## 1 Introduction  
Cadmium (Cd) is one of the most widespread problematic non-essential priority pollutants of soils due to a multitude of historic and contemporary geogenic and anthropogenic diffuse pollution sources. These include air emissions and waste from smelting/miningindustries and fossil fuel or biomass-fired power plants, fertilizers, agrochemicals and manure, sewage sludge, waste-based soil amendments applied to soil within circular economy implementation, and often contaminated irrigation water [1]. High mobility in the environment and the long-range transmission of pollutants (LRTP) with air fluxes have caused the occurrence of high diffuse soil pollution with Cd far away from emission sources [2]. High susceptibility to accumulation in tissues of some basic agricultural products (e.g., rice and root and leafy vegetables) may increase daily Cd intake by consumers to a level exceeding daily tolerable limits, thus particularly strongly endangering human health. Widespread Cd pollution of soils has induced searching for non-invasive, efficient, and cost-effective methods for its reduction to a level ensuring food and environmental security. In farmland, this is dictated mostly by cultivated crops and their ability to accumulate Cd in the edible parts. The development in recent decades of phytoremediation methods with the use of plants that hyperaccumulate pollutants creates promising prospects for non-invasive, low-cost, operationally simple, and environmentally acceptable reductions in soil pollution. However, due to the non-essentiality and strong toxicity of Cd to plants, only scarce species display tolerance to Cd and hyperaccumulation properties. In a hyperaccumulator database from 2017, only seven species were registered as Cd hyperaccumulators [3], and few species, mostly endemic or exotic, have been added to this list in the following years. Among them, only _Solanum nigrum_ L. that has been recently discovered as an efficient Cd hyperaccumulator [4] occurs as a common wild plant worldwide. Moreover, it appeared that different ecotypes of _S. nigrum_ grown far away from each other display significantly different Cd tolerance and hyperaccumulation capacity, thus creating the possibility of selecting the most efficient hyperaccumulator by comparing different ecotypes [5]. Surprisingly enough, despite high interest to phytoremediation of soils polluted with Cd and hundreds of studies on _S. nigrum_, they are concentrated mostly in the Far East region, while reports originating from other parts of the world are scarce and often not related to Cd accumulation [6; 7; 8; 9; 10; 11; 12; 13; 14]. Simultaneously, since the beginning of the industrial revolution at the end of 18 th century, Cd remains one of the most problematic pollutants in the Northern Hemisphere, and specifically in Europe, due to it having historically the longest and strongest impact of agrochemicals and emissions from heavy industry and power production. Long-range transport with LRTP strongly contributes to the diffuse pollution of agricultural lands, and Poland and Germany are currently responsible for the biggest Cd emission and deposition in many European countries [15]. In surface soils of mining areas, heavy industrial regions, urban areas, and farmlands with extensive agriculture in Europe, the USA, and Canada, Cd occurs in concentrations at the level of 10 mg kg\({}^{-1}\) up to roughly 100-300 mg kg\({}^{-1}\), occasionally reaching even bigger values [1]. This leads to excessive levels of Cd in edible plants, which is dangerous to human health. Such areas, mostly thickly populated (e.g., in the Upper Silesia industrial region in Poland or in Ruhr in Germany), must be efficiently remediated. Phytoremediation appears to be the best non-invasive, cost-effective, and sustainable option, and _S. nigrum_ as a common cosmopolitan weed plant with well confirmed Cd hyperaccumulative properties seems to be a perfect candidate for this purpose. _Solanum nigrum_ L. ssp.