Quantify the response of growth and chlorophyll fluorescence parameters of lemon balm (Melissa officinalis L.) medicinal plant to cadmium concentration in the soil

In order to quantify and simulate the response of lemon balm (Melissa officinalis L.) plants to cadmium (Cd) concentrations in the soil, a pot experiment was conducted using completely randomized design (CRD) with three replicates during 2014. The treatment consisted of twelve Cd concentrations from 0 to 300 mg kg-1 of Cd in the soil which was mixed with the experimental soil before planting. Lemon balm seeds were sown in pots containing different concentrations of cadmium. After eight weeks, chlorophyll fluorescence parameters, SPAD value and finally morphological characteristics and fresh and dry weights of plant organs were measured. The results showed that response of vegetative and chlorophyll fluorescence parameters in lemon balm to Cd concentrations described by either linear (five parameters) or segmented (14 parameters) models. Vegetative traits, SPAD value and some chlorophyll fluorescence such as Fv/Fm, Fv/Fo and Y(II) decreased while Fo, Y(NPQ) and Y(NO) increased when Cd concentration increased in the soil. Lemon balm was more sensitive to lower concentration of Cd. The most studied parameters of lemon balm such as shoot fresh and dry weights, Fo, Fv/Fm and Fv/Fo reflect a sharp slope in lower Cd concentrations at the range of 0 to 75 mg kg-1 of Cd and then changed with a slight slope. In conclusion, lemon balm plants are tolerant to 175 to 200 mg kg-1 of Cd in the soil, however, its dry matter accumulation was decreased up to 50% when Cd levels in the soil was nearly 30 mg kg-1.