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.
Heavy metals disturb the redox homeostasis of the plant cell. The indolamine hormone, melatonin, protects plants from oxidative damage by directly scavenging reactive oxygen species or by stimulating the activity of antioxidant enzymes. The antioxidative role of melatonin in the leaves of two medicinal plants, lemon balm (Melissa officinalis L.) and valerian (Valeriana officinalis L.) that were treated with increased concentrations of Zn and Cd 24 h after sowing at an open field, were investigated. The plants were treated with Zn, Cd, melatonin and a mixture of melatonin with the mentioned metals. Exogenously added melatonin increased the endogenous melatonin concentration in lemon balm leaves. However, in the valerian leaves, lower or the same endogenous melatonin level was detected. The significantly higher concentration of endogenous melatonin in both plants was measured after treatment with Zn. As the results showed, changes in superoxide dismutase (SOD) and peroxidase (POD) activities are species-specific and change depending on the plant development phase, and the type of treatment. Melatonin pretreatment induced alternation in SOD isoenzyme profiles and activities as well as POD activity in both plant species treated with heavy metals.
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