Heavy metals, due to their inability to degrade, pose a serious environmental and nutritional problem. The accumulation of essential and non-essential heavy metals in living organisms reduces normal growth and development, resulting in acute poisoning, disease and even death of organisms. Melatonin is a very important multifunctional molecule in protecting plants from oxidative stress due to its ability to directly neutralize reactive oxygen species (ROS). Also, melatonin has a chelating property, which may contribute in reducing metal-induced toxicity. In this paper, the protective role of melatonin in counteracting metal-induced free radical generation was highlighted. Using the HPLC-FLD technique melatonin was identified and quantified in the roots and leaves of lemon balm (Melissa officinalis L.), grown under photoperiod conditions. Furthermore, the response of plants pre-treated with exogenous 0.1 mM melatonin to the increased zinc (Zn) and cadmium (Cd) concentrations was observed, with changes in mineral (Ca, Mg), physiological and antioxidant status of the plant during heavy metals stress. The obtained melatonin concentrations were the highest published for dry plants so far. Elevated Cd and Zn levels in soil caused alternation in biochemical and physiological parameters of lemon balm leaves and roots. However, melatonin pre-treatment increased plant tolerance to heavy metals stress. Increased Cd 3 and Zn uptake and their translocation into the leaves were also improved, indicating the possible use of melatonin in phytoremediation.

Recently, plant remediation techniques -phytoremediation -have been developed. Japanese Knotweed (Reynoutria japonica), is an invasive species with negative impacts on the environment and the economy. In order to assess the tolerance of highly invasive weeds to heavy metals, an experiment was conducted in which this plant was grown in control soil and in soils contaminated with different concentrations of Pb, Cd. The content of heavy metals in the soil did not eliminate the ability of Reynoutria japonica roots to regenerate. In soil contaminated with any concentration of Pb, the shoots of this plant grew at a similar rate as the control plants, and had the same morphological characteristics. Higher concentrations of cadmium and lead Cd (100, 200 mg kg -1 ), Pb (2000 mg kg -1 ) have extended rhizome regeneration compared to control plants. In soils contaminated with lower concentrations of Cd or Pb shoots grew at a similar rate as control plants. Chemical analysis of heavy metal content showed that this weed accumulated large amounts of metals when grown in soil contaminated with heavy metals A relatively high intake of Cd, aboveground plants. At a cadmium concentration of 100 mg kg -1 , more than 537 times the amount of cadmium accumulated in the aboveground part of the plant, as opposed to that in control. The ability of Japanese Knotweed (Reynoutria japonica), to regenerate from parts of the rhizome, to grow and develop under stressful conditions in the presence of heavy metals indicates a high tolerance to heavy metals.

Phytoremediation has proven to be a suitable method for removing heavy metals from the soil with the help of plants. To examine the phytoremediation potential, the experimental study monitored the influence of high and low concentrations of heavy metals (Pb, Cd, and Zn) on Helianthus annuus L., as well as their accumulation in seeds, roots, stems and leaves. The experiment was carried out during one growing season in outdoor conditions, in pots with a volume of 5L in which Helianthus annuus L. was planted, and the soil was contaminated with these heavy metals in concentrations below and above the maximum allowable concentration prescribed by the Rulebook on Determining Permissible Quantities of Harmful and Dangerous Substances in Soil and Methods of Their Testing. After growth and development of the plant, the experimental research examined the accumulation potential of the plant, the growth of the plant itself and its ability to survive depending on different concentrations of heavy metals. By processing the obtained results, statistically significant differences of heavy metals were determined in individual parts of the plants, depending on the applied concentration. The highest concentration of Zn was recorded in the leaves of the plant in the amount of 18.21 mg/kg, and the lowest concentration in the stem, 3.92 mg/kg. The measured values of heavy metals Cd and Pb differ from the above because the lowest concentrations of these heavy metals were recorded in the seed, and were 12.02 mg/kg for Pb and 9.20 mg/kg for Cd, which is a statistically significantly lower determined value relative to other parts of the plant.

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