# Role of Melatonin to Enhance Phytoremediation Capacity
## 1 Introduction
For the above described systems, aquatic plant species are of interest for use in phytoremediation processes because they can accumulate more than 1450-fold the concentration of the heavy elements in water [13]. In free-floating macrophytes, the entire plant body is above water except the roots, while in submerged macrophytes, the whole plant body remains submerged in water. For their part, emergent macrophytes are plants rooted in soil, but which emerge to reach significant heights above the water. A wide range of wetland plant species, such as water hyacinths _Eichhornia_ spp., _Salvinia_ spp., water lettuce (_Pistia stratiotes_), giant duckweed, duckweed (_Lemma minor_), and _Azolla_ spp., submerged species such as _Potamogeton_ spp. and _Myriophyllum_ spp., and emergent species like _Typha_ spp., _Scirpus_ spp., _Limnocharis flava_, _Spartina_ spp., _Cyperus_ spp., and _Phragmites_ spp. have shown their potential for removing metals from various type of wastewaters [14; 15]. An extensive web-list of plant hyperaccumulators, classified by the metals they absorb, can be consulted [16]. Selecting the appropriate plant species is one of the most important considerations in the phytoremediation process. The plant species chosen should be capable of tolerating high metal levels and extreme soil conditions, such as high acidity, salinity, or alkalinity [17]. In addition, plants for revegetation should have other favorable attributes such as dense rooting systems, relatively fast growth rates, and high biomass production [7]. Furthermore, in semiarid mining regions, plant species should also be able to adapt to drought. Metal tolerant native plants are usually selected because they demonstrate tolerance to local environmental conditions and usually grow well and proliferate [18]. Suitable plants for phytoremediation can be divided into two groups based on their function: metal hyperaccumulators and biomass producers [19; 20]. Metal hyperaccumulators are plants that exhibit higher levels of metal ion absorption in their tissues, but usually do not produce a high amount of biomass and have a slow growth rate. _Brassicaceae_ spp. are known to have exceptionally high metal accumulating capacities [21]. Some authors have reported that _Thlaspi_ species (typical hyperaccumulators) can accumulate over 1% Zn, 0.1% Ni, and 0.1% Cd in dry tissues. In addition, other heavy metals, including arsenic (As), Cu, Co, Mn, and Pb, can potentially be hyperaccumulated from mine tailings [19; 20]. Biomass producers include plants that have a high biomass production and growth rate, but a relatively low metal uptake capacity, such as _Brassica juncea_ (Indian mustard) [21]. Nutrient shortages are one of the main limitations of plants used for phytoremediation in mining areas, so it is generally necessary to amend the soils. Other biotechniques include: (i) enrichment with microorganisms, which can play an important role in solubilizing minerals such as P and potassium (K), releasing nutrients, and supplying them to plants through in situ bioremediation processes [22]; (ii) the use of metabolites such as organic acids, amino acids, and vitamins, which have also been demonstrated to enhance plant growth [23]; (iii) the addition of plant hormones, siderophores, and some enzymes synthesized by microorganisms, which may help plant growth [24; 25; 26]. In general, plants inoculated with plant growth promoting bacteria (PGPB) accumulate greater amounts of heavy metals than non-inoculated plants [27].