# Role of Melatonin to Enhance Phytoremediation Capacity
## 3 Metals as a Severe Abiotic Stress and Effects Induced by Melatonin in Plants
Boron has also been assayed in the presence of melatonin (Table 1). In pepper plants whose roots were exposed to a high B concentration (100 \(\upmu\)M), melatonin treatment restored plant growth and photosynthesis compared with control plants. Plants treated with melatonin displayed no visible B toxicity symptoms, and leaves and fruits showed moderate B accumulation and high carbohydrate, carotenoid, and flavonoid contents. The authors took this as a demonstration of the clear protective activity of melatonin in reducing B absorption, suggesting its physiological relevance in B homeostasis [76]. Similar results and conclusions have been obtained in spinach plants exposed to high B concentrations (Table 1). Higher concentrations of fluoride ions (F\({}^{-}\)) in the soil and irrigation water can disturb both the physiological and biochemical processes of plants [96]. Melatonin acts as an ROS scavenger, improving many biochemical parameters in pigeon pea (_Cajanus cajan_) (Table 1), helping in diminishing F\({}^{-}\) toxicity [78]. Although soil alkalization is often associated with soil salinity, the former is considered much more hazardous to plants. This condition is generally linked with high pH stress and sodium toxicity caused by an excess of Na\({}_{2}\)CO\({}_{3}\) and NaHCO\({}_{3}\) in the soil, as well as osmotic stress. The comprehensive stress caused by alkaline soils directly affects physiological homeostasis at the cellular and whole-plant levels [97]. In this context, melatonin has been seen to protect apple and tomato plants against alkalinization (Table 1). In the case of apple, exogenous melatonin enhanced tolerance to alkaline stress by regulating the biosynthesis of polyamines, while in tomato plants, a strong resistance to dehydration, ROS homeostasis, and Na\({}^{+}\) detoxification has been described [80; 81]. In tomato also, melatonin treatment increased simulated acid rain (SAR) stress tolerance by repairing the grana lamella of the chloroplast, improving photosynthesis and antioxidant enzyme activities compared with the reactions recorded in SAR stressed plants without melatonin. Such positive effects of melatonin are concentration dependent [82]. The positive role of melatonin against soil mineral deficiency has been described in several works (Table 1), resulting in an improvement in chemical element deficiency tolerance. For example, Fe deficiency induced chlorosis in seedlings of _Arabidopsis thaliana_ was alleviated by melatonin. Exogenous melatonin significantly increased the soluble Fe content of shoots and roots and decreased the levels of root cell wall Fe bound to pectin and hemicellulose, remobilizing cell wall Fe and alleviating Fe deficiency induced chlorosis. Furthermore, Fe deficiency quickly induced melatonin biosynthesis in _Arabidopsis_ plants, acting synergistically with exogenous treatments. In mutant plants deficient in polyamine and nitric oxide (NO) biosynthesis, the protective role of melatonin was not observed, indicating that the process is dependent on the polyamine induced NO production under Fe deficient conditions [84]. In a similar work, tomato seedlings grown in an S deficient medium suffered serious growth inhibition as a result of a reduced chlorophyll content, photosynthesis, and biomass accumulation; it also led to cell structural alterations and DNA damage. Melatonin supplementation of S deprived plants resulted in a significant diminution in ROS content, alleviating all the described symptoms. Melatonin promoted S uptake and assimilation by regulating the expression of genes encoding enzymes involved in S transport and metabolism, supporting a role for melatonin as a molecule that improves primary metabolism and redox homeostasis [85]. In winter wheat grown in an N deficient medium, the application of melatonin in hydroponic solution significantly improved seedling growth under both N sufficient and deficient conditions, but the effect of melatonin in promoting seedling growth was particularly evident in the N deficient conditions.