Effects of bulk and nanoparticles of zinc oxide on the tissue differentiation in Melissa officinalis

This study was performed to identify the anatomical responses of lemon balm to zinc oxide nanoparticles (nZnO) at the concentrations of 100 and 300 mg/l. The microscopic observation showed that the nZnO300 had the largest diameter of the central cylinder and the diameter of xylem and phloem in the root compared to the control. The study of stem cross-section revealed that the highest stem diameter was observed in response to the nZnO100 group. The maximum thickness of collenchyma and the highest diameter of the central cylinder and the diameter of phloem and xylem were also recorded in the nZnO100 group. Comparing the leaf cross sections indicated that the thickness of the upper and lower epidermis, the diameter of the xylem, the thickness of the spongy parenchyma in the nZnO100 group were significantly increased relative to the control. Comparison of petiole cross sections showed that the thickness of the upper and lower epidermis and the diameter of the central/lateral phloem and xylem in all nZnO-treated seedlings were significantly reduced compared to the control. This study provided anatomical evidence that indicates fundamental changes in the fate of cell differentiation and tissue formation in response to nZnO. Future research is needed to link physiological, molecular, and anatomical evidence to accurately understand the impact of nanoproducts on the entire life of plants, especially crops and medicines.