## 1 Introduction
Of course, plants cultivated on contaminated soil absorb different metals, some of which are known to be toxic at low concentrations: because plants constitute the basis of the food chain, the quality of the soils is of primary importance [14]. Metals cannot be degraded, and cleanup generally requires their removal from the soil by \"dig and dump\" which is expensive and inhibits soil fertility, causing negative impacts on the ecosystem. Despite its potential, phytoremediation is yet to become a commercially viable technology because of the cost and the time needed. However, it would be one of the best approaches to remove contamination from the soil, without destroying the soil structure and fertility. In the phytoremediation process, major mechanisms involve the roots and rhizosphere (filtration, degradation) and the whole plant (degradation, volatilization, stabilization, extraction). Hence, the phytoremediation process could be the best technique for the remediation of widespread polluted areas, in particular where metals are present at relatively low concentrations in the surface layers of the soil [15; 16]. The main approaches proposed for phytoremediation are \"chelate-assisted phytoextraction\" and \"continuous phytoextraction.\" In the first case the mobility and the uptake of metals in soil are increased by artificial chelates added to soil, in the second case the natural ability of plants to absorb metals is used [17]. Hyperaccumulator plant species could represent a winning solution if they did not still have various limits to their use, such as low biomass production and shallow root systems [18; 19]. To overcome these limits we will have to create plants capable of hyperaccumulating metals and at the same time producing a large amount of biomass; the removed heavy metals could be recycled from the harvested contaminated plant biomass [20]. Hence, phytoextraction potential is mainly influenced by two factors:(i) element concentration in aboveground biomass and (ii) yield of aboveground dry biomass. Several pilot experiments have been performed in recent years using annual and perennial woody plants, showing the potential of this technique [21]. Producing energy from biomass increases considerably the sustainability of phytoremediation action [22] and generates a positive impact due to fossil fuel substitution. Remediation techniques that use soil-plant systems cannot be limited to one decontamination mechanism. Many processes occur simultaneously in every in situ application. In a field trial, different strategies can be adopted to obtain the desired effect on each soil pollutant. Degradation and volatilization through plant stomata of arsenic [23] and cadmium phytoextraction and adsorption on raw leaves [24], for example, can be maximized with the same crop management choices. Plants can have morphological and physiological responses to organic pollutants. Phenantrene, for example, shows high toxicity in maize and leads to water stress symptoms, with higher calcium, phosphorus, magnesium and zinc contents in leaves [25]. Thus, the interfering of some pollutants with transpiration and micro-element balance needs to be taken into account. Until now, phytoremediation has been limited by the long period of time plants require to reduce contaminant levels [26; 27]. Optimal plants for phytoremediation should have the following characteristics: (i) high biomass production harvested per year with economic value; (ii) extensive root systems; (iii) tolerance and accumulations of heavy metals in harvestable biomass. This description resembles the characteristics of giant need, which is a perennial rhizomatous grass, widely diffused in subtropical and temperate regions; due to its great adaptability and well-documented tolerance to heavy metals contamination, it is considered as one of the most promising energy crops [28].