# Role of Melatonin to Enhance Phytoremediation Capacity
## 1 Introduction  
Phytoremediation may be described as a technology that uses the ability of certain plants, known as hyperaccumulators, to take up pollutants from soil or water. Phytoremediation is of particular interest because it is a promising green technology, low cost, and eco-friendly [1]. However, hyperaccumulator plants are rare, and generally, their slow growth and small biomass limits their efficiency for phytoremediation purposes. The hyperaccumulator plants used for phytoremediation mainly accumulate metals in the shoot rather than the root [2]. Compared with non-accumulator plants, hyperaccumulating plants can concentrate 100 to 1000 times more heavy metals such as copper (Cu), zinc (Zn), cobalt (Co), manganese (Mn), nickel (Ni), and lead (Pb) [3]. Each plant species acts in a specific way in phytoremediation, taking up heavy metals by many mechanisms, includingaccumulation, exclusion, translocation, osmoregulation, and distribution. Of these, the most common is via the accumulation, translocation, and concentration of heavy metals in the aerial parts [4]. Various processes have been used as phytoremediation techniques, including phytoextraction, phytodegradation, rhizfiltration, phytostabilization, phytovolatilization, phytodesalination, and phytofiliration [5]. Phytoremediation can be carried out on site, thus reducing exposure risks for cleanup personnel or secondary contamination during transport. However, the physical, chemical, and biological properties of mine tailings (or other contaminated soils) may limit plant growth and their subsequent use in agriculture [6]. Three major phytoremediation techniques can be distinguished, depending on different plant properties: phytoextraction, phytostabilization, and phytovolatilization [7]. In the first method, plants are used to take up contaminants or metals via their roots. This type of extraction involves the accumulation or hyperaccumulation of metals in the above ground plant biomass. Contaminants or metals are then stored in the harvestable parts of plants and disposed of as hazardous waste or incinerated for metal recovery. In the case of phytostabilization, plants are used as a vegetation \"cap\" to not only reduce the mobility and bioavailability of contaminants in the natural environment, but also the availability for entry into the human food chain. The plant canopy serves to reduce air dispersion, while plant roots prevent water erosion, immobilize heavy metals, and prevent leaching. Thus, phytostabilization is a promising technique for the long term stabilization of tailings by creating a vegetation cap. Finally, phytovolatilization involves plants taking up pollutants (including organic contaminants) together with water and releasing them into the atmosphere through the stomata; some of these pollutants pass through the plants and reach the leaves and thereby evaporate into the atmosphere [8; 9]. In constructed wetlands, the interaction between water/soil, plants, and microorganisms occurs through chemical, physical, and biological processes. A wide range of wastewaters such as municipal, industrial, agricultural, and storm waters can be remediated in constructed wetlands. The efficiency of heavy metal uptake by the plants they contain has been demonstrated [10]. The rate of metal removal by plants varies widely and is related to plant growth rate, plant species, and the concentration of heavy metals in the wastewater [11]. In wetlands constructed for phytoremediation or wastewater treatment, the residence time of metals in plants and the potential release of metals closely depend on the distribution of metals within the plants. Artificial floating islands, another type of constructed wetlands, are a soil-less planting structure constructed with floating mats, floating aquatic plants, sediment rooted emergent wetland plants, and related ecological communities. In pilot studies, this system was seen to improve the quality of polluted waters by removing organic matter, suspended solids, nutrients, and metals [12].