In T0-T30 range it was >1, while the non-adapted N0 variety complied with this requirement only within T0-T10 range. Therefore, the overall effect of adaptation of the Middle European ecotype to high Cd contents in the soil manifested itself in higher tolerance of the A50 variety to Cd stress. This resulted in bigger shoot biomass and either higher Cd concentrations in xylem sap at the loading stage from roots, or greater long-range xylem sap flow rate from roots to leave vacuoles. Much lesser Cd accumulation in stem than in leaves observed in the South-West European [12] and also in the Asian (SY) [5] ecotypes suggests rather this last mechanism. Moreover, despite considerably higher Cd concentrations in tissues of the Asian _S. nigrum_ ecotypes SY, NZ and KT [5], they display evidently weaker protection mechanisms against stress and strong reduction of biomass preceded by a physiological breakdown. Thiscaused that the total Cd accumulation capacity of the Middle European ecotype at critical Cd contents in the soil appeared to be visibly higher (by 49%, 26% and 15% in N0, and by 52%, 32% and 22% in A50, respectively). Effect of Adaptation to High Cd Contents in Soil on Physiological Properties of the Middle European Ecotype of Solanum nigrum  
3.3.1. Photosynthetic Pigments  
Tends in the contents of photosynthetic pigments in leaves of the non-adapted N0 variety of the studied Middle European ecotype of _S. nigrum_ L., despite lack of visual changes in the plant/leaves status, showed general similarity to these observed in the Asian ecotypes [5].