The Effect of Different Concentrations of Cadmium Chloride on Oxidative Stress in Shoot Cultures of Lemon Balm

Melissa officinalis is a medicinal plant belonging to the Lamiaceae family. The essential oil of this plant is used in many fields. Although more than 100 types of chemical substances have been identified in this plant, the most important compositions of its essential oil include citral, citronella, geraniol and linalool. Heavy metals are one of the main causes of non-biotic stress for living organisms due to increased use in the field of industrial and agricultural development and its high accumulation and toxicity. Cadmium is an unnecessary heavy metal, which, due to its high mobility and low concentration, easily enters the food chain from the soil. Cadmium induces oxidative stress by stimulating the synthesis of free oxygen radicals in the plant. In this study, the effect of cadmium chloride on oxidative stress induction in lemon balm was investigated. This experiment was carried out in 2016-2017 at Shahrekord University.

This research was conducted in a completely randomized design with three replications in vitro condition. We investigated the effects of different concentrations of cadmium chloride (0, 10, 20 and 40μm) on the biochemical parameters of the lemon balm. In this study, sterile stems of lemon balm propagated on the medium (1/2 MS) were used. After 60 days, seedlings were removed from solid MS medium and the roots were cut and cultured in liquid medium of 1/2 MS with different concentrations of cadmium. The cultivation was carried out in Erlenmeyer flask (250 cc) and was kept in an incubator shaker device. Sampling for the experiment was conducted one week after the growth of the stems in the medium.

Based on the results obtained in this study, fresh weight of shoots grown in 10 and 20μM of cadmium chloride increased compared to the control. This heavy metal significantly reduced the amount of chlorophyll a, b and carotenoids, while the total chlorophyll content decreased only at 40μm concentration relative to control. Cadmium significantly increased hydrogen peroxide levels in all treatments. Also, it increased the activity of catalase and superoxide dismutase enzymes. Malondialdehyde increased in all treatment concentrations, but this increase was not significant at 40μm of cadmium chloride. Chloride cadmium reduced the absorption of molybdenum and iron, yet increased the adsorption of manganese and zinc.

The results of this study revealed that the cadmium chloride in lemon balm shoots may have the potential to accumulate compounds such as hydrogen peroxide which, as a messenger molecule, produces antioxidants and can help the plant tolerate stressed conditions. It seems that two concentrations of 10 and 20 μm of cadmium chloride have no toxic effects for the stems and the plant has managed to withstand this tension with defense mechanisms.