Antioxidative and Structural Responses of Melissa officinalis to Salt Stress

Salt stress is a common environmental factor that influences the growth and productivity of plants. However, various plants have evolved structural, physiological, and biochemical mechanisms to cope with salt stress. The anatomical changes and the involvement of the antioxidant system in relation to salt stress tolerance were investigated in Melissa officinalis during vegetative stage. The plants were subjected to NaCl stress (0, 50, 100, 150, 200 mM). After 40 days of treatments, the obtained results showed that the M. officinalis plants responded to salt stress anatomically, biochemically and physiologically. Biomass and photosynthetic pigments were reduced in the salt-exposed plants compared with the unexposed plants. Conversely, salt stress improved enzymatic antioxidants such as superoxide dismutase, polyphenole oxidase and peroxidase. Malondialdehyde content decreased in M. officinalis plants under salt stress that showed suitable antoxidative capacity against salinity-induced oxidative damage. Proline as a compatible osmolyte and non-enzymatic antioxidant heightened under salt conditions. Salt stress increased vascular area, ratio of xylem on phelom, central cylinder thickness and the thickness of xylem, phloem, pith and cortical parenchyma M. officinalis plants for overcome on stress conditions. Salt stress caused increase of xylem and phloem denser, enhancement of tracheid in roots and deformation of leaf cells. The length of the dorsal surface trichomes increased significantly under salinity. These findings displayed the different defense mechanisms in M. officinalis plants against salt stress and confirmed M. officinalis is a natrophobe plant.