Investigating the Effect of Cadmium and Lead on Growth Parameters and Quality Characteristics of Lemon Balm (Melissa officinalis L.)

Introduction: Pollution of the biosphere with toxic levels of metals has accelerated dramatically since the beginning of the industrial revolution. Soil pollution by heavy metals including cadmium (Cd) and lead (Pb) is a global problem, which can cause agricultural lands to become hazardous for wildlife and human populations. Accumulated Cd and Pb in their roots and shoots may also be negatively affect their photosynthesis, growth, and reproduction. Cd and Pb uptake and their effects on plants may be influenced by a variety of factors, e.g. the plant species, cultivar, soil characteristics and etc. Heavy metals such as Cd and Pb may cause the formation of reactive oxygen species (ROS), damage plant tissue membranes, and inhibit photosynthesis, carbon dioxide assimilation, and growth. Photosynthesis is the fundamental process of energy metabolism and therefore, closely related to the plants growth and productivity. However, it is a sensitive process and an important target of environmental stresses. Plants counteract the harmful effects of heavy metals by a variety of protective mechanisms including immobilization, exclusion, chelation, compartmentalization, osmotic regulation and elevating antioxidant system.Lemon balm (Melissa officinalis L.) is a widely grown aromatic and medicinal plant of the Labiatae family. The plant has various therapeutic properties and is also used to flavor different food products due to its particular taste. Little scientific data exist on the response of this medicinal plant to Cd and Pb stress. Therefore, the objective of this work was to investigate the biological and physiological responses of lemon balm (Melissa officinalis L.) under cadmium and lead stress conditions.Materials and Methods: A factorial randomized complete block design experiment with four replications was used to study the effect of Cd in four concentrations (0, 6, 12 and 24 mg.kg-1 soil) as well as Pb in four concentrations (0, 150 300 and 450 mg. Kg-1 soil). Before harvesting, chlorophyll fluorescence, photosynthetic and transpiration rates were measured with fluorescence meter and photosynthetic meter, respectively. Thereafter, plants were harvested and the roots were washed in distilled water. Then, half of the plants were separated into roots and shoots, which were dried at 105 °C for 24 h to determine the dry weight, Cd and Pb concentrations, Cd transfer factor and essential oil content. The other half of the plants were separated into roots and shoots, kept in liquid nitrogen, and then stored in freezer for one week to determine proline and shoot MDA concentrations. Data were subjected to two-way analysis of variance (ANOVA) and the difference between means was compared using LSD test. A significance level of 95% was applied by SAS 9.2.Results and Discussion: According to the results, root and shoot dry weight, protein content, photosynthetic rate, transpiration rate and chlorophyll fluorescence were decreased by increasing Cd and Pb concentrations. However, Cd concentration decreased these traits more than Pb. The shoot MDA concentration, essential oil and proline content were enhanced by increasing Cd and Pbconcentrations. The combination of two pollutants (Cd and Pb) together reduces the negative impact of each element alone. Shoot and root Cd and Pb concentrations were increased by increasing Cd and Pbconcentrations. The interaction effect of Cd×Pbapplication showed that increasing Cd concentration decreased root and shoot Pb concentrations and increasing Pb concentration decreased root and shoot Cd concentrations. Cd and Pb were not detected in essential oil. Moreover, the Cd transfer factor was increased with increasing Pb concentration under high Cd concentration levels (12 and 24 mg.kg-1).Conclusion:  This study demonstrated that Cd toxicity is more than Pb toxicity due to its high mobility. There was an antagonistic relationship between Cd and Pb. Furthermore, due to the absence of Cd and Pb in the essential oil, lemon balm may be a suitable plant for areas contaminated with heavy metals especial Cd and Pb.