Antioxidative response of Melissa officinalis L. and Valeriana officinalis L. leaves exposed to exogenous melatonin and excessive zinc and cadmium levels

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.